"Pluto Now Called a Plutoid"
Erik Max Francis
http://www.space.com/scienceastronomy/080611-plutoid-planets.html
Here's the official IAU press release:
http://www.iau.org/public_press/news/release/iau0804/
--
Erik Max Francis && m...@alcyone.com && http://www.alcyone.com/max/
San Jose, CA, USA && 37 18 N 121 57 W && AIM, Y!M erikmaxfrancis
There is another world, which is not of men.
-- Li Bai
Bryan Derksen
> Yay, here we go again ...
There's already a class of object called "Plutinos", which overlaps
significantly (though apparently not completely) with these Plutoids.
That's likely to be confusing.
One particularly odd feature of this new classification is the absolute
magnitude criterion. A Plutoid has to have an absolute magnitude
brighter than +1. I imagine it'd be possible to have an object that's a
Plutoid when its atmosphere is frozen out as a bright layer of frost
during the outer part of its orbit and then loses Plutoid status (but
not dwarf planet status) when it approaches perihelion and the dark
organic crust is exposed. :)
Bryan Derksen
> One particularly odd feature of this new classification is the absolute
> magnitude criterion. A Plutoid has to have an absolute magnitude
> brighter than +1. I imagine it'd be possible to have an object that's a
> Plutoid when its atmosphere is frozen out as a bright layer of frost
> during the outer part of its orbit and then loses Plutoid status (but
> not dwarf planet status) when it approaches perihelion and the dark
> organic crust is exposed. :)
I amend myself, it's not _quite_ that silly. Turns out on further
reading that the absolute magnitude criterion is just a crude "first
pass" used to determine whether an object should go through the Plutino
naming process, since it's easier to determine absolute magnitude than
it is to determine mass and diameter and roundness.
Erik Max Francis
> Erik Max Francis wrote:
>> Yay, here we go again ...
>
> There's already a class of object called "Plutinos", which overlaps
> significantly (though apparently not completely) with these Plutoids.
> That's likely to be confusing.
Yes. Not to mention it is, yet again, remarkably pointless.
--
Erik Max Francis && m...@alcyone.com && http://www.alcyone.com/max/
San Jose, CA, USA && 37 18 N 121 57 W && AIM, Y!M erikmaxfrancis
Convictions are more dangerous enemies of truth than lies.
-- Friedrich Nietzsche
Bryan Derksen
> Bryan Derksen wrote:
>
>> Erik Max Francis wrote:
>>> Yay, here we go again ...
>> There's already a class of object called "Plutinos", which overlaps
>> significantly (though apparently not completely) with these Plutoids.
>> That's likely to be confusing.
>
> Yes. Not to mention it is, yet again, remarkably pointless.
At least they didn't stick with Pluton, which is already in use by
geologists for something else entirely.
IMO if they really needed some specific term for those bodies,
"Trans-Neptunian Dwarf Planet" or "TNDP" would fit quite nicely within
the existing "Trans-Neptunian Object" and "Dwarf Planet"
classifications. A nice, simple intersection.
Or maybe "DPEC", for "Dwarf Planets Except Ceres." Assuming Vesta et al
don't make the cut once they refine their roundness criteria a bit. :)
VGer47
Why not 'planetoid' ? An existing term that would seem to fit airly
well.
Bryan Derksen
I'm assuming they wanted to differentiate between rocky and icy bodies.
"Planetoid" is pretty generic, and since it's already in use redefining
it would probably not work well.
Zombie Elvis
wrote:
>Yay, here we go again ...
>
>http://www.space.com/scienceastronomy/080611-plutoid-planets.html
>
>Here's the official IAU press release:
>
>http://www.iau.org/public_press/news/release/iau0804/
Here's what I don't get. They mention Pluto and Eris and explain why
they are considered Plutoids. They mention Ceres and why it's not a
Plutoid. So what about the big, round Transneptunian objects? What
about Sedna, perhaps the first Oort Cloud refugee ever seen? What
about Orcus? What about Varuna? What about Quauar, the Scrabble World?
Most of these would seem to fit the definition for a Plutoid except
for the brightness criterion which seems to have been added to cause
confusion rather than to remove it.
--
Cause, really, nothing says "I'm a counter culture
rebel, fighting the establishment" like an Aibo on
a skateboard.
- Seen on Slashdot
Roberto Castillo
roberto...@ameritech.net
http://mind-grapes.blogspot.com/
http://zombie-gulch.myminicity.com/
Erik Max Francis
> Here's what I don't get. They mention Pluto and Eris and explain why
> they are considered Plutoids. They mention Ceres and why it's not a
> Plutoid. So what about the big, round Transneptunian objects? What
> about Sedna, perhaps the first Oort Cloud refugee ever seen? What
> about Orcus? What about Varuna? What about Quauar, the Scrabble World?
> Most of these would seem to fit the definition for a Plutoid except
> for the brightness criterion which seems to have been added to cause
> confusion rather than to remove it.
Don't know what to tell you, except welcome to the IAU's continuing
stupid and pointless declarations. More to follow, surely.
--
Erik Max Francis && m...@alcyone.com && http://www.alcyone.com/max/
San Jose, CA, USA && 37 18 N 121 57 W && AIM, Y!M erikmaxfrancis
The conviction of wisdom is the plague of man.
-- Montaigne
mcv
I propose a new subcategory: the seasonal Plutoids.
mcv.
--
Science is not the be-all and end-all of human existence. It's a tool.
A very powerful tool, but not the only tool. And if only that which
could be verified scientifically was considered real, then nearly all
of human experience would be not-real. -- Zachriel
bal...@gmail.com
> Zombie Elvis wrote:
> > Here's what I don't get. They mention Pluto and Eris and explain why
> > they are considered Plutoids. They mention Ceres and why it's not a
> > Plutoid. So what about the big, round Transneptunian objects? What
> > about Sedna, perhaps the first Oort Cloud refugee ever seen? What
> > about Orcus? What about Varuna? What about Quauar, the Scrabble World?
> > Most of these would seem to fit the definition for a Plutoid except
> > for the brightness criterion which seems to have been added to cause
> > confusion rather than to remove it.
>
> Don't know what to tell you, except welcome to the IAU's continuing
> stupid and pointless declarations. More to follow, surely.
>
> --
> San Jose, CA, USA && 37 18 N 121 57 W && AIM, Y!M erikmaxfrancis
> The conviction of wisdom is the plague of man.
> -- Montaigne
I agree the name is not that important.
But I think this objects are a fantastic environment for SF, because
if colonized enable a space civilization with many worlds and
constantly expnading without space warp. They are dark and cold,but
probably contains all the required elements for life and industy,
including volatiles.
Sorry for my broken english.
TBerk
> Yay, here we go again ...
>
> http://www.space.com/scienceastronomy/080611-plutoid-planets.html
>
> Here's the official IAU press release:
>
> http://www.iau.org/public_press/news/release/iau0804/
>
> --
> Â San Jose, CA, USA && 37 18 N 121 57 W && AIM, Y!M erikmaxfrancis
> Â Â There is another world, which is not of men.
> Â Â -- Li Bai
Screw that. (Pardon my exclamation.)
Pluto is a planet.
TBerk
Space Cadet
Here! Here!
I second that motion!
All Agreed!
Just my $0.02
Keith W of St Louis aka Space Cadet
Quadibloc
> There's already a class of object called "Plutinos", which overlaps
> significantly (though apparently not completely) with these Plutoids.
> That's likely to be confusing.
Since Plutinos are *only* those bodies which share the same 2:3
resonance with Neptune that Pluto has, but regardless of size, while
there will be confusion due to the similarity in the names, they still
refer to very different (although overlapping) classes of objects.
We do already have terms for "gas giants" and "terrestrial planets";
some have noted that the whole idea of a 'planet' is flawed, because
the gap between those two is much greater than the gap between the
terrestrial planets and either the Kuiper Belt bodies or even the
asteroids.
A tiny chunk of rock in space is a meteoroid; when it plunges through
the Earth's atmosphere to be a light in our sky, it's a meteor, and
when it hits the ground, it's a meteorite.
By analogy, perhaps a comet should be called a cometoid when it's too
far from the Sun to have a tail - or, at least, an icy body, similar
to a comet, but the orbit of which never goes near enoough to the Sun
to allow a tail to be manifest, should be called a cometoid. This term
would apply, for example, to Chiron and the other Centaurs.
Starting from that, Pluto and the other large Kuiper Belt objects
would become quasi-planetary cometoids, or QPCs. However, this
wouldn't quite be synonymous with Plutoid, since the size threshold
would be lower, perhaps at 1.5 times the radius of Ceres.
If the Kuiper Belt is found to have bodies in it with a diameter equal
to or greater than that of Mercury, but which are not desired to be
called planets because they're in the same belt with other objects of
varying size, those objects could simply be called PCs; planetary
cometoids.
As for Pluto, however, since it _is_ much larger than any other
Plutino, it _has_ cleared its own orbit of everything but debris, even
if there are similar bodies in orbits only a little further out, so it
could be left as a planet (but then its diameter, rather than
Mercury's, would mark the dividing line between a QPC and a PC). That
would also mean that Eris would become a planet after we knew that it
was significantly bigger than all the Erisinos out there...
John Savard
Quadibloc
System bodies, Centaurs much bigger than Chiron, I did a web search,
and found, at
http://www.expreso.co.cr/centaurs/posts/notes/chariklo.html
that at least *some* astrologers are consistent enough to think of
Chariklo - and Pholus, for that matter - as well if they're going to
add Chiron to their astrological charts!
John Savard
Brett Paul Dunbar
<4b025605-faeb-465b...@j22g2000hsf.googlegroups.com>,
Quadibloc <jsa...@ecn.ab.ca> writes
>As for Pluto, however, since it _is_ much larger than any other
>Plutino, it _has_ cleared its own orbit of everything but debris, even
>if there are similar bodies in orbits only a little further out, so it
>could be left as a planet (but then its diameter, rather than
>Mercury's, would mark the dividing line between a QPC and a PC). That
>would also mean that Eris would become a planet after we knew that it
>was significantly bigger than all the Erisinos out there...
Pluto isn't a planet because it hasn't cleared its orbit Neptune has.
Neptune has cleared that region of space of debris, what's left either
orbits Neptune (e.g. Triton the second largest object in that region),
is in a resonance with Neptune (e.g. Pluto, the third largest object) or
is a transient (e.g. various Comets).
Eris is not a planet either as it is not much more massive than the
potentially colliding mass in its region of space. Ceres is about one
third the mass of the potentially colliding mass, Eris is about one
tenth the mass of the potentially colliding mass, and Pluto is about one
thirteenth the mass of the potentially colliding mass
The definition might seem a little fuzzy but nothing lies remotely close
to the boundary so it is absolutely unambiguous in practice. Ceres, the
most planet like dwarf planet is about one third of the mass of the
potentially colliding bodies, Neptune the least planet like of all the
Planets is 24 thousand times as massive as the potentially colliding
mass. The gap is about four orders of magnitude.
If the Kuiper wall is created by an object, this object will have
cleared its orbit and will therefore be a planet. It might lie closer to
the boundary than any of the currently known objects.
--
Great Internet Mersenne Prime Search http://www.mersenne.org/prime.htm
Livejournal http://brett-dunbar.livejournal.com/
Brett Paul Dunbar
To email me, use reply-to address
Bryan Derksen
> On Jun 12, 8:15 am, Bryan Derksen <bryan.derk...@shaw.ca> wrote:
>
>> There's already a class of object called "Plutinos", which overlaps
>> significantly (though apparently not completely) with these Plutoids.
>> That's likely to be confusing.
>
> Since Plutinos are *only* those bodies which share the same 2:3
> resonance with Neptune that Pluto has, but regardless of size, while
> there will be confusion due to the similarity in the names, they still
> refer to very different (although overlapping) classes of objects.
Which was my point. :)
Erik Max Francis
> In message
> <4b025605-faeb-465b...@j22g2000hsf.googlegroups.com>,
> Quadibloc <jsa...@ecn.ab.ca> writes
>
>> As for Pluto, however, since it _is_ much larger than any other
>> Plutino, it _has_ cleared its own orbit of everything but debris, even
>> if there are similar bodies in orbits only a little further out, so it
>> could be left as a planet (but then its diameter, rather than
>> Mercury's, would mark the dividing line between a QPC and a PC). That
>> would also mean that Eris would become a planet after we knew that it
>> was significantly bigger than all the Erisinos out there...
>
> Pluto isn't a planet because it hasn't cleared its orbit Neptune has.
> Neptune has cleared that region of space of debris, what's left either
> orbits Neptune (e.g. Triton the second largest object in that region),
> is in a resonance with Neptune (e.g. Pluto, the third largest object) or
> is a transient (e.g. various Comets).
More accurately and succinctly: Pluto is no longer a planet because the
IAU said so. Their "objective" definition was so vague that they had to
include a list of planets to make sure people knew what modifications
that entailed, and Pluto wasn't on it.
--
Erik Max Francis && m...@alcyone.com && http://www.alcyone.com/max/
San Jose, CA, USA && 37 18 N 121 57 W && AIM, Y!M erikmaxfrancis
Fear is an emotion indispensible for survival.
-- Hannah Arendt
Quadibloc
But it would have been even *more* confusing if the overlap was
bigger. I hadn't been sure what your precise point was, so I thought
this was worth mentioning.
John Savard
Brett Paul Dunbar
Francis <m...@alcyone.com> writes
>Brett Paul Dunbar wrote:
>
>> In message <4b025605-faeb-465b...@j22g2000hsf.googleg
>>roups.com>, Quadibloc <jsa...@ecn.ab.ca> writes
>>
>>> As for Pluto, however, since it _is_ much larger than any other
>>> Plutino, it _has_ cleared its own orbit of everything but debris, even
>>> if there are similar bodies in orbits only a little further out, so it
>>> could be left as a planet (but then its diameter, rather than
>>> Mercury's, would mark the dividing line between a QPC and a PC). That
>>> would also mean that Eris would become a planet after we knew that it
>>> was significantly bigger than all the Erisinos out there...
>> Pluto isn't a planet because it hasn't cleared its orbit Neptune
>>has. Neptune has cleared that region of space of debris, what's left
>>either orbits Neptune (e.g. Triton the second largest object in that
>>region), is in a resonance with Neptune (e.g. Pluto, the third
>>largest object) or is a transient (e.g. various Comets).
>
>More accurately and succinctly: Pluto is no longer a planet because
>the IAU said so. Their "objective" definition was so vague that they
>had to include a list of planets to make sure people knew what
>modifications that entailed, and Pluto wasn't on it.
>
No that is actually totally wrong. The definition is entirely
unambiguous, given the sheer size of the gap. On either the
Stern-Levinson parameter or Soter's Planetary discriminant, the gap is
about five orders of magnitude.
Name Stern-Levinson Planetary Mass (kg)
parameter discriminant
(Earth=1)
Earth 1.00 1.7×10^6 (5.9736×10^24 kg)
Venus 1.08 1.35×10^6 (4.8685×10^24 kg)
Jupiter 8510 6.25×10^5 (1.899×10^27 kg)
Saturn 308 1.9×10^5 (5.6846×10^26 kg)
Mars 0.0061 1.8×10^5 (6.4185×10^23 kg)
Mercury 0.0126 9.1×10^4 (3.302×10^23 kg)
Uranus 2.51 2.9×10^4 (8.6832×10^25 kg)
Neptune 1.79 2.4×10^4 (1.0243×10^26 kg)
Ceres 8.7×10^-9 0.33 (9.5×10^20 kg)
Eris 3.5×10^-8 0.10 (1.66×10^22 kg)
Pluto 1.95×10^-8 0.077 (1.29×10^22 ± 10% kg)
The number of objects classified as planets was small enough that it was
possible to list all the known objects that meet the criteria, listing
examples can be useful and there are only eight known so it was entirely
practical to list all of them. The definition of a Dwarf planet on the
other hand is far less clear for example Vesta appears to have become
spherical, solidified then been distorted by as massive impact, it is
unclear whether that is a dwarf planet or a small solar system body.
Stern's objections are more terminology than content, the IAU definition
matches what he calls an überplanet, clearly he finds the class itself
useful even if he isn't happy about the name.
In fact this postulated Kuiper wall object would also clearly be a
planet, indeed its existence is postulated as there is evidence that the
region of space just outside the Kuiper belt has been cleared, an object
which has cleared its region of space is a planet therefore if it is
responsible for the Kuiper wall then it is a planet.
Quadibloc
wrote:
> In message <nI2dnQje4vHry_3VnZ2dnUVZ_srin...@speakeasy.net>, Erik Max
> Francis <m...@alcyone.com> writes
> >More accurately and succinctly: Pluto is no longer a planet because
> >the IAU said so. Their "objective" definition was so vague that they
> >had to include a list of planets to make sure people knew what
> >modifications that entailed, and Pluto wasn't on it.
>
> No that is actually totally wrong. The definition is entirely
> unambiguous, given the sheer size of the gap. On either the
> Stern-Levinson parameter or Soter's Planetary discriminant, the gap is
> about five orders of magnitude.
So there is nothing fishy going on here at all; Soter's Planetary
Discriminant has saved us all from any uncertainty concerning what is,
or isn't, a planet.
John Savard
(Find the hidden pun in my post above!)
Bryan Derksen
> The definition of a Dwarf planet on the
> other hand is far less clear for example Vesta appears to have become
> spherical, solidified then been distorted by as massive impact, it is
> unclear whether that is a dwarf planet or a small solar system body.
Sadly, I don't think it'll be as easy to come up with a sharply-defined
criterion for dwarf planets as it was with planets. With planets there's
a solid physical reason why a distinction exists; once they're big
enough to start dominating their orbital neighborhoods it tends to
become a self-reinforcing situation, with them becoming even larger in
the process of sweeping up the debris. But there's nothing in particular
to distinguish a dwarf planet from any of the other debris that share
its orbit beyond just happening to be larger than most of it.
IMO it would perhaps have been better to just stick with orbit-based
categories for the small stuff - Main Belt, Kuiper Belt, etc. - and if
any particular objects within those belts distinguished themselves from
the others for some reason we could simply refer to them by name. If we
can't remember their names they must not be all that significant. But I
suppose I can see the necessity of the politics behind it considering
how unsatisfied some folks are even with dwarf planethood for Pluto and
company.
> Stern's objections are more terminology than content, the IAU definition
> matches what he calls an überplanet, clearly he finds the class itself
> useful even if he isn't happy about the name.
Considering how over-the-top some of Stern's objections are I'm not sure
I can guess what is really the root cause of them. You'd think the IAU
kicked his dog sometimes. :)
> In fact this postulated Kuiper wall object would also clearly be a
> planet, indeed its existence is postulated as there is evidence that the
> region of space just outside the Kuiper belt has been cleared, an object
> which has cleared its region of space is a planet therefore if it is
> responsible for the Kuiper wall then it is a planet.
Perhaps having a dramatically larger-than-Pluto ninth planet show up
would help settle things. It'd at least help to illustrate that the
definition of planet isn't as arbitrary as it may seem at first glance.
Erik Max Francis
> In message <nI2dnQje4vHry_3V...@speakeasy.net>, Erik Max
> Francis <m...@alcyone.com> writes
>> Brett Paul Dunbar wrote:
>>> Pluto isn't a planet because it hasn't cleared its orbit Neptune
>>> has. Neptune has cleared that region of space of debris, what's left
>>> either orbits Neptune (e.g. Triton the second largest object in that
>>> region), is in a resonance with Neptune (e.g. Pluto, the third
>>> largest object) or is a transient (e.g. various Comets).
>> More accurately and succinctly: Pluto is no longer a planet because
>> the IAU said so. Their "objective" definition was so vague that they
>> had to include a list of planets to make sure people knew what
>> modifications that entailed, and Pluto wasn't on it.
>
> No that is actually totally wrong. The definition is entirely
> unambiguous, given the sheer size of the gap. On either the
> Stern-Levinson parameter or Soter's Planetary discriminant, the gap is
> about five orders of magnitude.
Neither of these measures is mentioned, even indirectly, in the IAU's
resolution regarding Pluto or any subsequent IAU resolutions. So, so what?
> The number of objects classified as planets was small enough that it was
> possible to list all the known objects that meet the criteria, listing
> examples can be useful and there are only eight known so it was entirely
> practical to list all of them. The definition of a Dwarf planet on the
> other hand is far less clear for example Vesta appears to have become
> spherical, solidified then been distorted by as massive impact, it is
> unclear whether that is a dwarf planet or a small solar system body.
You're making my point for me.
> Stern's objections are more terminology than content, the IAU definition
> matches what he calls an überplanet, clearly he finds the class itself
> useful even if he isn't happy about the name.
So the references you're using to back up the IAU disagree with the
IAU's resolution and new classification system. Ironic.
--
Erik Max Francis && m...@alcyone.com && http://www.alcyone.com/max/
San Jose, CA, USA && 37 18 N 121 57 W && AIM, Y!M erikmaxfrancis
Experience is a good school, but the fees are high.
-- Heinrich Heine
Bryan Derksen
> Brett Paul Dunbar wrote:
>> No that is actually totally wrong. The definition is entirely
>> unambiguous, given the sheer size of the gap. On either the
>> Stern-Levinson parameter or Soter's Planetary discriminant, the gap is
>> about five orders of magnitude.
>
> Neither of these measures is mentioned, even indirectly, in the IAU's
> resolution regarding Pluto or any subsequent IAU resolutions. So, so what?
They don't mention any other specific method of measuring
neighborhood-clearing either. Or several other potentially ambiguous
parts of the definition, such as what it means to be "orbiting the sun"
(there are various co-orbital bodies that could go either way if you
play with the definition a bit). I think they assumed that most people
reading their resolution would not be trying to lawyerly nitpick holes
in it but would instead be more interested in the basic spirit it conveyed.
Do you know of any methods of quantifying "neighborhood-clearing" that
don't make the division between Pluto and the rest obvious?
Why not assume that they intend one of the previously-published methods
that _do_ make this division obvious, since they explicitly say in the
resolution that it makes that division? Do you have some reason to
believe that they picked an ambiguous definition and then made an
arbitrary decision to exclude Pluto?
>> The number of objects classified as planets was small enough that it was
>> possible to list all the known objects that meet the criteria, listing
>> examples can be useful and there are only eight known so it was entirely
>> practical to list all of them. The definition of a Dwarf planet on the
>> other hand is far less clear for example Vesta appears to have become
>> spherical, solidified then been distorted by as massive impact, it is
>> unclear whether that is a dwarf planet or a small solar system body.
>
> You're making my point for me.
The only thing being called into question as imprecise here is the
distinction between dwarf planets and small solar system bodies, not the
distinction between planets and dwarf planets.
In the real-life solar system there's a smooth continuum of examples
from "almost perfectly round" to "not at all round", so it's to be
expected that there'll be fuzzy edges if you base a definition on that
characteristic. Orbital cleanliness, on the other hand, shows a large
discontinuity between the eight biggest objects of the solar system and
all the rest of the swarming masses. So using that as a criterion is
much clearer.
>> Stern's objections are more terminology than content, the IAU definition
>> matches what he calls an überplanet, clearly he finds the class itself
>> useful even if he isn't happy about the name.
>
> So the references you're using to back up the IAU disagree with the
> IAU's resolution and new classification system. Ironic.
Bwah? The division given in the Stern and Levison paper agrees exactly
with the results of the IAU's resolution, dividing solar system objects
along the same line that the IAU did. It's only the words used to label
the resulting groups that differs, as Brett said. Which reference did
you think he was using?
The Stern-Levison paper is at
http://www.boulder.swri.edu/~hal/PDF/planet_def.pdf in case anyone's new
to this iteration of the debate.
Eivind
> They don't mention any other specific method of measuring
> neighborhood-clearing either. Or several other potentially ambiguous
> parts of the definition, such as what it means to be "orbiting the sun"
> (there are various co-orbital bodies that could go either way if you
> play with the definition a bit). I think they assumed that most people
> reading their resolution would not be trying to lawyerly nitpick holes
> in it but would instead be more interested in the basic spirit it conveyed.
People knew the "basic spirit" of "planet" before they started this
mess. So that means they're back at square one, yes ?
Before 'definition' everyone knows -aproximately- what "planet" means,
but there's some fuzziness about the edge-cases, prompting a pointless
quest for a single, clear, agreed-upon definition of the word.
After the 'definition' everyone knows -aproximately- what "planet"
means, but there's some fuzziness about the edge-cases.
So, everyone knows precisely what they knew all along. The only real
difference is, pluto got demoted.
Eivind Kjørstad
Bryan Derksen
> Bryan Derksen skreiv:
>
>> They don't mention any other specific method of measuring
>> neighborhood-clearing either. Or several other potentially ambiguous
>> parts of the definition, such as what it means to be "orbiting the sun"
>> (there are various co-orbital bodies that could go either way if you
>> play with the definition a bit). I think they assumed that most people
>> reading their resolution would not be trying to lawyerly nitpick holes
>> in it but would instead be more interested in the basic spirit it conveyed.
>
> People knew the "basic spirit" of "planet" before they started this
> mess. So that means they're back at square one, yes ?
No, not really. With the discovery of a whole class of Pluto-sized
iceballs looming there was great confusion over whether they should be
called "planets" or not. There wasn't any _general_ definition of
"planet" that could be used to make the determination, just an arbitrary
historical list, so everyone had their own "basic spirit" and the IAU
had to figure out some more official criteria for whether new objects
should be on the list or not.
> After the 'definition' everyone knows -aproximately- what "planet"
> means, but there's some fuzziness about the edge-cases.
Again, no, not really. The definition is much less ambiguous now,
there's a gap of many orders of magnitude in "orbital cleanliness"
between the objects considered planets and the objects not considered
planets. Unless any future discoveries happen to fall right in the
middle of that gap (which for theoretical reasons is unlikely) it'll be
quite obvious which way to classify them.
Erik Max Francis
> Erik Max Francis wrote:
>> Brett Paul Dunbar wrote:
>>> No that is actually totally wrong. The definition is entirely
>>> unambiguous, given the sheer size of the gap. On either the
>>> Stern-Levinson parameter or Soter's Planetary discriminant, the gap is
>>> about five orders of magnitude.
>>
>> Neither of these measures is mentioned, even indirectly, in the IAU's
>> resolution regarding Pluto or any subsequent IAU resolutions. So, so
>> what?
>
> They don't mention any other specific method of measuring
> neighborhood-clearing either. Or several other potentially ambiguous
> parts of the definition, such as what it means to be "orbiting the sun"
> (there are various co-orbital bodies that could go either way if you
> play with the definition a bit). I think they assumed that most people
> reading their resolution would not be trying to lawyerly nitpick holes
> in it but would instead be more interested in the basic spirit it conveyed.
>
> Do you know of any methods of quantifying "neighborhood-clearing" that
> don't make the division between Pluto and the rest obvious?
Sure, there are plenty of trivial ones. "An object has cleared its
orbit if there are no orbits intersecting it (after projecting both
their orbits into the ecliptic) that are greater in mass than Ceres."
Or, "There are no other objects greater in mass than Ceres within 2 au."
Or any number of other quantified definitions.
> Why not assume that they intend one of the previously-published methods
> that _do_ make this division obvious, since they explicitly say in the
> resolution that it makes that division? Do you have some reason to
> believe that they picked an ambiguous definition and then made an
> arbitrary decision to exclude Pluto?
Since they never gave or hinted at a definition, and excluded Pluto
anyway, yes.
Remember, the IAU has never said word one about what definition they
were using. It's pretty obvious, because just like the other
definitions, they didn't give one. (What is sufficiently "round"?) All
the discussion you've carried on about _which_ definition they might
have meant is pure speculation, because as you've already acknowledged,
there are several.
They were giving vague measures of what it means to be a planet. But
that was stupid; we all already _knew_ vaguely what it means to be a
planet -- it's something that's big, round, and gravitational
influential on its neighbors. The IAU definition accomplished nothing,
except strike one of the previous members off the list.
--
Erik Max Francis && m...@alcyone.com && http://www.alcyone.com/max/
San Jose, CA, USA && 37 18 N 121 57 W && AIM, Y!M erikmaxfrancis
Do we really want to go to Mars / Do we really want to try
-- Cassandra Wilson
Erik Max Francis
Yep, well-summarized.
--
Erik Max Francis && m...@alcyone.com && http://www.alcyone.com/max/
San Jose, CA, USA && 37 18 N 121 57 W && AIM, Y!M erikmaxfrancis
Brett Paul Dunbar
Francis <m...@alcyone.com> writes
That is a rather contrived and obviously inappropriate definition. I.e.
it is obviously wrong.
Soter's paper <http://arxiv.org/ftp/astro-ph/papers/0608/0608359.pdf>
gives a fairly decent basis for determining whether the neighbourhood
has been cleared in the concept of the planetary discriminant; the ratio
between the mass of the body and the total mass of the other
non-resonating and non-satellite bodies in the same orbital zone. He
defined the orbital zone as follows "Two bodies share an orbital zone if
their orbits cross a common radial distance from the primary, and their
non-resonant periods differ by less than an order of magnitude." This
seems a good definition for neighbourhood.
>
>> Why not assume that they intend one of the previously-published
>>methods that _do_ make this division obvious, since they explicitly
>>say in the resolution that it makes that division? Do you have some
>>reason to believe that they picked an ambiguous definition and then
>>made an arbitrary decision to exclude Pluto?
>
>Since they never gave or hinted at a definition, and excluded Pluto
>anyway, yes.
They did, they resolved on clearing the neighbourhood and hydrostatic
equilibrium. The two different papers give markedly different
definitions of clearing the neighbourhood but identical results,
indicating that what we have done is apply the label planet to a natural
class. We have reason to believe that there is a critical mass above
which an object will eject or absorb all of the non-resonant mass in the
vicinity of its orbit. This means that once a body passes the planet
threshold it will grow rapidly and quickly be an entirely unambiguous
case.
What we have a clear-cut separation based on the orbital dynamics of the
body.
Stern and Levinson's paper
<http://www.boulder.swri.edu/~hal/PDF/planet_def.pdf> attempted to find
which objects would do that, while Soter's measured whether a given
object actually had.
>
>Remember, the IAU has never said word one about what definition they
>were using. It's pretty obvious, because just like the other
>definitions, they didn't give one. (What is sufficiently "round"?) All
>the discussion you've carried on about _which_ definition they might
>have meant is pure speculation, because as you've already acknowledged,
>there are several.
There are two currently, it is possible that there are a few others that
differ slightly, but all would produce a division at the same point
>
>They were giving vague measures of what it means to be a planet. But
>that was stupid; we all already _knew_ vaguely what it means to be a
>planet -- it's something that's big, round, and gravitational
>influential on its neighbors. The IAU definition accomplished nothing,
>except strike one of the previous members off the list.
>
Pluto didn't fit, it had been called a planet when it was first
discovered as it was thought to be far more massive than it actually
was, eventually it became a tiny anomaly, it didn't fit with the planets
but it didn't fit any other category either, while there was only the
one object it just got left. The discovery of other Plutonian bodies
allowed the definition of a more appropriate category for Pluto and its
removal from the class of planets, much as the discovery of other
Asteroids had resulted in Ceres being re classified.
Quadibloc
> The IAU definition accomplished nothing,
> except strike one of the previous members off the list.
Actually, it did accomplish something else besides that.
It kept Eris off the list.
Which was, of course, the whole point. Pluto was previously believed
to be larger and heavier than it actually was. But it was still
considerably larger and heavier than Ceres. So there was no real
urgency to decide that Pluto was, or was not, a planet as Pluto
shrank, because there was still a clear threshhold at a size smaller
than Pluto that would keep the number of planets limited.
But when the Kuiper Belt Objects started being discovered, and
especially once Eris was discovered and found to be larger than Pluto,
matters came to a head. If a new category of object existed which
would number in the hundreds, whose members would qualify as planets,
this would create a planet "gold rush", as it were, and seriously
compromise the value of the designation of a body as a planet.
Four new planets in the solar system - Ceres, Pallas, Juno, and Vesta
- was one thing. Thousands of new planets is another.
There's a big enough size gap between Mercury and Pluto that excluding
Pluto is not arbitrary, and the existence of Plutinos, showing that
Pluto's orbit is not "clear", for some value of clear that allows
Jupiter (despite its Trojans) to be a planet - and here numerical
things like Soter's Planetary Discriminant *help* give a quantifiable
justification - adds to the ability to... rationalize the decision.
But the main thing is that while it would be pleasing to add new
planets to the Solar System by ones and twos, if they bid fair to be
added by the hundreds, anything that might be so added ought to be
excluded from our definition of a planet.
However much I regret the downgrading of Pluto, after so many years of
being accepted as a planet, on an emotional level, I still understand
why it was felt to be an unavoidable decision after the discovery of
Eris.
John Savard
Bryan Derksen
> Bryan Derksen wrote:
>> don't make the division between Pluto and the rest obvious?
>
> Sure, there are plenty of trivial ones. "An object has cleared its
> orbit if there are no orbits intersecting it (after projecting both
> their orbits into the ecliptic) that are greater in mass than Ceres."
> Or, "There are no other objects greater in mass than Ceres within 2 au."
> Or any number of other quantified definitions.
These are pretty silly definitions. If you include moons in
consideration (since you didn't mention anything that excluded them from
consideration) the first definition eliminates Earth, Jupiter, Saturn,
Uranus, Neptune, Pluto, and Eris. The second eliminates every candidate
_except_ Eris, and even then Eris may sometimes temporarily lose
planethood when other large Kuiper belt objects pass near it.
Not considering moons, the first definition eliminates Neptune, Pluto
and Eris as planets and the second definition eliminates Mercury, Venus,
Earth, and Mars (both due to their nearness to each other and their
nearness to the Sun, which is more massive than Ceres). In the second
definition Pluto and Eris are sometimes planets and sometimes
non-planets depending on how the other large Kuiper belt objects are
arranged on any given day. Ceres itself is also sometimes a planet and
sometimes not depending on where it is in its orbit relative to Mercury,
Venus, Earth and Mars.
So clearly these are not what the IAU had in mind. Where did these come
from? Or did you just make them up on the spot?
>> Why not assume that they intend one of the previously-published methods
>> that _do_ make this division obvious, since they explicitly say in the
>> resolution that it makes that division? Do you have some reason to
>> believe that they picked an ambiguous definition and then made an
>> arbitrary decision to exclude Pluto?
>
> Since they never gave or hinted at a definition, and excluded Pluto
> anyway, yes.
Could you please explain _why_, though? I already know that you're
assuming this, I asked what reason you had.
Definitions of planet clearing had been previously published. The
members of the IAU who voted on this resolution are, generally speaking,
well-read on such matters. I don't see any reason to assume they didn't
have those definitions in mind, especially since they give the same
results as the IAU's unspoken definition.
> Remember, the IAU has never said word one about what definition they
> were using. It's pretty obvious, because just like the other
> definitions, they didn't give one. (What is sufficiently "round"?) All
> the discussion you've carried on about _which_ definition they might
> have meant is pure speculation, because as you've already acknowledged,
> there are several.
The only two orbit-clearing definitions I've seen that weren't contrived
on the spot (like those other two you gave above, I suspect) give the
same clear and unambiguous results, results which agree with the IAU's
list of what the results are supposed to be. Perhaps it's speculation to
say that they probably meant one of those but it seems like pretty
well-founded speculation to me.
> They were giving vague measures of what it means to be a planet. But
> that was stupid; we all already _knew_ vaguely what it means to be a
> planet -- it's something that's big, round, and gravitational
> influential on its neighbors. The IAU definition accomplished nothing,
> except strike one of the previous members off the list.
They also struck down a bunch of candidates that nobody was entirely
sure would be planets or not, because the previous "definition" of
planet was far _more_ vague than the one you're complaining about. It
also provided some measurable basis for arguing whether new discoveries
are planets - the hypothetical object responsible for the Kuiper "cliff"
mentioned elsewhere in this thread, for example. Those both seem like
accomplishments to me.
Erik Max Francis
> On Jun 26, 9:32 pm, Erik Max Francis <m...@alcyone.com> wrote:
>> The IAU definition accomplished nothing,
>> except strike one of the previous members off the list.
>
> Actually, it did accomplish something else besides that.
>
> It kept Eris off the list.
>
> Which was, of course, the whole point. Pluto was previously believed
> to be larger and heavier than it actually was. But it was still
> considerably larger and heavier than Ceres. So there was no real
> urgency to decide that Pluto was, or was not, a planet as Pluto
> shrank, because there was still a clear threshhold at a size smaller
> than Pluto that would keep the number of planets limited.
>
> But when the Kuiper Belt Objects started being discovered, and
> especially once Eris was discovered and found to be larger than Pluto,
> matters came to a head. If a new category of object existed which
> would number in the hundreds, whose members would qualify as planets,
> this would create a planet "gold rush", as it were, and seriously
> compromise the value of the designation of a body as a planet.
>
> Four new planets in the solar system - Ceres, Pallas, Juno, and Vesta
> - was one thing. Thousands of new planets is another.
This is a slippery slope argument to something that never happened, and
was never in any remote danger of happening.
The IAU's previous "definition" of a planet was merely a list:
Something isn't a planet unless the IAU said it was, and the IAU hadn't
declared anything to be a planet since the discovery of Pluto. Eris was
not a planet because the IAU never said it was. Quite simple, really,
and required absolutely no action to take place.
--
Erik Max Francis && m...@alcyone.com && http://www.alcyone.com/max/
San Jose, CA, USA && 37 18 N 121 57 W && AIM, Y!M erikmaxfrancis
Let me not seem to have lived in vain.
-- (last words of Tycho Brahe)
Erik Max Francis
The point is, there are an _infinite_ number of possible
quantifications. Anyone could come up with some that they think are
useful or not. You could argue that this is part of what the
Bode-Titius law is.
The fundamental point here, which you're skipping over, is the IAU has
never said what they mean. You finding a few papers where people were
talking about similar concepts _doesn't mean that's what the iAU had in
mind when they made that declaration_, since there's absolutely zero
evidence whatsoever that they did. One could certainly come up with
others that don't match the list they chose.
In fact, I would doubt very much that there was a quantifiable
definition that they had in mind. As I've said, all the IAU's ultimate
declaration is basically a paragraph of, "Planets are big, round, and
gravitationally influential on bodies around them." That much is
blatantly obvious and was _already_ the de facto "definition" of a
planet, for those who've tried to come up with generalizations to, say,
other planetary systems, as we've seen many times over the years.
Except, of course, that part of the IAU's definition was, "Oh yeah,
here's an updated list; Pluto isn't on it. Sorry we had to make this
list because the exclusion wasn't even obvious enough by our old
definition."
>>> Why not assume that they intend one of the previously-published
>>> methods that _do_ make this division obvious, since they explicitly
>>> say in the resolution that it makes that division? Do you have some
>>> reason to believe that they picked an ambiguous definition and then
>>> made an arbitrary decision to exclude Pluto?
>>
>> Since they never gave or hinted at a definition, and excluded Pluto
>> anyway, yes.
>
> Could you please explain _why_, though? I already know that you're
> assuming this, I asked what reason you had.
>
> Definitions of planet clearing had been previously published. The
> members of the IAU who voted on this resolution are, generally speaking,
> well-read on such matters. I don't see any reason to assume they didn't
> have those definitions in mind, especially since they give the same
> results as the IAU's unspoken definition.
That those papers exist doesn't mean that ~400 people who voted on that
resolution (not voted to accept; voted total) were specifically familiar
with them in detail. In fact, I'd seriously doubt that they were, as
I've said above (and before), the IAU's resolution proposal was nothing
more than common sense: namely, that planets tend to clear out other,
smaller objects from their orbits.
I'm sure you were aware of this notion long before the IAU proposal came
out, or before you found those papers after the proposal came out.
Surely that doesn't mean that your (presumed) acknowledgement of that
obvious abstract feature of what we tend to call planets doesn't mean
that you were specifically familiar with those papers? That doesn't
make much logical sense.
Remember, the only person linking the IAU's non-specific resolution with
those particular papers is _you_. There's zero evidence anyone who
voted for that IAU resolution specifically had those papers in mind when
they voted for it, is there?
Humans are smart. I'm sure you could find evidence of _any_ vague,
non-specific notion that pops up in any proposal after the fact. It
ain't hard.
>> They were giving vague measures of what it means to be a planet. But
>> that was stupid; we all already _knew_ vaguely what it means to be a
>> planet -- it's something that's big, round, and gravitational
>> influential on its neighbors. The IAU definition accomplished
>> nothing, except strike one of the previous members off the list.
>
> They also struck down a bunch of candidates that nobody was entirely
> sure would be planets or not, because the previous "definition" of
> planet was far _more_ vague than the one you're complaining about. It
> also provided some measurable basis for arguing whether new discoveries
> are planets - the hypothetical object responsible for the Kuiper "cliff"
> mentioned elsewhere in this thread, for example. Those both seem like
> accomplishments to me.
There was no Kuiper "cliff." The previous IAU definition was merely a
list; the IAU would have to specifically _add_ objects to the list for
them to qualify, and there was zero risk of this happening at any point.
This is simply a false slippery slope argument.
--
Erik Max Francis && m...@alcyone.com && http://www.alcyone.com/max/
San Jose, CA, USA && 37 18 N 121 57 W && AIM, Y!M erikmaxfrancis
Bryan Derksen
> This is a slippery slope argument to something that never happened, and
> was never in any remote danger of happening.
>
> The IAU's previous "definition" of a planet was merely a list:
> Something isn't a planet unless the IAU said it was, and the IAU hadn't
> declared anything to be a planet since the discovery of Pluto. Eris was
> not a planet because the IAU never said it was. Quite simple, really,
> and required absolutely no action to take place.
Earlier you were complaining about the "arbitrariness" of the new IAU
definition, though. Seems like it doesn't get any more arbitrary than
what you're proposing here. :)
Orbit clearance distribution suggests a "natural" division, with some
physically-founded difference between the eight planets and the other
smaller objects of the solar system. When you've got a division like
that it seems to me that it's simply good scientific practice to tailor
your classification scheme to conform to it. Biologists are rejiggering
their taxonomic trees all the time for similar reasons as new
information comes to light about the relationships between species, as
another example.
Furthermore this definition of planethood will be quite easily
generalizable to other solar systems (using the Stern-Levison method of
determining orbit-clearance rather than the Soter method, that is, since
detecting asteroid-sized objects will remain extremely hard for quite
some time to come). Since the number of known extrasolar planet
candidates is likely to go into the thousands in the near future this
will make a good basis for classifying those objects without having to
put a list of new candidates up for a vote at the IAU every year.
Erik Max Francis
> Earlier you were complaining about the "arbitrariness" of the new IAU
> definition, though. Seems like it doesn't get any more arbitrary than
> what you're proposing here. :)
All definitions are arbitrary: We're applying categorical labels to the
natural world that has no obligation or even interest to conform to the
categories we prefer to choose.
> Furthermore this definition of planethood will be quite easily
> generalizable to other solar systems (using the Stern-Levison method of
> determining orbit-clearance rather than the Soter method, that is, since
> detecting asteroid-sized objects will remain extremely hard for quite
> some time to come). Since the number of known extrasolar planet
> candidates is likely to go into the thousands in the near future this
> will make a good basis for classifying those objects without having to
> put a list of new candidates up for a vote at the IAU every year.
This kind of misses the point that it's _still_ the IAU that decides
what objects are named and what their official designations are. If,
say, another very large object is found in the outer Solar System
(however likely that is) that _you_ think meets these particular
quantified definitions of a planet that you've chosen (and the IAU
hasn't mentioned), then it's _still_ up to the IAU to declare whether or
not it's a planet.
That is my point: Nothing has changed, except the change of status of
an existing planet, and a list of vague criteria that doesn't really
bring anything new to the table other than stating the obvious; that it
was pretty vague is pretty self-evidence given that they had to provide
_a new list_ in the resolution footnotes to make it clear that their new
definition excluded Pluto.
Brett Paul Dunbar
Francis <m...@alcyone.com> writes
Both papers were the subject of discussion at the time and do seem to
have been what was in mind. There wasn't much need to specifically
quantify the definition or to specify exactly hoe orbital dominance was
defined given that two rather different methods give a several orders of
magnitude difference between planets and everything else.
>
>In fact, I would doubt very much that there was a quantifiable
>definition that they had in mind. As I've said, all the IAU's ultimate
>declaration is basically a paragraph of, "Planets are big, round, and
>gravitationally influential on bodies around them." That much is
>blatantly obvious and was _already_ the de facto "definition" of a
>planet, for those who've tried to come up with generalizations to, say,
>other planetary systems, as we've seen many times over the years.
>Except, of course, that part of the IAU's definition was, "Oh yeah,
>here's an updated list; Pluto isn't on it. Sorry we had to make this
>list because the exclusion wasn't even obvious enough by our old
>definition."
Despite obviously not meeting the sort of common sense definition most
astronomers were using, Pluto was often referred to as a planet, the
redefinition restricted the name to just the class of large orbitally
dominant objects. There was a bit of a problem with general statements
about planets having to specifically exclude Pluto so much as with one
and prime numbers the definition was narrowed to exclude it.
The list was simply a footnote exhaustively stating the known bodies
that meet the definition. It is not part of the definition it is a
footnote. The IAU has a number of catalogues; the footnote lists the
members of the planet catalogue. Most classes have enough members that
this is impractical.
The definition was intended to reflect the consensus on what a planet
was while being self consistent. So it should match general usage of the
term, that was the point of the exercise. As there was a lot of press
interest it seemed to be a good idea to make sure that the press were
absolutely clear that the definition excluded Pluto, given Journalists
usual level of competence at reporting science stories this seems a good
idea.
The definition itself is as follows:
The IAU therefore resolves that planets and other bodies in our
Solar System, except satellites, be defined into three distinct
categories in the following way:
(1) A "planet"[1] is a celestial body that: (a) is in orbit
around the Sun, (b) has sufficient mass for its
self-gravity to overcome rigid body forces so that it
assumes a hydrostatic equilibrium (nearly round) shape,
and (c) has cleared the neighbourhood around its orbit.
(2) A "dwarf planet" is a celestial body that: (a) is in
orbit around the Sun, (b) has sufficient mass for its
self-gravity to overcome rigid body forces so that it
assumes a hydrostatic equilibrium (nearly round)
shape[2], (c) has not cleared the neighbourhood around
its orbit, and (d) is not a satellite.
(3) All other objects[3] except satellites orbiting the Sun
shall be referred to collectively as "Small Solar System
Bodies".
Footnotes:
[1] The eight planets are: Mercury, Venus, Earth,
Mars, Jupiter, Saturn, Uranus, and Neptune.
[2] An IAU process will be established to assign
borderline objects into either "dwarf planet"
and other categories.
[3] These currently include most of the Solar System
asteroids, most Trans-Neptunian Objects (TNOs),
comets, and other small bodies.
The IAU further resolves:
Pluto is a "dwarf planet" by the above definition and is
recognised as the prototype of a new category of
trans-Neptunian objects.
>
>>>> Why not assume that they intend one of the previously-published
>>>>methods that _do_ make this division obvious, since they explicitly
>>>>say in the resolution that it makes that division? Do you have some
>>>>reason to believe that they picked an ambiguous definition and then
>>>>made an arbitrary decision to exclude Pluto?
>>>
>>> Since they never gave or hinted at a definition, and excluded Pluto
>>>anyway, yes.
>> Could you please explain _why_, though? I already know that you're
>>assuming this, I asked what reason you had.
>> Definitions of planet clearing had been previously published. The
>>members of the IAU who voted on this resolution are, generally
>>speaking, well-read on such matters. I don't see any reason to assume
>>they didn't have those definitions in mind, especially since they
>>give the same results as the IAU's unspoken definition.
>
>That those papers exist doesn't mean that ~400 people who voted on that
>resolution (not voted to accept; voted total) were specifically
>familiar with them in detail. In fact, I'd seriously doubt that they
>were, as I've said above (and before), the IAU's resolution proposal
>was nothing more than common sense: namely, that planets tend to clear
>out other, smaller objects from their orbits.
And Pluto isn't one.
Given the nature of the discussion at the time it is fairly clear that
an object that met the definitions in those two papers was a planet. The
definition is intended to put into words roughly what everyone tended to
think of as a planet.
Rather than requiring papers to include a foot note stating the local
definition there is now a general definition which can be assumed.
>
>I'm sure you were aware of this notion long before the IAU proposal
>came out, or before you found those papers after the proposal came out.
>Surely that doesn't mean that your (presumed) acknowledgement of that
>obvious abstract feature of what we tend to call planets doesn't mean
>that you were specifically familiar with those papers? That doesn't
>make much logical sense.
>
>Remember, the only person linking the IAU's non-specific resolution
>with those particular papers is _you_. There's zero evidence anyone
>who voted for that IAU resolution specifically had those papers in mind
>when they voted for it, is there?
They were topics of discussion at the time. The definition is explicitly
based on orbital dynamics not planetary mechanics. Two markedly
different definitions of dominance give the exact same list very
clearly. That the definition might sound a little fuzzy does not make it
any way ambiguous. The definition also leaves to option of finding other
ways of quantifying orbital dominance.
>
>Humans are smart. I'm sure you could find evidence of _any_ vague,
>non-specific notion that pops up in any proposal after the fact. It
>ain't hard.
>
>>> They were giving vague measures of what it means to be a planet.
>>>But that was stupid; we all already _knew_ vaguely what it means to
>>>be a planet -- it's something that's big, round, and gravitational
>>>influential on its neighbors. The IAU definition accomplished
>>>nothing, except strike one of the previous members off the list.
>> They also struck down a bunch of candidates that nobody was entirely
>>sure would be planets or not, because the previous "definition" of
>>planet was far _more_ vague than the one you're complaining about. It
>>also provided some measurable basis for arguing whether new
>>discoveries are planets - the hypothetical object responsible for the
>>Kuiper "cliff" mentioned elsewhere in this thread, for example. Those
>>both seem like accomplishments to me.
>
>There was no Kuiper "cliff." The previous IAU definition was merely a
>list; the IAU would have to specifically _add_ objects to the list for
>them to qualify, and there was zero risk of this happening at any
>point. This is simply a false slippery slope argument.
>
The IAU now has a definition with which to rule on the nature of a body.
Which class a body belongs to affects the procedure for formally naming
it. The Kuiper cliff was suspected to be real at the time and there was
speculation that a fairly massive body had created it. If such a body
exists and is responsible for the cliff then it is a planet.
Bryan Derksen
> Bryan Derksen wrote:
>> So clearly these are not what the IAU had in mind. Where did these come
>> from? Or did you just make them up on the spot?
>
> The point is, there are an _infinite_ number of possible
> quantifications. Anyone could come up with some that they think are
> useful or not. You could argue that this is part of what the
> Bode-Titius law is.
And yet most of those possible quantifications are not what professional
astronomers would likely consider to be good ones, putting it mildly.
You still haven't mentioned where you got those examples you gave, I
presume you made them up yourself. That's not going to be particularly
representative of the scientific consensus.
Citing papers, on the other hand, is a good way to get an idea of what
professional astronomers think on the subject.
> The fundamental point here, which you're skipping over, is the IAU has
> never said what they mean.
And for some reason that you haven't explained you're assuming that they
meant something dumb.
I guess I just see no reason not to give them the benefit of the doubt.
> In fact, I would doubt very much that there was a quantifiable
> definition that they had in mind. As I've said, all the IAU's ultimate
> declaration is basically a paragraph of, "Planets are big, round, and
> gravitationally influential on bodies around them." That much is
> blatantly obvious and was _already_ the de facto "definition" of a
> planet, for those who've tried to come up with generalizations to, say,
> other planetary systems, as we've seen many times over the years.
If that was the de facto definition already then Pluto was already off
the list and it just hadn't been officialized yet. Pluto is not
particularly influential over the bodies around it.
If you accept that I'm not even sure what you're arguing about any more.
Would you have preferred that the IAU simply release an updated list
of planets with Pluto struck off of it, without explaining why? I
somehow doubt that would have been better received.
>> Definitions of planet clearing had been previously published. The
>> members of the IAU who voted on this resolution are, generally speaking,
>> well-read on such matters. I don't see any reason to assume they didn't
>> have those definitions in mind, especially since they give the same
>> results as the IAU's unspoken definition.
>
> That those papers exist doesn't mean that ~400 people who voted on that
> resolution (not voted to accept; voted total) were specifically familiar
> with them in detail. In fact, I'd seriously doubt that they were, as
> I've said above (and before), the IAU's resolution proposal was nothing
> more than common sense: namely, that planets tend to clear out other,
> smaller objects from their orbits.
Perhaps most of them hadn't read those specific papers (and perhaps they
had, I don't know for sure and neither do you). But they are a decent
way of estimating what the general attitude among astronomers was. Do
you know of any other papers on the subject that suggested alternative
methods of classification?
> Remember, the only person linking the IAU's non-specific resolution with
> those particular papers is _you_.
Hardly. I don't read those journals, I only came to them when I read up
on stuff written by other people about the IAU's resolution.
> There was no Kuiper "cliff." The previous IAU definition was merely a
> list; the IAU would have to specifically _add_ objects to the list for
> them to qualify, and there was zero risk of this happening at any
> point. This is simply a false slippery slope argument.
No, it's real:
http://en.wikipedia.org/wiki/Kuiper_belt#.22Kuiper_cliff.22
There could be Mars-sized or Earth-sized objects out there in the far
reaches of the solar system that are yet to be discovered, and
continuing to rely on an arbitrary list-based definition at that point
would be silly. You'd have to have the IAU debate over each one with no
explicit common basis amongst the debators for what they were actually
arguing about.
Bryan Derksen
> Bryan Derksen wrote:
>
>> Earlier you were complaining about the "arbitrariness" of the new IAU
>> definition, though. Seems like it doesn't get any more arbitrary than
>> what you're proposing here. :)
>
> All definitions are arbitrary: We're applying categorical labels to the
> natural world that has no obligation or even interest to conform to the
> categories we prefer to choose.
If that's so then what's the problem? I don't agree and consider the new
definition better than the old one because it makes the physical basis
for planethood explicit and measurable, but by this argument it should
make no difference one way or the other to you.
> This kind of misses the point that it's _still_ the IAU that decides
> what objects are named and what their official designations are. If,
> say, another very large object is found in the outer Solar System
> (however likely that is) that _you_ think meets these particular
> quantified definitions of a planet that you've chosen (and the IAU
> hasn't mentioned), then it's _still_ up to the IAU to declare whether or
> not it's a planet.
Perhaps, but this resolution means that at least there's now an explicit
basis for making that decision. If an object comes up for consideration
and its orbit is swarming with other bodies, the IAU can point to them
and say "not planet because it hasn't cleared its neighborhood" rather
than "not planet because we say not planet."
If they do this to some object that obviously _has_ cleared its
neighborhood, _then_ we can start debating whether the IAU was taking
stupid pills and has some secret definition of their own that they're
using behind closed doors. I see no reason to expect something like this
to happen, though.
> That is my point: Nothing has changed, except the change of status of
> an existing planet, and a list of vague criteria that doesn't really
> bring anything new to the table other than stating the obvious; that it
> was pretty vague is pretty self-evidence given that they had to provide
> _a new list_ in the resolution footnotes to make it clear that their new
> definition excluded Pluto.
Why _shouldn't_ they include a list of known planets as a footnote?
There's only eight of them, it's not a major use of space.
And as I mention in my other response to you in this thread today, if
you believe that orbit-clearing is already an "obvious" criterion for
planethood I really don't see what you're arguing about at this point.
Jens Egon Nyborg
> (2) A "dwarf planet" is a celestial body that: (a) is in
> orbit around the Sun, (b) has sufficient mass for its
> self-gravity to overcome rigid body forces so that it
> assumes a hydrostatic equilibrium (nearly round)
> shape[2], (c) has not cleared the neighbourhood around
> its orbit, and (d) is not a satellite.
If I may, (c) is were /I/ think the definition becomes downright silly.
Extend this definition to other systems (we will have to, someday) by
substituting "exoplanet" for planet and "the local star" for "the Sun".
Now we can ask "How many 'dwarf exoplanets' with a mass greater than
Jupiter are there in the Milkyway?"
We may or may not be able to give a sensible answer, but I see it as a
problem that the question actually makes sense.
Consider rogue planets, planets in independent orbit around the galaxy
(if they exit), are they "Dwarfs"? If not then Ceres would become a
"normal-sized" rogue planet if expelled from the solar system, but if
they are dwarfs, then surely there are dwarfs out there that are just
shy of being stars themselves.
In short, taking a word that denotes 'smallness' and using it to define
something often associated with smallness invites problem of
communication further down the line
Bryan Derksen
> Brett Paul Dunbar skrev:
>
>> (2) A "dwarf planet" is a celestial body that: (a) is in
>> orbit around the Sun, (b) has sufficient mass for its
>> self-gravity to overcome rigid body forces so that it
>> assumes a hydrostatic equilibrium (nearly round)
>> shape[2], (c) has not cleared the neighbourhood around
>> its orbit, and (d) is not a satellite.
>
> If I may, (c) is were /I/ think the definition becomes downright silly.
>
> Extend this definition to other systems (we will have to, someday) by
> substituting "exoplanet" for planet and "the local star" for "the Sun".
>
> Now we can ask "How many 'dwarf exoplanets' with a mass greater than
> Jupiter are there in the Milkyway?"
I'd be highly surprised if there were any, at least any that weren't in
a transitory state. An object the size of Jupiter will clear its
neighborhood of other objects in very short order.
IMO the trickier bit is measuring the roundness of extrasolar planets.
We can't detect stuff down at the size where this is relevant yet, but I
can imagine it becoming relevant in the forseeable future.
> Consider rogue planets, planets in independent orbit around the galaxy
> (if they exit), are they "Dwarfs"? If not then Ceres would become a
> "normal-sized" rogue planet if expelled from the solar system, but if
> they are dwarfs, then surely there are dwarfs out there that are just
> shy of being stars themselves.
"Rogue" planets don't revolve around any star, so neither the definition
of planet nor the definition of dwarf planet is generalizable to them.
The IAU will have to come up with some other way of classifying these
free-floating objects. We've already got "brown dwarf" for stuff that's
just shy of being stars, though, so we're good for a while yet there too.
Brett Paul Dunbar
<bryan....@shaw.ca> writes
>IMO the trickier bit is measuring the roundness of extrasolar planets.
>We can't detect stuff down at the size where this is relevant yet, but
>I can imagine it becoming relevant in the forseeable future.
There is only one known extra solar object too small to be a planet, PSR
B1257+12 D, a tiny object in orbit around a pulsar. It is less than
0.0004 Earth masses. Nothing so small could be detected around a normal
star.
je...@puggaard-nyborg.dk
> Jens Egon Nyborg wrote:
> > Brett Paul Dunbar skrev:
>
> >> (2) A "dwarf planet" is a celestial body that: (a) is in
> >> orbit around the Sun, (b) has sufficient mass for its
> >> self-gravity to overcome rigid body forces so that it
> >> assumes a hydrostatic equilibrium (nearly round)
> >> shape[2], (c) has not cleared the neighbourhood around
> >> its orbit, and (d) is not a satellite.
>
> > If I may, (c) is were /I/ think the definition becomes downright silly.
>
> > Extend this definition to other systems (we will have to, someday) by
> > substituting "exoplanet" for planet and "the local star" for "the Sun".
>
> > Now we can ask "How many 'dwarf exoplanets' with a mass greater than
> > Jupiter are there in the Milkyway?"
>
> I'd be highly surprised if there were any, at least any that weren't in
> a transitory state. An object the size of Jupiter will clear its
> neighborhood of other objects in very short order.
Well, sure, if an object the size of Jupiter is a dwarf planet it's
almost surely in a transitory state.
Now, imagine trying to explain that to a fifth grader without giving a
false impression.
> IMO the trickier bit is measuring the roundness of extrasolar planets.
> We can't detect stuff down at the size where this is relevant yet, but I
> can imagine it becoming relevant in the forseeable future.
>
> > Consider rogue planets, planets in independent orbit around the galaxy
> > (if they exit), are they "Dwarfs"? If not then Ceres would become a
> > "normal-sized" rogue planet if expelled from the solar system, but if
> > they are dwarfs, then surely there are dwarfs out there that are just
> > shy of being stars themselves.
>
> "Rogue" planets don't revolve around any star, so neither the definition
> of planet nor the definition of dwarf planet is generalizable to them.
> The IAU will have to come up with some other way of classifying these
> free-floating objects. We've already got "brown dwarf" for stuff that's
> just shy of being stars, though, so we're good for a while yet there too.
Right, neither definition is generalizable to this special case that
is likely to exit.
That's not good.
Erik Max Francis
> And yet most of those possible quantifications are not what professional
> astronomers would likely consider to be good ones, putting it mildly.
> You still haven't mentioned where you got those examples you gave, I
> presume you made them up yourself. That's not going to be particularly
> representative of the scientific consensus.
>
> Citing papers, on the other hand, is a good way to get an idea of what
> professional astronomers think on the subject.
Of course I made them up. I made them up to demonstrate a point.
You have taken the position from the beginning that these two papers
represent the view of the IAU consensus. In fact, there is no evidence
whatsoever that that is the case.
Furthermore, you seem to be taking the position that these are the only
two reasonable definitions. That, too, is without basis; there are an
_infinite_ number of ways of defining such criteria. Obviously the
examples I gave were partly in jest, but they were to make a point:
It's not hard to come up with criteria that are perfectly quantifiable,
easy to understand, and differ from those two papers.
Furthermore, the two papers aren't independent; the Soter paper
references the Stern and Levison paper.
>> The fundamental point here, which you're skipping over, is the IAU has
>> never said what they mean.
>
> And for some reason that you haven't explained you're assuming that they
> meant something dumb.
>
> I guess I just see no reason not to give them the benefit of the doubt.
I've never said they meant something dumb. I'm saying _we don't know
what they meant_, because they didn't say. Your supposition that they
were specifically thinking of these two obscure papers does not seem
very compelling.
> If you accept that I'm not even sure what you're arguing about any more.
> Would you have preferred that the IAU simply release an updated list
> of planets with Pluto struck off of it, without explaining why? I
> somehow doubt that would have been better received.
It would have been more honest, at least.
>> There was no Kuiper "cliff." The previous IAU definition was merely a
> > list; the IAU would have to specifically _add_ objects to the list for
> > them to qualify, and there was zero risk of this happening at any
> > point. This is simply a false slippery slope argument.
>
> No, it's real:
> http://en.wikipedia.org/wiki/Kuiper_belt#.22Kuiper_cliff.22
>
> There could be Mars-sized or Earth-sized objects out there in the far
> reaches of the solar system that are yet to be discovered, and
> continuing to rely on an arbitrary list-based definition at that point
> would be silly. You'd have to have the IAU debate over each one with no
> explicit common basis amongst the debators for what they were actually
> arguing about.
Wikipedia reference aside, the bottom line is that the former IAU
"definition" of a planet was just a list. If the IAU does not
explicitly add something to the list, then it is not a planet. It's
really quite simple, and does not lead to a slippery slope; if the IAU
elects to not do anything, then nothing changes.
--
Erik Max Francis && m...@alcyone.com && http://www.alcyone.com/max/
San Jose, CA, USA && 37 18 N 121 57 W && AIM, Y!M erikmaxfrancis
Have you ever loved somebody / Who didn't know
-- Zhane
Erik Max Francis
> Erik Max Francis wrote:
>> Bryan Derksen wrote:
>>
>>> Earlier you were complaining about the "arbitrariness" of the new IAU
>>> definition, though. Seems like it doesn't get any more arbitrary than
>>> what you're proposing here. :)
>>
>> All definitions are arbitrary: We're applying categorical labels to
>> the natural world that has no obligation or even interest to conform
>> to the categories we prefer to choose.
>
> If that's so then what's the problem? I don't agree and consider the new
> definition better than the old one because it makes the physical basis
> for planethood explicit and measurable, but by this argument it should
> make no difference one way or the other to you.
This is silly. That someone is engaging in pointless box shifting
doesn't mean that objecting to the box shifting is inappropriate and
wasteful.
>> That is my point: Nothing has changed, except the change of status of
>> an existing planet, and a list of vague criteria that doesn't really
>> bring anything new to the table other than stating the obvious; that
>> it was pretty vague is pretty self-evidence given that they had to
>> provide _a new list_ in the resolution footnotes to make it clear that
>> their new definition excluded Pluto.
>
> Why _shouldn't_ they include a list of known planets as a footnote?
> There's only eight of them, it's not a major use of space.
The quintessential example of the vagueness of the new IAU definition is
this: Had they not included the revised list, no one would really be
sure if it was meant that Pluto was still supposed to be on the list.
That doesn't make for very good objective criteria.
Furthermore, nothing about the IAU's position on the blessing of planet
status has really changed. If a new object is discovered, it will still
be the IAU that weighs in on whether or not is a planet, namely by
saying, "Yes, this thing is a planet." So it's still a list, but now
with added vague criteria that were already basically obvious, except
they wanted to change the list. Yes, I call that not being terribly useful.
--
Erik Max Francis && m...@alcyone.com && http://www.alcyone.com/max/
San Jose, CA, USA && 37 18 N 121 57 W && AIM, Y!M erikmaxfrancis
Erik Max Francis
> The list was simply a footnote exhaustively stating the known bodies
> that meet the definition. It is not part of the definition it is a
> footnote. The IAU has a number of catalogues; the footnote lists the
> members of the planet catalogue. Most classes have enough members that
> this is impractical.
If you're saying that footnotes in IAU resolutions aren't normative (I
don't know if they are, as was discussed the first time the planethood
status came up), then that's even sillier. The only way anyone would
have been able to know for sure that Pluto was to be stricken from the
list of planetary bodies by the resolution is that they _included an
updated list_ in the footnotes. If the footnotes aren't normative,
then, wow, that's silly. The only way people know what intended effect
the resolution has is by reading the footnotes, which aren't normative?
--
Erik Max Francis && m...@alcyone.com && http://www.alcyone.com/max/
San Jose, CA, USA && 37 18 N 121 57 W && AIM, Y!M erikmaxfrancis
To be adult is to be alone.
-- Jean Rostand
Bryan Derksen
> Well, sure, if an object the size of Jupiter is a dwarf planet it's
> almost surely in a transitory state.
>
> Now, imagine trying to explain that to a fifth grader without giving a
> false impression.
Seems fairly straightforward to me, not sure what the problem is. Some
solar systems are fresh and dynamic, still sorting themselves out as
they form, and once they understand this it should be obvious that
transitory oddities like this can exist.
If it's really a problem the IAU can probably head most of these
outliers off at the pass with a definition for "protoplanet" that
overrides dwarf planethood.
>> "Rogue" planets don't revolve around any star, so neither the definition
>> of planet nor the definition of dwarf planet is generalizable to them.
>> The IAU will have to come up with some other way of classifying these
>> free-floating objects. We've already got "brown dwarf" for stuff that's
>> just shy of being stars, though, so we're good for a while yet there too.
>
> Right, neither definition is generalizable to this special case that
> is likely to exit.
>
> That's not good.
No, but IMO it's much better than not having a definition that can
generalize to _any_ case. Special cases are always odd and don't quite
fit right, that's why they're called special cases.
Fortunately we don't know of any such cases yet and probably won't for
quite some time, so again I don't see it as a pressing issue.
Eivind
> That is a rather contrived and obviously inappropriate definition. I.e.
> it is obviously wrong.
Here's a hint: Definitions are not WRONG, (By definition, they cannot
be!) they are merely more or less PRACTICAL.
Eivind
Eivind
> Both papers were the subject of discussion at the time and do seem to
> have been what was in mind.
Based on precisely -what- evidence does this "seem" to be the case ?
Your hunch ?
Eivind
Brett Paul Dunbar
Francis <m...@alcyone.com> writes
>Bryan Derksen wrote:
>
>> Erik Max Francis wrote:
>>> Bryan Derksen wrote:
>>>
>>>> Earlier you were complaining about the "arbitrariness" of the new
>>>>IAU definition, though. Seems like it doesn't get any more
>>>>arbitrary than what you're proposing here. :)
>>>
>>> All definitions are arbitrary: We're applying categorical labels to
>>>the natural world that has no obligation or even interest to conform
>>>to the categories we prefer to choose.
>> If that's so then what's the problem? I don't agree and consider the
>>new definition better than the old one because it makes the physical
>>basis for planethood explicit and measurable, but by this argument it
>>should make no difference one way or the other to you.
>
>This is silly. That someone is engaging in pointless box shifting
>doesn't mean that objecting to the box shifting is inappropriate and
>wasteful.
It isn't pointless, it is defining a widely used term (Planet) to cover
a rather distinct natural class of objects (massive orbitally dominant
bodies). The term was generally used to cover this class but for
historical reasons (massive initial overestimate of size and nothing
else similar being known) an object which was not a member had been
added. In line with what happened in the 1840s with Ceres, Pluto was
excluded once several other members of its class of objects were
discovered. As there was now a better place to classify it.
Planet is a distinct class the way that for example Star and Brown Dwarf
are distinct classes, there are distinct fundamental physical
characteristics to which the labels are attached. Making the definitions
non-arbitrary.
>
>>> That is my point: Nothing has changed, except the change of status
>>>of an existing planet, and a list of vague criteria that doesn't
>>>really bring anything new to the table other than stating the
>>>obvious; that it was pretty vague is pretty self-evidence given that
>>>they had to provide _a new list_ in the resolution footnotes to make
>>>it clear that their new definition excluded Pluto.
>> Why _shouldn't_ they include a list of known planets as a footnote?
>>There's only eight of them, it's not a major use of space.
>
>The quintessential example of the vagueness of the new IAU definition
>is this: Had they not included the revised list, no one would really
>be sure if it was meant that Pluto was still supposed to be on the
>list. That doesn't make for very good objective criteria.
Yes they would have done. Any reasonable definition of cleared the
neighbourhood would exclude Pluto, as there is a large mass of other
objects orbiting in its vicinity. The other Plutinos, which are in
resonance with Neptune but have no special harmonic relationship with
Pluto. There are two fairly distinct definitions now, both give
identical results on rather different basis.
>
>Furthermore, nothing about the IAU's position on the blessing of planet
>status has really changed. If a new object is discovered, it will
>still be the IAU that weighs in on whether or not is a planet, namely
>by saying, "Yes, this thing is a planet." So it's still a list, but
>now with added vague criteria that were already basically obvious,
>except they wanted to change the list. Yes, I call that not being
>terribly useful.
>
That is true of all classes of bodies. One of the IAUs functions is
nomenclature cataloguing which class bodies belong to and formally
naming them. This doesn't mean that there is necessarily any dispute
about which class an object belongs to.
Brett Paul Dunbar
Francis <m...@alcyone.com> writes
>Brett Paul Dunbar wrote:
>
>> The list was simply a footnote exhaustively stating the known bodies
>> that meet the definition. It is not part of the definition it is a
>> footnote. The IAU has a number of catalogues; the footnote lists the
>> members of the planet catalogue. Most classes have enough members that
>> this is impractical.
>
>If you're saying that footnotes in IAU resolutions aren't normative (I
>don't know if they are, as was discussed the first time the planethood
>status came up), then that's even sillier. The only way anyone would
>have been able to know for sure that Pluto was to be stricken from the
>list of planetary bodies by the resolution is that they _included an
>updated list_ in the footnotes. If the footnotes aren't normative,
>then, wow, that's silly. The only way people know what intended effect
>the resolution has is by reading the footnotes, which aren't normative?
>
The definition is perfectly adequate for determining the membership of
the class, the footnote is a complete list of known examples. It isn't
the definition, the definition is the definition. A footnote can give
clarification or examples, in this specific case the number of examples
was small enough that all the examples could be listed. As the
resolution was of great interest to the general press it was felt
appropriate to make it as clear as possible so that non-specialist
journalists would be clear what had been resolved. The footnotes were
for the benefit of journalists who might be entirely ignorant of
astronomy, that didn't mean that they were needed for the usual audience
of these things, this one however had a much wider audience.
Bryan Derksen
> Bryan Derksen wrote:
>
>> And yet most of those possible quantifications are not what professional
>> astronomers would likely consider to be good ones, putting it mildly.
>> You still haven't mentioned where you got those examples you gave, I
>> presume you made them up yourself. That's not going to be particularly
>> representative of the scientific consensus.
>>
>> Citing papers, on the other hand, is a good way to get an idea of what
>> professional astronomers think on the subject.
>
> Of course I made them up. I made them up to demonstrate a point.
>
> You have taken the position from the beginning that these two papers
> represent the view of the IAU consensus. In fact, there is no evidence
> whatsoever that that is the case.
There is, actually. The papers have been published in fora that are
likely to be read by astronomers, used language similar to the IAU's
resolution, and the definitions presented give the same results that the
IAU indicated their unspoken definition gave.
It's not airtight evidence that'd hold up in the court of law, but
you're providing no evidence whatsoever that they _didn't_ have those
definitions in mind so it's a reasonable default assumption.
> Furthermore, you seem to be taking the position that these are the only
> two reasonable definitions.
No, I'm taking the position that I find those definitions to be
reasonable and that I haven't seen any other definitions that are
particularly likely to be what the IAU had in mind. That's a reasonable
position. If you could find some other definitions that the IAU might
have had in mind (that aren't made up whole-cloth out of straw) then by
all means let me know.
> That, too, is without basis; there are an
> _infinite_ number of ways of defining such criteria. Obviously the
> examples I gave were partly in jest, but they were to make a point:
> It's not hard to come up with criteria that are perfectly quantifiable,
> easy to understand, and differ from those two papers.
But I pointed out that both of the examples you gave produced ridiculous
results, with one of them actually having objects oscillate back and
forth between planet and not-planet over the course of their orbits (an
issue I raised way earlier in this thread about the newer "Plutoid"
definition before I realized a moment later that I'd misread part of
it). There's not nearly so many ways of defining such criteria that
actually make sense.
> Furthermore, the two papers aren't independent; the Soter paper
> references the Stern and Levison paper.
I don't see why this is a problem. Indeed, it's kind of my point;
astronomers who are interested in these sorts of things will likely have
known about these papers, or at least been aware of the general concepts
that the papers were based on.
> I've never said they meant something dumb. I'm saying _we don't know
> what they meant_, because they didn't say. Your supposition that they
> were specifically thinking of these two obscure papers does not seem
> very compelling.
Why do you say the papers are obscure? One of them was coauthored by
Alan Stern, who was NASA's Associate Administrator for the Science
Mission Directorate (their top "science officer). As you point out
above, Soter referenced it. And I notice now it was actually presented
at the XXIVth General Assembly of the IAU back in 2000.
>> No, it's real:
>> http://en.wikipedia.org/wiki/Kuiper_belt#.22Kuiper_cliff.22
>>
>> There could be Mars-sized or Earth-sized objects out there in the far
>> reaches of the solar system that are yet to be discovered, and
>> continuing to rely on an arbitrary list-based definition at that point
>> would be silly. You'd have to have the IAU debate over each one with no
>> explicit common basis amongst the debators for what they were actually
>> arguing about.
>
> Wikipedia reference aside,
The Wikipedia article is merely intended as a convenient summary of the
issue, as always. That section of the article references five different
scientific journal articles discussing the Kuiper cliff as the source of
that summarized information.
> the bottom line is that the former IAU
> "definition" of a planet was just a list. If the IAU does not
> explicitly add something to the list, then it is not a planet. It's
> really quite simple, and does not lead to a slippery slope; if the IAU
> elects to not do anything, then nothing changes.
The result would be that that objects larger and more gravitationally
influential than Pluto get classified as non-planets while Pluto
remained a planet for no reason other than the historical accident that
it was discovered first. That doesn't sit right with me, the result
makes the definition of "planet" useless for physically-relevant
analysis. Evidently it didn't sit right with the IAU either, since they
changed it. It also doesn't give any useful basis for coming up with a
definition for exoplanets.
Bryan Derksen
The evidence that comes to mind is similar language, identical results
to what the IAU indicated their definition came to, lack of similarly
plausible alternatives, and (I now notice) one of the papers was
actually presented at an IAU general assembly several years earlier.
Brett Paul Dunbar
<e104a6d6-f145-48ab...@d45g2000hsc.googlegroups.com>,
je...@puggaard-nyborg.dk writes
There is not, as yet, any consensus on whether an object that formed in
a star-like manner that did not reach sufficient mass to be a Brown
Dwarf is a planet. An object that size that was formed from part of a
gas cloud in association with a star is a planet, if it was the main
object formed from a small independent gas cloud there is no agreement
as to whether it is a planet.
<http://en.wikipedia.org/wiki/Sub-brown_dwarf> The definition is unclear
as to whether a sub-brown dwarf is a planet, however so are astronomers
at this point.
More generally a significant proportion of astronomers believe that a
planet must be in orbit around a star, much as a moon must be in orbit
around a body orbiting a star. Therefore an interstellar planetary mass
object would be in a different class, other astronomers disagree. As of
yet there is no agreement on the issue.
Bryan Derksen
> There is not, as yet, any consensus on whether an object that formed in
> a star-like manner that did not reach sufficient mass to be a Brown
> Dwarf is a planet. An object that size that was formed from part of a
> gas cloud in association with a star is a planet, if it was the main
> object formed from a small independent gas cloud there is no agreement
> as to whether it is a planet.
> <http://en.wikipedia.org/wiki/Sub-brown_dwarf> The definition is unclear
> as to whether a sub-brown dwarf is a planet, however so are astronomers
> at this point.
There's another class of objects that could fall under the "interstellar
planet" moniker; planet-sized objects that formed in the conventional
planet-like way in orbit around a star, but that were ejected from orbit
into interstellar space at some point. Current models of solar system
formation suggest that this sort of thing might actually happen fairly
frequently as gas giants migrate inward, scattering smaller objects in
their path.
I expect such objects would probably start out with much less hydrogen
and helium in their atmospheres than objects that collapsed directly,
and perhaps have fewer orbiting companion objects of their own, but I'm
not sure whether it'd be all that easy to distinguish them at long range.
Erik Max Francis
> There is not, as yet, any consensus on whether an object that formed in
> a star-like manner that did not reach sufficient mass to be a Brown
> Dwarf is a planet.
Psst. Brown dwarf isn't objectively defined by the IAU either.
--
Erik Max Francis && m...@alcyone.com && http://www.alcyone.com/max/
San Jose, CA, USA && 37 18 N 121 57 W && AIM, Y!M erikmaxfrancis
Only those who dare to fail greatly can ever achieve greatly.
-- Robert F. Kennedy
Erik Max Francis
> There's another class of objects that could fall under the "interstellar
> planet" moniker; planet-sized objects that formed in the conventional
> planet-like way in orbit around a star, but that were ejected from orbit
> into interstellar space at some point. Current models of solar system
> formation suggest that this sort of thing might actually happen fairly
> frequently as gas giants migrate inward, scattering smaller objects in
> their path.
>
> I expect such objects would probably start out with much less hydrogen
> and helium in their atmospheres than objects that collapsed directly,
> and perhaps have fewer orbiting companion objects of their own, but I'm
> not sure whether it'd be all that easy to distinguish them at long range.
The usual name assigned to these objects is "rogue planet." Not
surprisingly, though, the IAU hasn't really weighed in.
--
Erik Max Francis && m...@alcyone.com && http://www.alcyone.com/max/
San Jose, CA, USA && 37 18 N 121 57 W && AIM, Y!M erikmaxfrancis
Brett Paul Dunbar
Francis <m...@alcyone.com> writes
>Brett Paul Dunbar wrote:
>
>> There is not, as yet, any consensus on whether an object that formed
>>in a star-like manner that did not reach sufficient mass to be a
>>Brown Dwarf is a planet.
>
>Psst. Brown dwarf isn't objectively defined by the IAU either.
>
That turns out not to be the case, there isn't a final definition, there
is a working definition.
<http://www.dtm.ciw.edu/boss/definition.html>
Working Group on Extrasolar Planets
Definition of a "Planet"
POSITION STATEMENT ON THE DEFINITION OF A "PLANET"
WORKING GROUP ON EXTRASOLAR PLANETS (WGESP) OF THE INTERNATIONAL
ASTRONOMICAL UNION
Created: February 28, 2001
Last Modified: February 28, 2003
Rather than try to construct a detailed definition of a planet
which is designed to cover all future possibilities, the WGESP
has agreed to restrict itself to developing a working definition
applicable to the cases where there already are claimed
detections, e.g., the radial velocity surveys of companions to
(mostly) solar-type stars, and the imaging surveys for
free-floating objects in young star clusters. As new claims are
made in the future, the WGESP will weigh their individual merits
and circumstances, and will try to fit the new objects into the
WGESP definition of a "planet", revising this definition as
necessary. This is a gradualist approach with an evolving
definition, guided by the observations that will decide all in
the end.
Emphasizing again that this is only a working definition,
subject to change as we learn more about the census of low-mass
companions, the WGESP has agreed to the following statements:
1) Objects with true masses below the limiting mass for
thermonuclear fusion of deuterium (currently calculated
to be 13 Jupiter masses for objects of solar
metallicity) that orbit stars or stellar remnants are
"planets" (no matter how they formed). The minimum
mass/size required for an extrasolar object to be
considered a planet should be the same as that used in
our Solar System.
2) Substellar objects with true masses above the limiting
mass for thermonuclear fusion of deuterium are "brown
dwarfs", no matter how they formed nor where they are
located.
3) Free-floating objects in young star clusters with masses
below the limiting mass for thermonuclear fusion of
deuterium are not "planets", but are "sub-brown dwarfs"
(or whatever name is most appropriate).
These statements are a compromise between definitions based
purely on the deuterium-burning mass or on the formation
mechanism, and as such do not fully satisfy anyone on the WGESP.
However, the WGESP agrees that these statements constitute the
basis for a reasonable working definition of a "planet" at this
time. We can expect this definition to evolve as our knowledge
improves.
End Quote.
So the IAU's working definition is a Brown Dwarf is an object big enough
to fuse Deuterium but too small to fuse Hydrogen. For the present a
rogue Planet is a sub-brown-dwarf, while an object of a similar size
orbiting a star or a stellar remnant is a planet, provided it would be a
planet were it in the solar system. This leaves one known object
unclassified, PSR B1257+12 D which is an object orbiting a pulsar, which
is small enough that it hasn't cleared its neighbourhood (we can't
actually see any debris, but for the Stern-Levinson parameter we don't
need to). It is usually thought of as an extra-solar comet or asteroid,
the first known. There are circumstellar disks known around other stars
but no other individually resolved sub-planetary, extra-solar masses.
K_h
"Jens Egon Nyborg" <je...@invalid.invalid> wrote in message
news:g4bdcl$kho$1...@registered.motzarella.org...
The IAU's definition of a planet is extremely flawed for a number of
reasons. For some examples, the IAU definition:
* defines a planet as "nearly round" suggesting that only nearly spherical
bodies qualify as planets. But Saturn is over 12% oblate and so it is not
clear if it qualifies as a planet.
* defines a planet as having "cleared the neighborhood around its orbit"
which would exclude Jupiter type planets in other solar systems. For
example, consider a solar system with a 6 solar-mass star orbited by a 25
Jupiter mass brown dwarf (at 20AU from the star) and one Jupiter mass planet
at each LaGrange point L4 and L5. Neither Jupiter mass planet would have
"cleared the neighborhood around its orbit" and so they would not qualify as
planets.
* requires that a Kuiper belt world with the same size and mass as the Earth
is not a planet. A world with 70 percent the mass of the Earth may very
well reside within the Kuiper belt.
* defines a dwarf planet as not being a planet and this contradicts English
usage. In English, an adjective like "dwarf" modifies a noun like "planet".
What the IAU has done is like defining a woman to be a young female and then
claiming that there are old women.
* defines a planet in such a way that it only applies to Earth's solar
system. This is bad; a definition should be applicable throughout nature.
Claiming one definition for Earth's solar system, and another definition for
the rest of the universe, implies there is something special about Earth's
solar system and that is a violation of the Copernican principle.
* has intrinsic ambiguity in its phrase "cleared the neighborhood around its
orbit". There is no way to know how much clearing is necessary.
* fails to specify what kind of orbit a planet must take. It is unclear if
a world, with the same size and mass as the Earth, traveling on a hyperbolic
orbit around the sun qualifies as a planet.
The IAU's definition of a planet is not based on good science and is not
very useful. The best way to define a planet is to define it in context
with other bodies in the universe. Below is an example of one possible way
of doing this.
-----------------------------------------------------------------------------
A star is a celestial body that sustains, has sustained, or is capable of
sustaining nuclear fusion.
A brown dwarf is a star that is only capable of fusing deuterium and/or
lithium even if neither is present.
A celestial body that has its barycenter continuously inside the body of
another celestial body is a satellite of that celestial body.
A planetary body is a celestial body that has sufficient mass for its self
gravity to overcome rigid body forces so that it assumes a hydrostatic
equilibrium, is nearly spherical or nearly spheroidal in shape, and is not a
star.
A planet is a planetary body that is not the satellite of another planetary
body.
A moon is a satellite of a planet.
-----------------------------------------------------------------------------
With the above definitions, there is the option of having additional
definitions that divide planets and moons into further categories:
Giant planet: Diameter >= 30,000 Km.
Midsize planet: 30,000 Km > Diameter >= 3,000 Km.
Dwarf planet: 3,000 Km > Diameter
Giant moon: Maximum width >= 3,000 Km.
Midsize moon: 3,000 Km > Maximum width >= 300 Km.
Dwarf moon: 300 Km > Maximum width
Primary planet: Giant or midsize planet
Secondary planet: dwarf planet
Primary moon: Giant or midsize moon
Secondary moon: dwarf moon
A classification scheme like this should satisfy the anti-Pluto people since
only 8 planets in the solar system qualify as primary planets. It also
prevents the number of primary planets from growing too large; which seemed
to be a big concern for the IAU.
There is my 2 cents on the planet definition problem and one possible way to
fix it.
K
Brett Paul Dunbar
<KHo...@SX729.com> writes
>The IAU's definition of a planet is extremely flawed for a number of
>reasons. For some examples, the IAU definition:
>
>
>
>* defines a planet as "nearly round" suggesting that only nearly spherical
>bodies qualify as planets. But Saturn is over 12% oblate and so it is not
>clear if it qualifies as a planet.
It does, the nearly round is hydrostatic equilibrium so it is round
modulo any distortions caused by spin.
>
>* defines a planet as having "cleared the neighborhood around its orbit"
>which would exclude Jupiter type planets in other solar systems. For
>example, consider a solar system with a 6 solar-mass star orbited by a 25
>Jupiter mass brown dwarf (at 20AU from the star) and one Jupiter mass planet
>at each LaGrange point L4 and L5. Neither Jupiter mass planet would have
>"cleared the neighborhood around its orbit" and so they would not qualify as
>planets.
That is an actual potential ambiguity, at the moment we don't know of
any such bodies if we find one then we will have to decide. This is true
of most possible definitions. How you might classify a giant Trojan can
await actually discovering one.
>
>* requires that a Kuiper belt world with the same size and mass as the Earth
>is not a planet. A world with 70 percent the mass of the Earth may very
>well reside within the Kuiper belt.
The minimum size for orbit clearing depends on distance from the
primary; and this is a problem why?
>
>* defines a dwarf planet as not being a planet and this contradicts English
>usage. In English, an adjective like "dwarf" modifies a noun like "planet".
>What the IAU has done is like defining a woman to be a young female and then
>claiming that there are old women.
In English the noun with the modifier often denotes a subset, it can
also denote a disjoint set. A few examples: In Alpine Skiing the Slalom
Giant Slalom and Supergiant Slalom are three distinct events and there
is no overlap between the types of courses used. Moles, Golden Moles and
Marsupial Moles are three quite distinct, if superficially similar,
groups of burrowing mammals again they are entirely disjoint sets and
are not closely related.
>
>* defines a planet in such a way that it only applies to Earth's solar
>system. This is bad; a definition should be applicable throughout nature.
>Claiming one definition for Earth's solar system, and another definition for
>the rest of the universe, implies there is something special about Earth's
>solar system and that is a violation of the Copernican principle.
We have a lot more information about the solar system than about any
other system, we can actually resolve individual objects that are too
small to be planets in the solar system, with one exception this is not
true of any exoplanetary system, so for the moment we don't need to
worry about the lower limit of planetary size outside the solar system.
>
>* has intrinsic ambiguity in its phrase "cleared the neighborhood around its
>orbit". There is no way to know how much clearing is necessary.
>
>* fails to specify what kind of orbit a planet must take. It is unclear if
>a world, with the same size and mass as the Earth, traveling on a hyperbolic
>orbit around the sun qualifies as a planet.
A hyperbola isn't actually an orbit, as it isn't closed so no that would
not have cleared its orbit. Orbits are either elliptical or circular and
repeat. Parabolas and hyperbolas do not repeat.
>
>
>
>The IAU's definition of a planet is not based on good science and is not
>very useful. The best way to define a planet is to define it in context
>with other bodies in the universe. Below is an example of one possible way
>of doing this.
The IAUs definition does this, a planet has achieved a particular status
in terms of its orbital dynamics, it has cleared a region of space of
pretty much all other bodies.
>-----------------------------------------------------------------------------
>
>
>
>A star is a celestial body that sustains, has sustained, or is capable of
>sustaining nuclear fusion.
>
>
>
>A brown dwarf is a star that is only capable of fusing deuterium and/or
>lithium even if neither is present.
>
>
>
>A celestial body that has its barycenter continuously inside the body of
>another celestial body is a satellite of that celestial body.
>
>
>
>A planetary body is a celestial body that has sufficient mass for its self
>gravity to overcome rigid body forces so that it assumes a hydrostatic
>equilibrium, is nearly spherical or nearly spheroidal in shape, and is not a
>star.
>
>
>
>A planet is a planetary body that is not the satellite of another planetary
>body.
That would include a bunch of small spherical bodies which are too
small, such as Ceres. Planet are large and should dominate their region
of space.
>
>
>
>A moon is a satellite of a planet.
Wrong, a fair number of non-planets have moons for example Dactyl is a
moon of the Asteroid Ida. Not acceptable for that reason
>
>
>
>-----------------------------------------------------------------------------
>With the above definitions, there is the option of having additional
>definitions that divide planets and moons into further categories:
>
>
>
> Giant planet: Diameter >= 30,000 Km.
> Midsize planet: 30,000 Km > Diameter >= 3,000 Km.
> Dwarf planet: 3,000 Km > Diameter
>
> Giant moon: Maximum width >= 3,000 Km.
> Midsize moon: 3,000 Km > Maximum width >= 300 Km.
> Dwarf moon: 300 Km > Maximum width
>
>
>
> Primary planet: Giant or midsize planet
> Secondary planet: dwarf planet
>
> Primary moon: Giant or midsize moon
> Secondary moon: dwarf moon
>
>
>
>A classification scheme like this should satisfy the anti-Pluto people since
>only 8 planets in the solar system qualify as primary planets. It also
>prevents the number of primary planets from growing too large; which seemed
>to be a big concern for the IAU.
>
It doesn't use any fundamental characteristic to define a Planet,
diameter is an arbitrary number while orbit clearing is a dynamic
process which produces a clear natural gap. Diameter depends a lot on
composition, Mercury has a smaller diameter than Ganymede but is almost
twice the mass. Icy bodies also achieve hydrostatic equilibrium at a
lower mass than rocky bodies.
>
>
>There is my 2 cents on the planet definition problem and one possible way to
>fix it.
There are fewer problems with the IAU definition than with yours. Orbit
clearing is a straightforward and significant orbital dynamic process.
Bryan Derksen
> In message <PYKdnToYLvdOlenV...@comcast.com>, K_h
> <KHo...@SX729.com> writes
>> is not a planet. A world with 70 percent the mass of the Earth may very
>> well reside within the Kuiper belt.
>
> The minimum size for orbit clearing depends on distance from the
> primary; and this is a problem why?
Actually, using the Stern-Levison method for determining orbit clearing
capacity, an Earth-mass object orbiting smack dab in the middle of the
Kuiper belt (42.5 AU radius, 277 year period) would probably qualify as
a planet.
The calculation I fed into Google:
(((1 earth mass)^2) / (277 years)) / (((1 earth mass)^2) / (1 year))
= 0.0036101083
According to the table at
<http://en.wikipedia.org/wiki/Cleared_the_neighbourhood> this is about
half the orbit-clearing oomph that Mars has but over a hundred thousand
times the orbit-clearing oomph of Eris, its next nearest competitor.
Dropping the object's mass to 0.7 Earths only lowers its parameter to
0.001, still within the same order of magnitude as Mars.
A five order of magnitude gap on the non-planet side versus a zero order
of magnitude gap on the planet side makes me think it's pretty clearly a
planet. :)
>> * has intrinsic ambiguity in its phrase "cleared the neighborhood around its
>> orbit". There is no way to know how much clearing is necessary.
The footnote that lists which objects qualify as planets helps to
clarify this, as well as the fact that by both known published methods
there's a gap of many orders of magnitude between planet and non-planet.
K_h
"Brett Paul Dunbar" <br...@nospam.demon.co.uk> wrote in message
news:FV$OyfDNg...@dimetrodon.demon.co.uk...
> In message <PYKdnToYLvdOlenV...@comcast.com>, K_h
> <KHo...@SX729.com> writes
>>The IAU's definition of a planet is extremely flawed for a number of
>>reasons. For some examples, the IAU definition:
>>
>>
>>
>>* defines a planet as "nearly round" suggesting that only nearly spherical
>>bodies qualify as planets. But Saturn is over 12% oblate and so it is not
>>clear if it qualifies as a planet.
>
> It does, the nearly round is hydrostatic equilibrium so it is round modulo
> any distortions caused by spin.
Well, then, the IAU definition should say that. It says nothing about
modulo spin distortions.
>>
>>* defines a planet as having "cleared the neighborhood around its orbit"
>>which would exclude Jupiter type planets in other solar systems. For
>>example, consider a solar system with a 6 solar-mass star orbited by a 25
>>Jupiter mass brown dwarf (at 20AU from the star) and one Jupiter mass
>>planet
>>at each LaGrange point L4 and L5. Neither Jupiter mass planet would have
>>"cleared the neighborhood around its orbit" and so they would not qualify
>>as
>>planets.
>
> That is an actual potential ambiguity, at the moment we don't know of any
> such bodies if we find one then we will have to decide. This is true of
> most possible definitions. How you might classify a giant Trojan can await
> actually discovering one.
Definitions should be generic enough to cover the kinds of cases that exist
in nature. With trillions of solar systems in the Milky Way galaxy alone,
there have to be configurations like this. The defintion of a planet should
be generic enough to cover whatever nature can produce.
>>
>>* requires that a Kuiper belt world with the same size and mass as the
>>Earth
>>is not a planet. A world with 70 percent the mass of the Earth may very
>>well reside within the Kuiper belt.
>
> The minimum size for orbit clearing depends on distance from the primary;
> and this is a problem why?
An Earth sized world in the Kuiper belt is a planet under any reasonable
definition. If / when and Earth sized world if found orbiting another star,
at 30 AU say, it will almost certainly be called a planet.
>>
>>* defines a dwarf planet as not being a planet and this contradicts
>>English
>>usage. In English, an adjective like "dwarf" modifies a noun like
>>"planet".
>>What the IAU has done is like defining a woman to be a young female and
>>then
>>claiming that there are old women.
>
> In English the noun with the modifier often denotes a subset, it can also
> denote a disjoint set. A few examples: In Alpine Skiing the Slalom Giant
> Slalom and Supergiant Slalom are three distinct events and there is no
> overlap between the types of courses used. Moles, Golden Moles and
> Marsupial Moles are three quite distinct, if superficially similar, groups
> of burrowing mammals again they are entirely disjoint sets and are not
> closely related.
On something as fundamental as defining planets, the usual rules of English
should be obeyed. The IAU defintion does not follow the usual rules and
that is bad.
>>
>>* defines a planet in such a way that it only applies to Earth's solar
>>system. This is bad; a definition should be applicable throughout nature.
>>Claiming one definition for Earth's solar system, and another definition
>>for
>>the rest of the universe, implies there is something special about Earth's
>>solar system and that is a violation of the Copernican principle.
>
> We have a lot more information about the solar system than about any other
> system, we can actually resolve individual objects that are too small to
> be planets in the solar system, with one exception this is not true of any
> exoplanetary system, so for the moment we don't need to worry about the
> lower limit of planetary size outside the solar system.
Never put off to tomorrow what you can do today. The IAU's bad defintion
should be cleared up as soon as possible.
>>
>>* has intrinsic ambiguity in its phrase "cleared the neighborhood around
>>its
>>orbit". There is no way to know how much clearing is necessary.
>>
>>* fails to specify what kind of orbit a planet must take. It is unclear
>>if
>>a world, with the same size and mass as the Earth, traveling on a
>>hyperbolic
>>orbit around the sun qualifies as a planet.
>
> A hyperbola isn't actually an orbit, as it isn't closed so no that would
> not have cleared its orbit. Orbits are either elliptical or circular and
> repeat. Parabolas and hyperbolas do not repeat.
No, they are hyperbolic orbits. Go to http://en.wikipedia.org/wiki/Orbits
and do a search for "hyperbolic orbit". Also check out:
http://en.wikipedia.org/wiki/Hyperbolic_trajectory
>>
>>A star is a celestial body that sustains, has sustained, or is capable of
>>sustaining nuclear fusion.
>>
>>
>>
>>A brown dwarf is a star that is only capable of fusing deuterium and/or
>>lithium even if neither is present.
>>
>>
>>
>>A celestial body that has its barycenter continuously inside the body of
>>another celestial body is a satellite of that celestial body.
>>
>>
>>
>>A planetary body is a celestial body that has sufficient mass for its self
>>gravity to overcome rigid body forces so that it assumes a hydrostatic
>>equilibrium, is nearly spherical or nearly spheroidal in shape, and is not
>>a
>>star.
>>
>>
>>
>>A planet is a planetary body that is not the satellite of another
>>planetary
>>body.
>
> That would include a bunch of small spherical bodies which are too small,
> such as Ceres. Planet are large and should dominate their region of space.
>
>>
>>
>>
>>A moon is a satellite of a planet.
>
> Wrong, a fair number of non-planets have moons for example Dactyl is a
> moon of the Asteroid Ida. Not acceptable for that reason.
Once astronomers settle on decent definitions, this won't be a problem.
Under the definitions I proposed, only planets would have moons and Dactyl
would be a satellite that is not a moon.
Yes, those numbers were arbitrary and I should have used "Equatorial
Diameter" instead of "Diameter" for the planets. The phrase "orbit
clearing" is ambiguous and that is one of the problems with the IAU's
definition.
>>
>>There is my 2 cents on the planet definition problem and one possible way
>>to
>>fix it.
>
>
> There are fewer problems with the IAU definition than with yours. Orbit
> clearing is a straightforward and significant orbital dynamic process.
No, the IAU definition has all the problems I mentioned but the definitions
I provided do not have any of them. Orbit clearing is too ambiguous to be
worthy of scientific nomenclature.
K
Bryan Derksen
> "Brett Paul Dunbar" <br...@nospam.demon.co.uk> wrote in message
> news:FV$OyfDNg...@dimetrodon.demon.co.uk...
>> In message <PYKdnToYLvdOlenV...@comcast.com>, K_h
>> <KHo...@SX729.com> writes
>>> bodies qualify as planets. But Saturn is over 12% oblate and so it is not
>>> clear if it qualifies as a planet.
>> It does, the nearly round is hydrostatic equilibrium so it is round modulo
>> any distortions caused by spin.
> Well, then, the IAU definition should say that. It says nothing about
> modulo spin distortions.
The IAU definition _does_ say that. Quoting it directly:
"(1) A planet [1] is a celestial body that (a) is in orbit around the
Sun, (b) has sufficient mass for its self-gravity to overcome rigid body
forces so that it assumes a hydrostatic equilibrium (nearly round)
shape, and (c) has cleared the neighbourhood around its orbit."
"Hydrostatic equilibrium" inherently includes oblateness due to
rotation. A rotating self-gravitating object that's in hydrostatic
equilibrium will be an oblate spheroid, just like Saturn is.
>>> * defines a planet as having "cleared the neighborhood around its orbit"
>>> which would exclude Jupiter type planets in other solar systems. For
>>> example, consider a solar system with a 6 solar-mass star orbited by a 25
>>> Jupiter mass brown dwarf (at 20AU from the star) and one Jupiter mass
>>> planet
>>> at each LaGrange point L4 and L5. Neither Jupiter mass planet would have
>>> "cleared the neighborhood around its orbit" and so they would not qualify
>>> as
>>> planets.
>> That is an actual potential ambiguity, at the moment we don't know of any
>> such bodies if we find one then we will have to decide. This is true of
>> most possible definitions. How you might classify a giant Trojan can await
>> actually discovering one.
>
> Definitions should be generic enough to cover the kinds of cases that exist
> in nature. With trillions of solar systems in the Milky Way galaxy alone,
> there have to be configurations like this. The defintion of a planet should
> be generic enough to cover whatever nature can produce.
Exactly how generic was the previous definition by comparison? In case
you weren't aware, the previous definition of "planet" was just the
following list: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus,
Neptune, and Pluto. It was arbitrary.
The new definition is still explicitly limited to just the solar system
but that's an easy limit to lift. I don't think a few hypothetical and
highly unlikely hard cases are enough of a problem to call this an
"extremely flawed" definition on that basis, you'll find hard cases for
practically any definition if you look hard enough.
Also, the Milky Way doesn't have trillions of stars, so I think you're a
bit off on the "trillions of solar systems" thing.
>>> * requires that a Kuiper belt world with the same size and mass as the
>>> Earth
>>> is not a planet. A world with 70 percent the mass of the Earth may very
>>> well reside within the Kuiper belt.
>> The minimum size for orbit clearing depends on distance from the primary;
>> and this is a problem why?
>
> An Earth sized world in the Kuiper belt is a planet under any reasonable
> definition. If / when and Earth sized world if found orbiting another star,
> at 30 AU say, it will almost certainly be called a planet.
And, according to the calculations I showed earlier, such an object is
approximately as capable of clearing its orbit as Mars is. So yes, it
would likely be called a planet, even under the IAU's definition. What's
the problem here?
>> In English the noun with the modifier often denotes a subset, it can also
>> denote a disjoint set. A few examples: In Alpine Skiing the Slalom Giant
>> Slalom and Supergiant Slalom are three distinct events and there is no
>> overlap between the types of courses used. Moles, Golden Moles and
>> Marsupial Moles are three quite distinct, if superficially similar, groups
>> of burrowing mammals again they are entirely disjoint sets and are not
>> closely related.
>
> On something as fundamental as defining planets, the usual rules of English
> should be obeyed. The IAU defintion does not follow the usual rules and
> that is bad.
I'd be in favor of dropping the "dwarf planet" category entirely and
just let them be asteroids and KBOs and whatnot, but I somehow doubt
that would go over better than the current definition did with most of
those who are objecting to it. :)
>> We have a lot more information about the solar system than about any other
>> system, we can actually resolve individual objects that are too small to
>> be planets in the solar system, with one exception this is not true of any
>> exoplanetary system, so for the moment we don't need to worry about the
>> lower limit of planetary size outside the solar system.
>
> Never put off to tomorrow what you can do today. The IAU's bad defintion
> should be cleared up as soon as possible.
Actually, it makes sense to put off decisions until tomorrow when you
haven't got enough information to make good ones today. When extrasolar
planets started being discovered the abundance of close-orbiting "hot
Jupiters" was a big surprise, turning our accepted theories of planet
formation on their heads. If the IAU had tried coming up with some
hard-and-fast definition before then that assumed any close-in bodies
would be small and rocky like they were in our solar system they could
wind up with misclassifications even sillier than Pluto's was.
The IAU's definition as it currently stands is explicitly limited to
just our solar system, and except in highly unusual situations we can't
yet detect extrasolar objects small enough and far enough out from their
primaries to get anywhere near the boundary of planethood anyway, so I
don't see what the rush is. We can wait until we know more about
planetary systems in general.
>>> * fails to specify what kind of orbit a planet must take. It is unclear
>>> if
>>> a world, with the same size and mass as the Earth, traveling on a
>>> hyperbolic
>>> orbit around the sun qualifies as a planet.
>> A hyperbola isn't actually an orbit, as it isn't closed so no that would
>> not have cleared its orbit. Orbits are either elliptical or circular and
>> repeat. Parabolas and hyperbolas do not repeat.
>
> No, they are hyperbolic orbits. Go to http://en.wikipedia.org/wiki/Orbits
> and do a search for "hyperbolic orbit".
The first time the phrase is mentioned in that article is in the
sentence "A comet in a parabolic or hyperbolic orbit about a central
star is not gravitationally bound to the star and therefore is not
considered part of the star's planetary system."
This is another one of those obscure edge cases that just isn't worth
fretting about, IMO. The likelihood of something like this coming up are
negligible. A better issue to think about is how to classify
free-floating "rogue" planets, which hyperbolic planets are a
subcategory of. But since we can't detect those yet there's no big rush
here either.
> Yes, those numbers were arbitrary and I should have used "Equatorial
> Diameter" instead of "Diameter" for the planets. The phrase "orbit
> clearing" is ambiguous and that is one of the problems with the IAU's
> definition.
At least orbit clearing has a large discontinuity between "very clear"
and "not at all clear" in practice, and a solid theoretical basis
indicating that this discontinuity will be seen in most other cases too.
So even if the exact numeric threshold is undefined one can say
"somewhere between Mars and Pluto" and it's unlikely to be ambiguous
which side a candidate object falls on.
Planetary diameter for any given mass, on the other hand, can vary
widely with composition and temperature and has no "natural" cutoff points.
Brett Paul Dunbar
<KHo...@SX729.com> writes
>
>"Brett Paul Dunbar" <br...@nospam.demon.co.uk> wrote in message
>news:FV$OyfDNg...@dimetrodon.demon.co.uk...
>> In English the noun with the modifier often denotes a subset, it can also
>> denote a disjoint set. A few examples: In Alpine Skiing the Slalom Giant
>> Slalom and Supergiant Slalom are three distinct events and there is no
>> overlap between the types of courses used. Moles, Golden Moles and
>> Marsupial Moles are three quite distinct, if superficially similar, groups
>> of burrowing mammals again they are entirely disjoint sets and are not
>> closely related.
>
>
>On something as fundamental as defining planets, the usual rules of English
>should be obeyed. The IAU defintion does not follow the usual rules and
>that is bad.
>
While the modifier may more often denote a subset, it very often doesn't
both possibilities are perfectly normal English. An objection on that
grammatical basis is frankly absurd.
A few other examples: Anteaters, Spiny Anteaters and Scaly Anteaters
(AKA Anteaters, Echidnas and Pangolins), or Mice and Marsupial Mice or
Crabs and Hermit Crabs.
K_h
"Bryan Derksen" <bryan....@shaw.ca> wrote in message
news:GCvdk.69025$Jx.48462@pd7urf1no...
> K_h wrote:
>> "Brett Paul Dunbar" <br...@nospam.demon.co.uk> wrote in message
>> news:FV$OyfDNg...@dimetrodon.demon.co.uk...
>>> In message <PYKdnToYLvdOlenV...@comcast.com>, K_h
>>> <KHo...@SX729.com> writes
>>>> * defines a planet as "nearly round" suggesting that only nearly
>>>> spherical
>>>> bodies qualify as planets. But Saturn is over 12% oblate and so it is
>>>> not
>>>> clear if it qualifies as a planet.
>
>>> It does, the nearly round is hydrostatic equilibrium so it is round
>>> modulo any distortions caused by spin.
>
>> Well, then, the IAU definition should say that. It says nothing about
>> modulo spin distortions.
>
> The IAU definition _does_ say that. Quoting it directly:
>
> "(1) A planet [1] is a celestial body that (a) is in orbit around the Sun,
> (b) has sufficient mass for its self-gravity to overcome rigid body forces
> so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c)
> has cleared the neighbourhood around its orbit."
>
> "Hydrostatic equilibrium" inherently includes oblateness due to rotation.
> A rotating self-gravitating object that's in hydrostatic equilibrium will
> be an oblate spheroid, just like Saturn is.
No, it does not say that. The phrase "modulo spin distortions" is contained
nowhere in the definition. A non-spinning planet in hydrostatic equilibrium
will be nearly spherical in shape but a spinning planet in hydrostatic
equilibrium will be nearly spheroidal in shape. Note, not "nearly round"
since "nearly round" suggests nearly spherical rather than nearly
spheroidal. Check out the definition of round at
http://en.wikipedia.org/wiki/Round.
It is hard to know how common these "few hypothetical and highly unlikely"
cases are. Even if only 1 percent of solar systems violate the IAU's
definition then that is bad. The definitions I suggested avoid this problem
totally.
> Also, the Milky Way doesn't have trillions of stars, so I think you're a
> bit off on the "trillions of solar systems" thing.
Yes, a typo on my part. The best estimates seem to be about a quarter
trillion stars.
>>>> * requires that a Kuiper belt world with the same size and mass as the
>>>> Earth
>>>> is not a planet. A world with 70 percent the mass of the Earth may
>>>> very
>>>> well reside within the Kuiper belt.
>>> The minimum size for orbit clearing depends on distance from the
>>> primary; and this is a problem why?
>>
>> An Earth sized world in the Kuiper belt is a planet under any reasonable
>> definition. If / when and Earth sized world if found orbiting another
>> star, at 30 AU say, it will almost certainly be called a planet.
>
> And, according to the calculations I showed earlier, such an object is
> approximately as capable of clearing its orbit as Mars is. So yes, it
> would likely be called a planet, even under the IAU's definition. What's
> the problem here?
New studies suggest that a world, about 70 percent as massive as the Earth,
could exist in the Kuiper built without clearing its orbit.
http://www.msnbc.msn.com/id/25274320/. A world that massive deserves to be
called a planet even though it hasn't cleared out its region in the Kuiper
belt.
Also, check out:
http://www.spacedaily.com/reports/The_Great_Planet_Debate_Dwarf_Planets_Are_Planets_Too_999.html
>>> In English the noun with the modifier often denotes a subset, it can
>>> also denote a disjoint set. A few examples: In Alpine Skiing the Slalom
>>> Giant Slalom and Supergiant Slalom are three distinct events and there
>>> is no overlap between the types of courses used. Moles, Golden Moles and
>>> Marsupial Moles are three quite distinct, if superficially similar,
>>> groups of burrowing mammals again they are entirely disjoint sets and
>>> are not closely related.
>>
>> On something as fundamental as defining planets, the usual rules of
>> English should be obeyed. The IAU defintion does not follow the usual
>> rules and that is bad.
>
> I'd be in favor of dropping the "dwarf planet" category entirely and just
> let them be asteroids and KBOs and whatnot, but I somehow doubt that would
> go over better than the current definition did with most of those who are
> objecting to it. :)
A bad idea to call "dwarf planets" asteroids. A world like Pluto has an
atmosphere, polar caps, three moons, and interesting geological activity at
its surface.
>>> We have a lot more information about the solar system than about any
>>> other system, we can actually resolve individual objects that are too
>>> small to be planets in the solar system, with one exception this is not
>>> true of any exoplanetary system, so for the moment we don't need to
>>> worry about the lower limit of planetary size outside the solar system.
>>
>> Never put off to tomorrow what you can do today. The IAU's bad defintion
>> should be cleared up as soon as possible.
>
> Actually, it makes sense to put off decisions until tomorrow when you
> haven't got enough information to make good ones today. When extrasolar
> planets started being discovered the abundance of close-orbiting "hot
> Jupiters" was a big surprise, turning our accepted theories of planet
> formation on their heads. If the IAU had tried coming up with some
> hard-and-fast definition before then that assumed any close-in bodies
> would be small and rocky like they were in our solar system they could
> wind up with misclassifications even sillier than Pluto's was.
>
> The IAU's definition as it currently stands is explicitly limited to just
> our solar system, and except in highly unusual situations we can't yet
> detect extrasolar objects small enough and far enough out from their
> primaries to get anywhere near the boundary of planethood anyway, so I
> don't see what the rush is. We can wait until we know more about planetary
> systems in general.
I disagree. The defintions I suggested solve the problem no matter what
discoveries are certain to come. Nature has a great deal of diversity and
so good definitions should be comprehensive enough to encompass all that
diversity. Defining a planet, based on its surroundings, is always bad
because there is bound to be such a wide range of environments surrounding
worlds like Mercury, Jupiter, etc..
>
>>>> * fails to specify what kind of orbit a planet must take. It is
>>>> unclear if
>>>> a world, with the same size and mass as the Earth, traveling on a
>>>> hyperbolic
>>>> orbit around the sun qualifies as a planet.
>>> A hyperbola isn't actually an orbit, as it isn't closed so no that would
>>> not have cleared its orbit. Orbits are either elliptical or circular and
>>> repeat. Parabolas and hyperbolas do not repeat.
>>
>> No, they are hyperbolic orbits. Go to
>> http://en.wikipedia.org/wiki/Orbits and do a search for "hyperbolic
>> orbit".
>
> The first time the phrase is mentioned in that article is in the sentence
> "A comet in a parabolic or hyperbolic orbit about a central star is not
> gravitationally bound to the star and therefore is not considered part of
> the star's planetary system."
>
> This is another one of those obscure edge cases that just isn't worth
> fretting about, IMO. The likelihood of something like this coming up are
> negligible. A better issue to think about is how to classify free-floating
> "rogue" planets, which hyperbolic planets are a subcategory of. But since
> we can't detect those yet there's no big rush here either.
It is not obscure at all, there are probably billions of "free-floating"
planets in the local group alone. The defintions I provided qualify them as
planets but, by the IAU's definition, they are not planets at all since they
are not "in orbit" around a star. If the definition of a planet depends on
its orbit being closed then the IAU definition should have used the phrase
"in a closed orbit" instead of "in orbit". Obviously the IAU's defintion
contains substantial weaknesses.
>> Yes, those numbers were arbitrary and I should have used "Equatorial
>> Diameter" instead of "Diameter" for the planets. The phrase "orbit
>> clearing" is ambiguous and that is one of the problems with the IAU's
>> definition.
>
> At least orbit clearing has a large discontinuity between "very clear" and
> "not at all clear" in practice, and a solid theoretical basis indicating
> that this discontinuity will be seen in most other cases too. So even if
> the exact numeric threshold is undefined one can say "somewhere between
> Mars and Pluto" and it's unlikely to be ambiguous which side a candidate
> object falls on.
It is too soon to claim that there is a "solid theoretical basis" indicating
that a large discontinuity will be seen in most cases. The aforementioned
issue with a 70 percent mass world not clearing out its Kuiper belt region
underscores this point.
> Planetary diameter for any given mass, on the other hand, can vary widely
> with composition and temperature and has no "natural" cutoff points.
Yes, that is certainly true. So the option I suggested, of categorizing
planets by three size ranges, has pluses and minuses.
K
Bryan Derksen
> "Bryan Derksen" <bryan....@shaw.ca> wrote in message
> news:GCvdk.69025$Jx.48462@pd7urf1no...
>>
>> "(1) A planet [1] is a celestial body that (a) is in orbit around the Sun,
>> (b) has sufficient mass for its self-gravity to overcome rigid body forces
>> so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c)
>> has cleared the neighbourhood around its orbit."
>>
>> "Hydrostatic equilibrium" inherently includes oblateness due to rotation.
>> A rotating self-gravitating object that's in hydrostatic equilibrium will
>> be an oblate spheroid, just like Saturn is.
>
> No, it does not say that. The phrase "modulo spin distortions" is contained
> nowhere in the definition.
This is pointless semantic quibbling and nitpickery, not worth any
further effort to refute. You're being silly.
>> And, according to the calculations I showed earlier, such an object is
>> approximately as capable of clearing its orbit as Mars is. So yes, it
>> would likely be called a planet, even under the IAU's definition. What's
>> the problem here?
>
> New studies suggest that a world, about 70 percent as massive as the Earth,
> could exist in the Kuiper built without clearing its orbit.
Please check back a few posts earlier in this thread. I showed my
calculations, a planet 70 percent as massive as the Earth _would_ clear
its region in the Kuiper belt. It would be approximately as capable of
clearing objects from its orbit as Mars is, and Mars is considered a planet.
It's at
http://groups.google.com/group/rec.arts.sf.science/msg/2cf5c261a6112032
in case it didn't make it to your newsserver.
> A world that massive deserves to be
> called a planet even though it hasn't cleared out its region in the Kuiper
> belt.
Agreed. Get back to me if they actually _do_ designate it a plutoid and
I'll share your annoyance. Until then you're getting bent out of shape
about some hypothetical scenario that hasn't occurred and based on the
numbers I've dug up is not particularly likely to occur.
> Also, check out:
> http://www.spacedaily.com/reports/The_Great_Planet_Debate_Dwarf_Planets_Are_Planets_Too_999.html
Alan Stern's been making some peculiar statements on this subject even
though he really should know better. He says in that article, for
example, that "Earth would not be considered a planet if it orbited the
Sun beyond Neptune, because its gravitational influence would be
insufficient to clear out the Kuiper Belt." By _his own method_ of
determining orbit clearing capacity, however, I've shown that it'd be
about as capable of clearing its orbit as Mars is (see my previous post
linked to above). So if Mars is considered a planet based on its
orbit-clearing prowess then Earth in the Kuiper belt would be too.
>> I'd be in favor of dropping the "dwarf planet" category entirely and just
>> let them be asteroids and KBOs and whatnot, but I somehow doubt that would
>> go over better than the current definition did with most of those who are
>> objecting to it. :)
>
> A bad idea to call "dwarf planets" asteroids. A world like Pluto has an
> atmosphere, polar caps, three moons, and interesting geological activity at
> its surface.
So come up with some additional subdivisions for asteroids and KBOs and
whatnot. I'm not too concerned what you do there, I'm merely pointing
out that I consider the division between dwarf planet and non-dwarf
planet to be a fairly arbitrary one (in contrast to the division between
planet and non-planet, which has a theoretical basis and is unlikely to
produce many ambiguous cases as a result).
>> This is another one of those obscure edge cases that just isn't worth
>> fretting about, IMO. The likelihood of something like this coming up are
>> negligible. A better issue to think about is how to classify free-floating
>> "rogue" planets, which hyperbolic planets are a subcategory of. But since
>> we can't detect those yet there's no big rush here either.
>
> It is not obscure at all, there are probably billions of "free-floating"
> planets in the local group alone.
And we're unlikely to detect any of them any time soon, and if we do
we're even less likely to find any of them in trajectories worth calling
"hyperbolic orbits." Hence not worth fretting about. Eris and her
friends, on the other hand, are there right now and so needed to be
dealt with.
> The defintions I provided qualify them as
> planets but, by the IAU's definition, they are not planets at all since they
> are not "in orbit" around a star. If the definition of a planet depends on
> its orbit being closed then the IAU definition should have used the phrase
> "in a closed orbit" instead of "in orbit". Obviously the IAU's defintion
> contains substantial weaknesses.
This is more semantic quibbling. The exact wording the IAU used is "is
in orbit around the Sun," which implies a closed orbit to everyone
except the most obsessive and lawyerly-minded - a hyperbolic orbit isn't
_around_ an object, it _passes by_ it. I think you've decided that this
definition is bad for some other reason and are just casting about for
every little nook you can hammer an objection into.
>> At least orbit clearing has a large discontinuity between "very clear" and
>> "not at all clear" in practice, and a solid theoretical basis indicating
>> that this discontinuity will be seen in most other cases too. So even if
>> the exact numeric threshold is undefined one can say "somewhere between
>> Mars and Pluto" and it's unlikely to be ambiguous which side a candidate
>> object falls on.
>
> It is too soon to claim that there is a "solid theoretical basis" indicating
> that a large discontinuity will be seen in most cases.
What objections do you have to the theory behind it? It's a fairly
straightforward positive feedback loop - once an object gets big enough
relative to the other objects sharing its orbit to start ejecting or
accreting them, it rapidly gets even bigger.
> The aforementioned
> issue with a 70 percent mass world not clearing out its Kuiper belt region
> underscores this point.
My earlier calculations showing that a 0.7-Earth mass _would_ clear an
orbit in the Kuiper belt aside, have you heard of the Kuiper cliff? It
was also mentioned earlier in this thread, you can read about it here:
<http://en.wikipedia.org/wiki/Kuiper_belt#.22Kuiper_cliff.22>. It may be
a case of orbit-clearing by a yet-undiscovered planet.
Brett Paul Dunbar
<KHo...@SX729.com> writes
The definition states that it must be in hydrostatic equilibrium and
then clarifies for laymen by inserting the parenthetical phrase "nearly
round". In context they are clearly including oblateness caused by spin
in nearly. This is at most a facetious grammatical quibble when the
meaning is clear.
We have seen several hundred planetary system and so far are rather
short on ambiguous systems.
From Soter's 2006 paper
<http://arxiv.org/ftp/astro-ph/papers/0608/0608359.pdf>
7. Exoplanetary Systems
For our proposed definition of a planet to have general
validity, exoplanets should also have non-overlapping orbits,
unless shielded from collisions by a mean motion resonance.
Among 164 exoplanetary systems catalogued by Schneider (2006),
20 are known to possess more than one planet. Figure 4 shows the
distance ranges (from pericenter to apocenter) of the known
secondaries in those systems, together with the inner five
planets of our own solar system.
Most of these exoplanetary systems have non-intersecting orbits,
with three exceptions. In HD 128311, the two orbits appear to
approach each other within 0.1 AU, and the uncertain
eccentricities allow that the orbits may actually cross.
However, these planets appear to share a 2:1 mean motion
resonance (Vogt et al. 2005, Sandor & Kley 2006), which would
prevent them from colliding. In HD 82943, the two planets have
overlapping orbits, but they are probably also in a 2:1
resonance (Ferraz-Mello et al. 2005).
For the system HD 160691, the best-fit solution to the Doppler
observations (McCarthy et al. 2005) allows the orbits of the two
outer planets to overlap, in which case the system would become
unstable in < 20,000 years (Gozdziewski et al. 2005). However,
uncertainties in the orbit of the outer planet allow for the
possibility that the orbits do not overlap and/or that a mean
motion resonance stabilizes the system (Bois et al. 2003,
Gozdziewski et al. 2005).
Orbital migration of growing planets interacting with the disk
planetesimals may establish mean motion resonances between
planets, which would allow them to survive on intersecting
orbits without collisions.
All known exoplanets of main sequence stars fall well above the
gap in Figure 1 and would be classified as planets by the
criterion of dynamical dominance.
To which I can only say Dwarf Planets in very distant orbits can get
quite big. That does not make them planets. The posited Kuiper wall
causing object would be a planet as it has been posited due to that
neighbourhood having been cleared.
>>>> In English the noun with the modifier often denotes a subset, it can
>>>> also denote a disjoint set. A few examples: In Alpine Skiing the Slalom
>>>> Giant Slalom and Supergiant Slalom are three distinct events and there
>>>> is no overlap between the types of courses used. Moles, Golden Moles and
>>>> Marsupial Moles are three quite distinct, if superficially similar,
>>>> groups of burrowing mammals again they are entirely disjoint sets and
>>>> are not closely related.
>>>
>>> On something as fundamental as defining planets, the usual rules of
>>> English should be obeyed. The IAU defintion does not follow the usual
>>> rules and that is bad.
>>
>> I'd be in favor of dropping the "dwarf planet" category entirely and just
>> let them be asteroids and KBOs and whatnot, but I somehow doubt that would
>> go over better than the current definition did with most of those who are
>> objecting to it. :)
>
>A bad idea to call "dwarf planets" asteroids. A world like Pluto has an
>atmosphere, polar caps, three moons, and interesting geological activity at
>its surface.
>
Asteroids are already Minor Planets, a term which denotes an entirely
disjoint set from Planet. As the new term Dwarf Planet does.
One Asteroid is definitely a Dwarf Planet, Ceres. Vesta is a marginal
case as it seems to have achieved hydrostatic equilibrium and then been
distorted by a massive impact after it had solidified.
We can wait having more information about what there is to classify
before classifying them, in other words we should wait and see what is
out there before deciding how to classify them.
It is pretty clear on this point, if an object doesn't have a closed
orbit it doesn't have a neighbourhood. In fact under the IAUs working
definition it isn't a planet at all, it is a Sub-Brown Dwarf if it is
too small to fuse Deuterium.
>
>
>>> Yes, those numbers were arbitrary and I should have used "Equatorial
>>> Diameter" instead of "Diameter" for the planets. The phrase "orbit
>>> clearing" is ambiguous and that is one of the problems with the IAU's
>>> definition.
>>
>> At least orbit clearing has a large discontinuity between "very clear" and
>> "not at all clear" in practice, and a solid theoretical basis indicating
>> that this discontinuity will be seen in most other cases too. So even if
>> the exact numeric threshold is undefined one can say "somewhere between
>> Mars and Pluto" and it's unlikely to be ambiguous which side a candidate
>> object falls on.
>
>It is too soon to claim that there is a "solid theoretical basis" indicating
>that a large discontinuity will be seen in most cases. The aforementioned
>issue with a 70 percent mass world not clearing out its Kuiper belt region
>underscores this point.
If it hasn't cleared its orbit it is the largest member of a potentially
colliding population it isn't a planet. A planet cannot be part of a
potentially colliding population, it must have either ejected, accreted
or captured virtually everything in its vicinity.
The orbital dynamics seem to indicate that there is a critical mass
above which a body will in fairly short order clear its orbit of debris,
once an object hits this mass it will be very clearly a planet very
quickly.
K_h
No, the context does not include oblateness; nearly round means nearly
spherical. Oblate planets like Saturn are not nearly round but they are
nearly spheroidal.
No excuse for not fixing the definitions now. Current lack of data does not
justify bad definitions.
> All known exoplanets of main sequence stars fall well above the
> gap in Figure 1 and would be classified as planets by the
> criterion of dynamical dominance.
A moot point since gas giants will be discovered first. Dynamic dominance
is a vague and ambiguous term and should not be used to define a planet.
Imagine a solar system in formation and a growing planet that has to
periodically "clear out" its neighborhood due to constantly influxing
material. Under the IAU's bad definition, it is alternately a planet at
sometimes and not at others. So the IAU's definition is really quite bad.
No, see the web sites I referenced. Big dwarf planets in distant orbits is
really a bad idea since "big dwarf" is a contradiction in terms.
This only underscores the need to get better definitions for things like
"Minor Planets" and "Asteroids". A minor planet is not a planet? Silly. A
great way to define asteroids is as objects small enough so that hydrostatic
equilibrium does not apply.
Bad idea. It means constantly updating the definitions as new information
comes along. The example definitions I provided do not require constant
reclassification as new data arrives.
>>>> Yes, those numbers were arbitrary and I should have used "Equatorial
>>>> Diameter" instead of "Diameter" for the planets. The phrase "orbit
>>>> clearing" is ambiguous and that is one of the problems with the IAU's
>>>> definition.
>>>
>>> At least orbit clearing has a large discontinuity between "very clear"
>>> and
>>> "not at all clear" in practice, and a solid theoretical basis indicating
>>> that this discontinuity will be seen in most other cases too. So even if
>>> the exact numeric threshold is undefined one can say "somewhere between
>>> Mars and Pluto" and it's unlikely to be ambiguous which side a candidate
>>> object falls on.
>>
>>It is too soon to claim that there is a "solid theoretical basis"
>>indicating
>>that a large discontinuity will be seen in most cases. The aforementioned
>>issue with a 70 percent mass world not clearing out its Kuiper belt region
>>underscores this point.
>
> If it hasn't cleared its orbit it is the largest member of a potentially
> colliding population it isn't a planet. A planet cannot be part of a
> potentially colliding population, it must have either ejected, accreted
> or captured virtually everything in its vicinity.
This is a bad way to define a planet. Try defining a building in such a
way that it is only a building unless it dominates its surroundings.
>
> The orbital dynamics seem to indicate that there is a critical mass
> above which a body will in fairly short order clear its orbit of debris,
> once an object hits this mass it will be very clearly a planet very
> quickly.
"Seems?" Definitions should not be justified by how things "seem". The
example I gave, with a star, a brown dwarf, and two jupiter type planets at
L4 and L5 clearly show that the "clearing its orbit of debris" is a very bad
way to define an word like "planet". Generally, objects are defined by the
properties intrinsic to themselves, not what is or is not in their
neighborhoods.
K
K_h
> K_h wrote:
>> "Bryan Derksen" <bryan....@shaw.ca> wrote in message
>> news:GCvdk.69025$Jx.48462@pd7urf1no...
>>> The IAU definition _does_ say that. Quoting it directly:
>>>
>>> "(1) A planet [1] is a celestial body that (a) is in orbit around the
>>> Sun, (b) has sufficient mass for its self-gravity to overcome rigid body
>>> forces so that it assumes a hydrostatic equilibrium (nearly round)
>>> shape, and (c) has cleared the neighbourhood around its orbit."
>>>
>>> "Hydrostatic equilibrium" inherently includes oblateness due to
>>> rotation. A rotating self-gravitating object that's in hydrostatic
>>> equilibrium will be an oblate spheroid, just like Saturn is.
>>
>> No, it does not say that. The phrase "modulo spin distortions" is
>> contained nowhere in the definition.
>
> This is pointless semantic quibbling and nitpickery, not worth any further
> effort to refute. You're being silly.
No, it is not "pointless semantic quibbling and nitpickery" and I am not
being silly. The IAU's definition is very bad for many reasons, including
those that I outlined in my first reply to this thread.
I agree with Stern that it would not clear out the Kuiper Belt. So, under
the IAU's definition it would not be a planet. That is very bad.
>>> I'd be in favor of dropping the "dwarf planet" category entirely and
>>> just let them be asteroids and KBOs and whatnot, but I somehow doubt
>>> that would go over better than the current definition did with most of
>>> those who are objecting to it. :)
>>
>> A bad idea to call "dwarf planets" asteroids. A world like Pluto has an
>> atmosphere, polar caps, three moons, and interesting geological activity
>> at its surface.
>
> So come up with some additional subdivisions for asteroids and KBOs and
> whatnot. I'm not too concerned what you do there, I'm merely pointing out
> that I consider the division between dwarf planet and non-dwarf planet to
> be a fairly arbitrary one (in contrast to the division between planet and
> non-planet, which has a theoretical basis and is unlikely to produce many
> ambiguous cases as a result).
As astronomers learn more and more about other solar systems, more and more
unexpected discoveries arise. It is premature to cleaim that it is
"unlikely to produce many ambiguous cases". The definitions I provided are
clear and comprehensive and do not require additional subdivisions for
asteroids and KBO's. In fact, the term "Plutoid" is not needed.
>>> This is another one of those obscure edge cases that just isn't worth
>>> fretting about, IMO. The likelihood of something like this coming up are
>>> negligible. A better issue to think about is how to classify
>>> free-floating "rogue" planets, which hyperbolic planets are a
>>> subcategory of. But since we can't detect those yet there's no big rush
>>> here either.
>>
>> It is not obscure at all, there are probably billions of "free-floating"
>> planets in the local group alone.
>
> And we're unlikely to detect any of them any time soon, and if we do we're
> even less likely to find any of them in trajectories worth calling
> "hyperbolic orbits." Hence not worth fretting about. Eris and her friends,
> on the other hand, are there right now and so needed to be dealt with.
The example definitions I provided clearly qualify these billions of
free-floating planets as planets. A good definition of a planet should
cover these now, rather than wait for lots of them to be discovered and then
have to modify the definition of a planet again.
>> The defintions I provided qualify them as planets but, by the IAU's
>> definition, they are not planets at all since they are not "in orbit"
>> around a star. If the definition of a planet depends on its orbit being
>> closed then the IAU definition should have used the phrase "in a closed
>> orbit" instead of "in orbit". Obviously the IAU's defintion contains
>> substantial weaknesses.
>
> This is more semantic quibbling. The exact wording the IAU used is "is in
> orbit around the Sun," which implies a closed orbit to everyone except the
> most obsessive and lawyerly-minded - a hyperbolic orbit isn't _around_ an
> object, it _passes by_ it. I think you've decided that this definition is
> bad for some other reason and are just casting about for every little nook
> you can hammer an objection into.
No. To define a planet as "in orbit around the Sun" fails to define planets
around other starts as well as planets that are in galactic or intergalactic
space. It is bad to have one definition of a planet for our solar system
and another for everywhere else. Yes, hyperbolic orbits around the sun will
be quite rare, for worlds the size we typically call planets, but that was
not the point. The point is that there is no excuse for such bad wording by
the IAU. If it meant a closed orbit around the sun then it should have
specified that kind of orbit in its definition so that it is clear that the
definition is not applying to open orbits around the sun.
>>> At least orbit clearing has a large discontinuity between "very clear"
>>> and "not at all clear" in practice, and a solid theoretical basis
>>> indicating that this discontinuity will be seen in most other cases too.
>>> So even if the exact numeric threshold is undefined one can say
>>> "somewhere between Mars and Pluto" and it's unlikely to be ambiguous
>>> which side a candidate object falls on.
>>
>> It is too soon to claim that there is a "solid theoretical basis"
>> indicating that a large discontinuity will be seen in most cases.
>
> What objections do you have to the theory behind it? It's a fairly
> straightforward positive feedback loop - once an object gets big enough
> relative to the other objects sharing its orbit to start ejecting or
> accreting them, it rapidly gets even bigger.
See my original post, I listed a case where Jupiter size and mass worlds
would not qualify as planets under the IAU's definition. Cases like this
will not eject or accrete the other worlds and it is too soon to know how
common or rare systems like these are.
>> The aforementioned issue with a 70 percent mass world not clearing out
>> its Kuiper belt region underscores this point.
>
> My earlier calculations showing that a 0.7-Earth mass _would_ clear an
> orbit in the Kuiper belt aside, have you heard of the Kuiper cliff? It was
> also mentioned earlier in this thread, you can read about it here:
> <http://en.wikipedia.org/wiki/Kuiper_belt#.22Kuiper_cliff.22>. It may be a
> case of orbit-clearing by a yet-undiscovered planet.
"May be the case" is the operative term. It may be that a 0.7 Earth mass
world has not cleared out its orbit, far out in the Kuiper Belt, as Stern
has pointed out. If that turns out to be the case then it will be yet
another example of why the IAU's definition is so flawed. The point is that
the definitions should be fixed so that it is clear what a planet is no
matter what its changing environmental conditions are. The example
definitions I provided do just that.
K
Bryan Derksen
>
> No, the context does not include oblateness; nearly round means nearly
> spherical. Oblate planets like Saturn are not nearly round but they are
> nearly spheroidal.
Something that's sphereoidal isn't round? This is silly.
>> All known exoplanets of main sequence stars fall well above the
>> gap in Figure 1 and would be classified as planets by the
>> criterion of dynamical dominance.
>
> A moot point since gas giants will be discovered first. Dynamic dominance
> is a vague and ambiguous term and should not be used to define a planet.
> Imagine a solar system in formation and a growing planet that has to
> periodically "clear out" its neighborhood due to constantly influxing
> material. Under the IAU's bad definition, it is alternately a planet at
> sometimes and not at others. So the IAU's definition is really quite bad.
Firstly, once again I remind you that the IAU's definition doesn't
currently apply to any solar system other than ours. So there's no big
rush to make sure it works with edge cases like these, especially edge
cases that are going to be rare and hard to detect.
Secondly, planet formation is a turbulent and dynamic time. It's planet
_formation_, after all, the transition from non-planet to planet. So
what's wrong with objects gaining and losing their planethood during the
process? That's what it's all about.
> No, see the web sites I referenced. Big dwarf planets in distant orbits is
> really a bad idea since "big dwarf" is a contradiction in terms.
Are you ignoring me? I gave you numbers using the method that Alan Stern
himself co-developed showing that the hypothetical example you gave is
as much a planet as Mars is. At this point I call straw man, you're
arguing against a problem that doesn't actually exist.
>> Asteroids are already Minor Planets, a term which denotes an entirely
>> disjoint set from Planet. As the new term Dwarf Planet does.
>
> This only underscores the need to get better definitions for things like
> "Minor Planets" and "Asteroids". A minor planet is not a planet? Silly. A
> great way to define asteroids is as objects small enough so that hydrostatic
> equilibrium does not apply.
Under the _original_ definition of "planet", way back when, the Moon was
a planet too. Planet just meant "wanderer", an astronomical feature
that's not fixed relative to the stars. So when the first asteroids were
discovered there was little to distinguish them from the other planets
except their relative dimness. Scientific definitions evolve over time
as our understanding of the subject grows.
>> We can wait having more information about what there is to classify
>> before classifying them, in other words we should wait and see what is
>> out there before deciding how to classify them.
>
> Bad idea. It means constantly updating the definitions as new information
> comes along. The example definitions I provided do not require constant
> reclassification as new data arrives.
You realize that measuring exoplanetary diameters is generally harder to
do with precision compared to mass and orbital period? As far as I'm
aware unless you're lucky enough to have the exoplanet's orbit line up
in such a way that it transits across the face of its primary there's no
way to even try.
> This is a bad way to define a planet. Try defining a building in such a
> way that it is only a building unless it dominates its surroundings.
Why? I don't see any connection between the two.
> "Seems?" Definitions should not be justified by how things "seem". The
> example I gave, with a star, a brown dwarf, and two jupiter type planets at
> L4 and L5 clearly show that the "clearing its orbit of debris" is a very bad
> way to define an word like "planet".
A hypothetical case with no indication that it's particularly likely to
occur. And if it does, so what if the two trojan objects aren't defined
as planets? They're behaving quite differently from all the other known
planets. They might even warrant a class of their own.
> Generally, objects are defined by the
> properties intrinsic to themselves, not what is or is not in their
> neighborhoods.
How do you define something as a "moon" if not in relation to what's in
its neighborhood? It's perfectly cromulent to define something based on
the context it's embedded in, among other characteristics. Lots of
things are defined that way.
Michael Ash
> No, it is not "pointless semantic quibbling and nitpickery" and I am not
> being silly. The IAU's definition is very bad for many reasons, including
> those that I outlined in my first reply to this thread.
> I agree with Stern that it would not clear out the Kuiper Belt. So, under
> the IAU's definition it would not be a planet. That is very bad.
I'm just curious, "very bad" generally implies dire consequences, like
people dying, massive financial loss, hardship and sadness and woe. What
exactly is going to happen to deserve the phrase "very bad" due to
disagreement or confusion over what label to stick on an astronomical
body?
--
Mike Ash
Radio Free Earth
Broadcasting from our climate-controlled studios deep inside the Moon
Candy West
> Eivind wrote:
> > Bryan Derksen skreiv:
>
> >> They don't mention any other specific method of measuring
> >> neighborhood-clearing either. Or several other potentially ambiguous
> >> parts of the definition, such as what it means to be "orbiting the sun"
> >> (there are various co-orbital bodies that could go either way if you
> >> play with the definition a bit). I think they assumed that most people
> >> reading their resolution would not be trying to lawyerly nitpick holes
> >> in it but would instead be more interested in the basic spirit it conveyed.
>
> > People knew the "basic spirit" of "planet" before they started this
> > mess. So that means they're back at square one, yes ?
>
> No, not really. With the discovery of a whole class of Pluto-sized
> iceballs looming there was great confusion over whether they should be
> called "planets" or not. There wasn't any _general_ definition of
> "planet" that could be used to make the determination, just an arbitrary
> historical list, so everyone had their own "basic spirit" and the IAU
> had to figure out some more official criteria for whether new objects
> should be on the list or not.
>
> > After the 'definition' everyone knows -aproximately- what "planet"
> > means, but there's some fuzziness about the edge-cases.
>
> Again, no, not really. The definition is much less ambiguous now,
> there's a gap of many orders of magnitude in "orbital cleanliness"
> between the objects considered planets and the objects not considered
> planets. Unless any future discoveries happen to fall right in the
> middle of that gap (which for theoretical reasons is unlikely) it'll be
> quite obvious which way to classify them.
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Bryan Derksen
>> Alan Stern's been making some peculiar statements on this subject even
>> though he really should know better. He says in that article, for example,
>> that "Earth would not be considered a planet if it orbited the Sun beyond
>> Neptune, because its gravitational influence would be insufficient to
>> clear out the Kuiper Belt." By _his own method_ of determining orbit
>> clearing capacity, however, I've shown that it'd be about as capable of
>> clearing its orbit as Mars is (see my previous post linked to above). So
>> if Mars is considered a planet based on its orbit-clearing prowess then
>> Earth in the Kuiper belt would be too.
>
> I agree with Stern that it would not clear out the Kuiper Belt. So, under
> the IAU's definition it would not be a planet. That is very bad.
Alright, one more time, in more detail.
Back in 2000 Alan Stern and Harold Levison presented a paper to the IAU
that described a method for calculating how rapidly an object would
clear other objects out of its orbit and proposed classifying planets
based on this. The paper can be found here:
<http://www.boulder.swri.edu/~hal/PDF/planet_def.pdf>
Calculating the full Stern-Levison parameter is complex, but it is
proportional to the mass of the object squared divided by its orbital
period so _comparing_ two objects is fairly straightforward. You can
find a table at <http://en.wikipedia.org/wiki/Cleared_the_neighbourhood>
that lists the Stern-Levison parameters for various objects in the solar
system normalized against Earth's Stern-Levison parameter. Notably, the
planet that's least able to clear objects from its orbit is Mars, with a
Stern-Levison parameter 0.0061 times that of Earth. The dwarf planet
that's most capable of clearing its orbit is Eris, with a Stern-Levison
parameter 3.5*10^−8 times that of Earth.
A hypothetical Earth-mass object whose orbit lies in the middle of the
Kuiper belt (the belt ranges from 30 to 55 AU, so the middle is
approximately 42.5 AU) has an orbital period of 277 years. So to compare
its Stern-Levison parameter to that of Earth's, you'd do the following
calculation:
(((1 earth mass)^2) / (277 years)) / (((1 earth mass)^2) / (1 year))
If you really don't trust me or can't otherwise work it out yourself,
Google Calculator can handle it via this link here:
<http://www.google.com/search?q=(((1+earth+mass)^2)+%2F+(277+years))+%2F+(((1+earth+mass)^2)+%2F+(1+year))>
The result is 0.0036. So the hypothetical planet is 0.0036 times as
capable of clearing its orbit of debris as Earth is capable of clearing
Earth's orbit. This is slightly more than half the orbit-clearing
capability of Mars, but more than 100000 times the orbit-clearing
capability of Eris. The object should be able to keep its orbit about as
clear as Mars keeps its orbit.
This is based on Stern's own work. I don't know why he seems to think
such an object wouldn't clear its orbit now, but until he (and you) can
present some actual support for the statement that contradicts the above
I can only conclude that it _just ain't so._ The numbers don't lie; that
hypothetical object is as good an orbit-clearer as an object that is
quite solidly classified as a planet, and so would itself be grouped in
with the planets on that basis.
So, either point out where I made a math error above, point out a
reference where someone's shown the Stern-Levison method to be in error,
or give it up. This statement you keep repeating is demonstrably incorrect.
> As astronomers learn more and more about other solar systems, more and more
> unexpected discoveries arise. It is premature to cleaim that it is
> "unlikely to produce many ambiguous cases".
It doesn't make much sense to claim that something is "unlikely" _after_
it's been thoroughly tested, does it? We'd already know then.
In this case, the theoretical basis for the prediction is quite well
understood. An object's gravity grows when it gains mass, which in turn
helps it to gain further mass. Do you have some reason to expect otherwise?
>> My earlier calculations showing that a 0.7-Earth mass _would_ clear an
>> orbit in the Kuiper belt aside, have you heard of the Kuiper cliff? It was
>> also mentioned earlier in this thread, you can read about it here:
>> <http://en.wikipedia.org/wiki/Kuiper_belt#.22Kuiper_cliff.22>. It may be a
>> case of orbit-clearing by a yet-undiscovered planet.
>
> "May be the case" is the operative term. It may be that a 0.7 Earth mass
> world has not cleared out its orbit, far out in the Kuiper Belt, as Stern
> has pointed out. If that turns out to be the case then it will be yet
> another example of why the IAU's definition is so flawed.
Howso? An object that's orbiting _that_ far out seems pretty distinct
from the other objects closer in that are called planets. And we're not
going to be detecting things like that any time soon, either. By my
calculations in order to reduce a 0.7-Earth mass's orbit-clearing
capabilities down to the level of Eris it'll have to have an orbital
period of about 10,000,000 years. That's a mean orbital radius of 46,400
AU, or 0.73 light-years. Well out into the outer Oort cloud.
Brett Paul Dunbar
<KHo...@SX729.com> writes
>
>"Brett Paul Dunbar" <br...@nospam.demon.co.uk> wrote in message
>news:8ZqAhGNb$9dI...@dimetrodon.demon.co.uk...
>> In message <NeudnfOCrshjWOrV...@comcast.com>, K_h
>> <KHo...@SX729.com> writes
>>>
>>>"Bryan Derksen" <bryan....@shaw.ca> wrote in message
>>>news:GCvdk.69025$Jx.48462@pd7urf1no...
>>>> K_h wrote:
>>>>> "Brett Paul Dunbar" <br...@nospam.demon.co.uk> wrote in message
>>>>> news:FV$OyfDNg...@dimetrodon.demon.co.uk...
>>>>>> In message <PYKdnToYLvdOlenV...@comcast.com>, K_h
>>>>>> <KHo...@SX729.com> writes
>> The definition states that it must be in hydrostatic equilibrium and
>> then clarifies for laymen by inserting the parenthetical phrase "nearly
>> round". In context they are clearly including oblateness caused by spin
>> in nearly. This is at most a facetious grammatical quibble when the
>> meaning is clear.
>
>
>No, the context does not include oblateness; nearly round means nearly
>spherical. Oblate planets like Saturn are not nearly round but they are
>nearly spheroidal.
They are quite clearly in hydrostatic equilibrium, and it is obvious
from the context that nearly round is simply a more colloquial way of
describing hydrostatic equilibrium.
>>>It is hard to know how common these "few hypothetical and highly
>>>unlikely" cases are. Even if only 1 percent of solar systems violate
>>>the IAU's definition then that is bad. The definitions I suggested
>>>avoid this problem totally.
>>
>> We have seen several hundred planetary system and so far are rather
>> short on ambiguous systems.
>
>No excuse for not fixing the definitions now. Current lack of data does not
>justify bad definitions.
>
A current lack of data can justify a wait and see attitude, we'll worry
about it when it becomes a live issue, at the moment it's a moot point.
>> All known exoplanets of main sequence stars fall well above the
>> gap in Figure 1 and would be classified as planets by the
>> criterion of dynamical dominance.
>
>A moot point since gas giants will be discovered first. Dynamic dominance
>is a vague and ambiguous term and should not be used to define a planet.
>Imagine a solar system in formation and a growing planet that has to
>periodically "clear out" its neighborhood due to constantly influxing
>material. Under the IAU's bad definition, it is alternately a planet at
>sometimes and not at others. So the IAU's definition is really quite bad.
The IAU definition currently applies only to one mature system, it can
easily be extended to other mature systems. The formation phase is
rather short but the definition cannot easily be applied to them. You
could explicitly include the phrase "in a mature system" in the
definition and classify protoplanetary systems separately. Or you could
include "has cleared, or is in the process of clearing, its
neighbourhood".
As things stand the Earth acquire planetary status early in the solar
system's formation, lost planetary status when Theia became so massive
that it became perturbed from either L4 or L5 and regained planetary
status when earth accreted Theia and the Moon formed.
>>
>> To which I can only say Dwarf Planets in very distant orbits can get
>> quite big. That does not make them planets. The posited Kuiper wall
>> causing object would be a planet as it has been posited due to that
>> neighbourhood having been cleared.
>
>No, see the web sites I referenced. Big dwarf planets in distant orbits is
>really a bad idea since "big dwarf" is a contradiction in terms.
If it caused the Kuiper wall it is a planet as it has cleared its
neighbourhood creating the Kuiper cliff. You are talking about a
somewhat smaller body which had not cleared its neighbourhood, that
would not be a planet.
>> Asteroids are already Minor Planets, a term which denotes an entirely
>> disjoint set from Planet. As the new term Dwarf Planet does.
>
>This only underscores the need to get better definitions for things like
>"Minor Planets" and "Asteroids". A minor planet is not a planet? Silly. A
>great way to define asteroids is as objects small enough so that hydrostatic
>equilibrium does not apply.
You may think it silly but it has been the case for more than a century
and a half that Minor Planet is a term for objects too small to be
Planets and which aren't comets. It is a synonym for Planetoid and
Asteroid. This has not caused any confusion. They are listed and named
in the Minor Planets Catalogue.
<http://en.wikipedia.org/wiki/Minor_planet_number>
>> We can wait having more information about what there is to classify
>> before classifying them, in other words we should wait and see what is
>> out there before deciding how to classify them.
>
>Bad idea. It means constantly updating the definitions as new information
>comes along. The example definitions I provided do not require constant
>reclassification as new data arrives.
What is wrong with developing the definitions based on observations?
If you over specify prior to observation you may find that your
definition is a poor match for what exists. This sort of thing has been
a persistent problem in biology, many folk classifications bear little
relation to the underlying nature of the species concerned, as they were
compiled in ignorance of the underlying evolutionary relationships.
>"Seems?" Definitions should not be justified by how things "seem". The
>example I gave, with a star, a brown dwarf, and two jupiter type planets at
>L4 and L5 clearly show that the "clearing its orbit of debris" is a very bad
>way to define an word like "planet". Generally, objects are defined by the
>properties intrinsic to themselves, not what is or is not in their
>neighborhoods.
Plenty of classes of bodies are defined by their context, a moon is a
tertiary body, a Trojan is a body orbiting in the L4 or L5 point of a
larger body (or, more narrowly, Jupiter), a Plutino is a body in a 2:3
orbital resonance with Neptune.
K_h
> K_h wrote:
>
> This is based on Stern's own work. I don't know why he seems to think such
> an object wouldn't clear its orbit now, but until he (and you) can present
> some actual support for the statement that contradicts the above I can
> only conclude that it _just ain't so._ The numbers don't lie; that
> hypothetical object is as good an orbit-clearer as an object that is quite
> solidly classified as a planet, and so would itself be grouped in with the
> planets on that basis.
This disagreement is something you and Stern need to iron out. Count me out
of the loop. The dynamics of "clearing" is still somehwat in its infancy.
If Stern is correct then your argument on this point is invalid. If your
claims are correct, then there still is a big problem with the IAU's
definition. To say that a planet must have "cleared its orbit" presumes
that there was material there to be cleared. What if there is a world that
is in a clear orbit but the orbit was not cleared out by that world? Is
such a world a planet (since it materially dominates its orbital space) or
is it not a planet (since it is incapable of clearing out its environment if
there had been material there)? It is unclear how much clearing is needed
in order to qualify for planethood.
>> As astronomers learn more and more about other solar systems, more and
>> more unexpected discoveries arise. It is premature to cleaim that it is
>> "unlikely to produce many ambiguous cases".
>
> It doesn't make much sense to claim that something is "unlikely" _after_
> it's been thoroughly tested, does it? We'd already know then.
>
> In this case, the theoretical basis for the prediction is quite well
> understood. An object's gravity grows when it gains mass, which in turn
> helps it to gain further mass. Do you have some reason to expect
> otherwise?
Since orbital dynamics is still very much in its infancy, it is premature to
say that the dynamic theory has been "thoroughly tested". It is too soon to
say what startling discoveries will be made as new solar systems are
explored.
>>> My earlier calculations showing that a 0.7-Earth mass _would_ clear an
>>> orbit in the Kuiper belt aside, have you heard of the Kuiper cliff? It
>>> was also mentioned earlier in this thread, you can read about it here:
>>> <http://en.wikipedia.org/wiki/Kuiper_belt#.22Kuiper_cliff.22>. It may be
>>> a case of orbit-clearing by a yet-undiscovered planet.
>>
>> "May be the case" is the operative term. It may be that a 0.7 Earth mass
>> world has not cleared out its orbit, far out in the Kuiper Belt, as Stern
>> has pointed out. If that turns out to be the case then it will be yet
>> another example of why the IAU's definition is so flawed.
>
> Howso? An object that's orbiting _that_ far out seems pretty distinct from
> the other objects closer in that are called planets. And we're not going
> to be detecting things like that any time soon, either. By my calculations
> in order to reduce a 0.7-Earth mass's orbit-clearing capabilities down to
> the level of Eris it'll have to have an orbital period of about 10,000,000
> years. That's a mean orbital radius of 46,400 AU, or 0.73 light-years.
> Well out into the outer Oort cloud.
Again, this is between you and Stern.
K
K_h
> K_h wrote:
>>
>> No, the context does not include oblateness; nearly round means nearly
>> spherical. Oblate planets like Saturn are not nearly round but they are
>> nearly spheroidal.
>
> Something that's sphereoidal isn't round? This is silly.
The definition of `round' is spherical. To say that something is nearly
round simply means that it is nearly spherical. So a slighly oblate planet
can be said to be "nearly round" but Saturn is not slightly oblate. It is
over 12% oblate and planets in other solar systems are certainly more
oblate. So, the IAU's definition has the problem of saying what the
percentage the cutoff point is for "nearly round" (less than 2% oblate?,
less than 4% oblate? ...etc).
>>> All known exoplanets of main sequence stars fall well above the
>>> gap in Figure 1 and would be classified as planets by the
>>> criterion of dynamical dominance.
>>
>> A moot point since gas giants will be discovered first. Dynamic
>> dominance is a vague and ambiguous term and should not be used to define
>> a planet. Imagine a solar system in formation and a growing planet that
>> has to periodically "clear out" its neighborhood due to constantly
>> influxing material. Under the IAU's bad definition, it is alternately a
>> planet at sometimes and not at others. So the IAU's definition is really
>> quite bad.
>
> Firstly, once again I remind you that the IAU's definition doesn't
> currently apply to any solar system other than ours. So there's no big
> rush to make sure it works with edge cases like these, especially edge
> cases that are going to be rare and hard to detect.
The definitions should be comprehensive. Claiming that the definitions
should not be fixed until arguably "rare" cases are found is bad. There is
no reason not to have definitions that cover everything now.
> Secondly, planet formation is a turbulent and dynamic time. It's planet
> _formation_, after all, the transition from non-planet to planet. So
> what's wrong with objects gaining and losing their planethood during the
> process? That's what it's all about.
Well, just think about it. Suppose we defined trees as only objects with
green leaves. In the fall, when the leaves turn yellow and then orange,
they are not trees. But next summer they are trees again. Such silly
definitions do not exist, or are at least very rare, in other scientific
disciplines (for example botany).
>> No, see the web sites I referenced. Big dwarf planets in distant orbits
>> is really a bad idea since "big dwarf" is a contradiction in terms.
>
> Are you ignoring me? I gave you numbers using the method that Alan Stern
> himself co-developed showing that the hypothetical example you gave is as
> much a planet as Mars is. At this point I call straw man, you're arguing
> against a problem that doesn't actually exist.
Not at all. "Big dwarf" is a contradiction in terms. Just think about it.
>>> We can wait having more information about what there is to classify
>>> before classifying them, in other words we should wait and see what is
>>> out there before deciding how to classify them.
>>
>> Bad idea. It means constantly updating the definitions as new
>> information comes along. The example definitions I provided do not
>> require constant reclassification as new data arrives.
>
> You realize that measuring exoplanetary diameters is generally harder to
> do with precision compared to mass and orbital period? As far as I'm aware
> unless you're lucky enough to have the exoplanet's orbit line up in such a
> way that it transits across the face of its primary there's no way to even
> try.
So what? My category of three kinds of planets was optional to the
definitions I provided. So they could be dropped or, if kept, then the
discovery of certain new planets would remain agnostic until further data
clarified their diameters. Yes, exoplanetary diameters are harder to do
with precision but checking to see if their orbits are "clear" is harder
still.
>> This is a bad way to define a planet. Try defining a building in such a
>> way that it is only a building unless it dominates its surroundings.
>
> Why? I don't see any connection between the two.
Please consider it.
>> "Seems?" Definitions should not be justified by how things "seem". The
>> example I gave, with a star, a brown dwarf, and two jupiter type planets
>> at L4 and L5 clearly show that the "clearing its orbit of debris" is a
>> very bad way to define an word like "planet".
>
> A hypothetical case with no indication that it's particularly likely to
> occur. And if it does, so what if the two trojan objects aren't defined as
> planets? They're behaving quite differently from all the other known
> planets. They might even warrant a class of their own.
Well, they would qualify as planets under the usual, informal, understanding
of "planet". A good definition of "planet" should be as comprehensive and
as simple as possible.
>> Generally, objects are defined by the properties intrinsic to themselves,
>> not what is or is not in their neighborhoods.
>
> How do you define something as a "moon" if not in relation to what's in
> its neighborhood? It's perfectly cromulent to define something based on
> the context it's embedded in, among other characteristics. Lots of things
> are defined that way.
I have already answered this in the previous postings.
K
K_h
> In message <sdudnY3Eef4CC-TV...@comcast.com>, K_h
> <KHo...@SX729.com> writes
>>
>>"Brett Paul Dunbar" <br...@nospam.demon.co.uk> wrote in message
>>news:8ZqAhGNb$9dI...@dimetrodon.demon.co.uk...
>>> In message <NeudnfOCrshjWOrV...@comcast.com>, K_h
>>> <KHo...@SX729.com> writes
>>>>
>>>>"Bryan Derksen" <bryan....@shaw.ca> wrote in message
>>>>news:GCvdk.69025$Jx.48462@pd7urf1no...
>>>>> K_h wrote:
>>>>>> "Brett Paul Dunbar" <br...@nospam.demon.co.uk> wrote in message
>>>>>> news:FV$OyfDNg...@dimetrodon.demon.co.uk...
>>>>>>> In message <PYKdnToYLvdOlenV...@comcast.com>, K_h
>>>>>>> <KHo...@SX729.com> writes
>
>>> The definition states that it must be in hydrostatic equilibrium and
>>> then clarifies for laymen by inserting the parenthetical phrase "nearly
>>> round". In context they are clearly including oblateness caused by spin
>>> in nearly. This is at most a facetious grammatical quibble when the
>>> meaning is clear.
>>
>>
>>No, the context does not include oblateness; nearly round means nearly
>>spherical. Oblate planets like Saturn are not nearly round but they are
>>nearly spheroidal.
>
> They are quite clearly in hydrostatic equilibrium, and it is obvious from
> the context that nearly round is simply a more colloquial way of
> describing hydrostatic equilibrium.
No. I have already answered this in previous posts; at least a couple times
now. "Nearly round" in not a colloquial way of describing hydrostatic
equilibirum. A world in hydrostatic equilibrium can be nearly spherical or
nearly spheroidal, the former for a non-rotating world and the latter for a
rotating world. So worlds can have things like mountains on them and thus
can be "nearly spherical" or "nearly spheroidal".
http://en.wikipedia.org/wiki/Spheroid
http://en.wikipedia.org/wiki/Sphere
>>>>It is hard to know how common these "few hypothetical and highly
>>>>unlikely" cases are. Even if only 1 percent of solar systems violate
>>>>the IAU's definition then that is bad. The definitions I suggested
>>>>avoid this problem totally.
>>>
>>> We have seen several hundred planetary system and so far are rather
>>> short on ambiguous systems.
>>
>>No excuse for not fixing the definitions now. Current lack of data does
>>not
>>justify bad definitions.
>>
>
> A current lack of data can justify a wait and see attitude, we'll worry
> about it when it becomes a live issue, at the moment it's a moot point.
Never put off to tomorrow what can be done today. As Sky and Telescope
magazine has rightly noted, it is only a matter of time before the IAU
changes its definition. There is such dissatisfaction with the IAU's
definition right now that many astronomers are simply not using it. The
aforementioned magazine refuses to recognize the IAU's definition as well as
lots of other astronomers.
>>> All known exoplanets of main sequence stars fall well above the
>>> gap in Figure 1 and would be classified as planets by the
>>> criterion of dynamical dominance.
>>
>>A moot point since gas giants will be discovered first. Dynamic dominance
>>is a vague and ambiguous term and should not be used to define a planet.
>>Imagine a solar system in formation and a growing planet that has to
>>periodically "clear out" its neighborhood due to constantly influxing
>>material. Under the IAU's bad definition, it is alternately a planet at
>>sometimes and not at others. So the IAU's definition is really quite bad.
>
> The IAU definition currently applies only to one mature system, it can
> easily be extended to other mature systems. The formation phase is rather
> short but the definition cannot easily be applied to them. You could
> explicitly include the phrase "in a mature system" in the definition and
> classify protoplanetary systems separately. Or you could include "has
> cleared, or is in the process of clearing, its neighbourhood".
Yes, you rightly point out other flaws in the IAU's definition; it does not
distinguish between mature and immature planetary systems. The example
definitions I suggested avoid this problem totally since they can be applied
at any time to any world.
> As things stand the Earth acquire planetary status early in the solar
> system's formation, lost planetary status when Theia became so massive
> that it became perturbed from either L4 or L5 and regained planetary
> status when earth accreted Theia and the Moon formed.
Under the IAU's definition, yes. There have been several documentaries on
the formation of the moon on the discovery channel. They all refer to Earth
and Theia as planets, even though they don't qualify as such under the
current IAU definition. Yet another problem for the IAU's definition.
>>>
>>> To which I can only say Dwarf Planets in very distant orbits can get
>>> quite big. That does not make them planets. The posited Kuiper wall
>>> causing object would be a planet as it has been posited due to that
>>> neighbourhood having been cleared.
>>
>>No, see the web sites I referenced. Big dwarf planets in distant orbits
>>is
>>really a bad idea since "big dwarf" is a contradiction in terms.
>
> If it caused the Kuiper wall it is a planet as it has cleared its
> neighbourhood creating the Kuiper cliff. You are talking about a somewhat
> smaller body which had not cleared its neighbourhood, that would not be a
> planet.
It should be called a planet. Pluto, for example, has polar caps, an
atmosphere, seasons, three moons, and interesting geological activity at its
surface.
>>> Asteroids are already Minor Planets, a term which denotes an entirely
>>> disjoint set from Planet. As the new term Dwarf Planet does.
>>
>>This only underscores the need to get better definitions for things like
>>"Minor Planets" and "Asteroids". A minor planet is not a planet? Silly.
>>A
>>great way to define asteroids is as objects small enough so that
>>hydrostatic
>>equilibrium does not apply.
>
> You may think it silly but it has been the case for more than a century
> and a half that Minor Planet is a term for objects too small to be Planets
> and which aren't comets. It is a synonym for Planetoid and Asteroid. This
> has not caused any confusion. They are listed and named in the Minor
> Planets Catalogue.
"A minor planet is not a planet" is a contradiction in terms. Yes, for the
last century there has been some very bad terminology.
>>> We can wait having more information about what there is to classify
>>> before classifying them, in other words we should wait and see what is
>>> out there before deciding how to classify them.
>>
>>Bad idea. It means constantly updating the definitions as new information
>>comes along. The example definitions I provided do not require constant
>>reclassification as new data arrives.
>
> What is wrong with developing the definitions based on observations?
>
> If you over specify prior to observation you may find that your definition
> is a poor match for what exists. This sort of thing has been a persistent
> problem in biology, many folk classifications bear little relation to the
> underlying nature of the species concerned, as they were compiled in
> ignorance of the underlying evolutionary relationships.
This is why the definitions should be such that they are observation
independent and will not change with new discoveries. The more science
learns about stars does not cause a change in the definition of what a star
is and this should be the case with any resonable definition of a planet.
That is the case with the examples I provided.
>>"Seems?" Definitions should not be justified by how things "seem". The
>>example I gave, with a star, a brown dwarf, and two jupiter type planets
>>at
>>L4 and L5 clearly show that the "clearing its orbit of debris" is a very
>>bad
>>way to define an word like "planet". Generally, objects are defined by
>>the
>>properties intrinsic to themselves, not what is or is not in their
>>neighborhoods.
K
Brett Paul Dunbar
<KHo...@SX729.com> writes
>
>"Brett Paul Dunbar" <br...@nospam.demon.co.uk> wrote in message
>news:zCTz3UGz...@dimetrodon.demon.co.uk...
>> In message <sdudnY3Eef4CC-TV...@comcast.com>, K_h
>> <KHo...@SX729.com> writes
>>>
>>>"Brett Paul Dunbar" <br...@nospam.demon.co.uk> wrote in message
>>>news:8ZqAhGNb$9dI...@dimetrodon.demon.co.uk...
>>>> In message <NeudnfOCrshjWOrV...@comcast.com>, K_h
>>>> <KHo...@SX729.com> writes
>>>>>
>>>>>"Bryan Derksen" <bryan....@shaw.ca> wrote in message
>>>>>news:GCvdk.69025$Jx.48462@pd7urf1no...
>>>>>> K_h wrote:
>>>>>>> "Brett Paul Dunbar" <br...@nospam.demon.co.uk> wrote in message
>>>>>>> news:FV$OyfDNg...@dimetrodon.demon.co.uk...
>>>>>>>> In message <PYKdnToYLvdOlenV...@comcast.com>, K_h
>>>>>>>> <KHo...@SX729.com> writes
>now. "Nearly round" in not a colloquial way of describing hydrostatic
>equilibirum. A world in hydrostatic equilibrium can be nearly spherical or
>nearly spheroidal, the former for a non-rotating world and the latter for a
>rotating world. So worlds can have things like mountains on them and thus
>can be "nearly spherical" or "nearly spheroidal".
And your point is still utterly absurd and by not backing down on this
you are demonstrating that you are an idiot. The parenthetical comment
can be omitted without altering the meaning of the phrase. Absolutely
nobody is confused about whether Saturn is in Hydrostatic Equilibrium.
The Dwarf Planet definition has some problems in relationship to smaller
spheroid bodies this does not affect the definition of Planet as any
body big enough to be orbitally dominant is going to be in hydrostatic
equilibrium (largely round due to its own gravity).
>> A current lack of data can justify a wait and see attitude, we'll worry
>> about it when it becomes a live issue, at the moment it's a moot point.
>
>Never put off to tomorrow what can be done today. As Sky and Telescope
>magazine has rightly noted, it is only a matter of time before the IAU
>changes its definition. There is such dissatisfaction with the IAU's
>definition right now that many astronomers are simply not using it. The
>aforementioned magazine refuses to recognize the IAU's definition as well as
>lots of other astronomers.
Don't get ahead of yourself, wait and see what is out there. We have a
working definition that allows all known systems to be categorised and
can wait and see how usage is developed.
>
>
>>>> All known exoplanets of main sequence stars fall well above the
>>>> gap in Figure 1 and would be classified as planets by the
>>>> criterion of dynamical dominance.
>>>
>>>A moot point since gas giants will be discovered first. Dynamic dominance
>>>is a vague and ambiguous term and should not be used to define a planet.
>>>Imagine a solar system in formation and a growing planet that has to
>>>periodically "clear out" its neighborhood due to constantly influxing
>>>material. Under the IAU's bad definition, it is alternately a planet at
>>>sometimes and not at others. So the IAU's definition is really quite bad.
>>
>> The IAU definition currently applies only to one mature system, it can
>> easily be extended to other mature systems. The formation phase is rather
>> short but the definition cannot easily be applied to them. You could
>> explicitly include the phrase "in a mature system" in the definition and
>> classify protoplanetary systems separately. Or you could include "has
>> cleared, or is in the process of clearing, its neighbourhood".
>
>Yes, you rightly point out other flaws in the IAU's definition; it does not
>distinguish between mature and immature planetary systems. The example
>definitions I suggested avoid this problem totally since they can be applied
>at any time to any world.
At the moment it only apples to a single system, which happens to be
mature. It apples to no immature systems and we have, so far, not
detected any immature systems so the working definition for exponents
has not had to deal with any as yet.
>
>
>> As things stand the Earth acquire planetary status early in the solar
>> system's formation, lost planetary status when Theia became so massive
>> that it became perturbed from either L4 or L5 and regained planetary
>> status when earth accreted Theia and the Moon formed.
>
>Under the IAU's definition, yes. There have been several documentaries on
>the formation of the moon on the discovery channel. They all refer to Earth
>and Theia as planets, even though they don't qualify as such under the
>current IAU definition. Yet another problem for the IAU's definition.
It isn't really applicable in an immature system, actually by the
Stern-Levinson parameter Earth remained a Planet by Soter's discriminant
is briefly wasn't. The formative stage is brief and dramatic It isn't
wholly surprising that definitions developed for a mature system aren't
really applicable. We can resolve the ambiguities at a later date, when
we know a bit more about planetary formation.
>> If it caused the Kuiper wall it is a planet as it has cleared its
>> neighbourhood creating the Kuiper cliff. You are talking about a somewhat
>> smaller body which had not cleared its neighbourhood, that would not be a
>> planet.
>
>It should be called a planet. Pluto, for example, has polar caps, an
>atmosphere, seasons, three moons, and interesting geological activity at its
>surface.
Being interesting does not make it a planet. Comets also have
atmospheres sometimes very big atmospheres. Pluto looks to be a
gargantuan comet.
>
>
>>>> Asteroids are already Minor Planets, a term which denotes an entirely
>>>> disjoint set from Planet. As the new term Dwarf Planet does.
>>>
>>>This only underscores the need to get better definitions for things
>>>like "Minor Planets" and "Asteroids". A minor planet is not a
>>>planet? Silly. A great way to define asteroids is as objects small
>>>enough so that hydrostatic equilibrium does not apply.
>>
>> You may think it silly but it has been the case for more than a century
>> and a half that Minor Planet is a term for objects too small to be Planets
>> and which aren't comets. It is a synonym for Planetoid and Asteroid. This
>> has not caused any confusion. They are listed and named in the Minor
>> Planets Catalogue.
>
>"A minor planet is not a planet" is a contradiction in terms. Yes, for the
>last century there has been some very bad terminology.
No it isn't, it is perfectly normal English. Minor Planets aren't
Planets, Beach Volleyball [1] isn't Volleyball [2], Koala Bears aren't
Bears. A modifier and a noun denoting an entirely separate class from
the unmodified noun is hardly unusual or remotely ungrammatical.
>> What is wrong with developing the definitions based on observations?
>>
>> If you over specify prior to observation you may find that your definition
>> is a poor match for what exists. This sort of thing has been a persistent
>> problem in biology, many folk classifications bear little relation to the
>> underlying nature of the species concerned, as they were compiled in
>> ignorance of the underlying evolutionary relationships.
>
>This is why the definitions should be such that they are observation
>independent and will not change with new discoveries. The more science
>learns about stars does not cause a change in the definition of what a star
>is and this should be the case with any resonable definition of a planet.
>That is the case with the examples I provided.
>
We know what a Star is, and have now based the definition on the
fundamental physical characteristics. It is however nothing like the
original definition. The oldest definition was "lights in the sky other
than the Sun", then "fixed lights in the Sky" then "big hot discrete
things" then we eventually worked out what made them glow and switched
to roughly "discrete bodies big enough to fuse hydrogen or which have in
the past fused hydrogen" it has since been revised to exclude black
holes so is now "discrete bodies big enough to fuse hydrogen or which
have in the past fused hydrogen which are not within an event horizon".
That is hardly supportive of your case, while the membership of the
class has been reasonably stable (of known bodies only the Moon and some
Planets have been excluded and the Sun added) the definition has changed
massively.
The working definition of exponent as it stands may change, the
planetary status of the objects currently assigned is in no real doubt.
[1] Played on sand, usually outdoors with teams of two. Recently
added to the Olympics.
[2] Played on an indoor court with teams of six. Long established
Olympic sport.
Bryan Derksen
> of the loop. The dynamics of "clearing" is still somehwat in its infancy.
If Stern wants to jump in on this thread that would be entirely welcome.
But until then it's you who's here arguing the issue.
> If Stern is correct then your argument on this point is invalid.
Funny thing is both sides of this particular point are based on Stern's
arguments. However, the side that says "it can clear its orbit" is from
a scientific journal article backed with detailed physics and rigorous
analysis, and the side that says "it can't clear its orbit" is just
something he said to some MSNBC reporter. I think he needs to support
the second point a bit better. :)
> If your
> claims are correct, then there still is a big problem with the IAU's
> definition. To say that a planet must have "cleared its orbit" presumes
> that there was material there to be cleared. What if there is a world that
> is in a clear orbit but the orbit was not cleared out by that world? Is
> such a world a planet (since it materially dominates its orbital space) or
> is it not a planet (since it is incapable of clearing out its environment if
> there had been material there)? It is unclear how much clearing is needed
> in order to qualify for planethood.
This is yet another of those unlikely edge cases but it's actually not
the problem you make it out to be. The Stern-Levison parameter doesn't
take into account what debris are actually present, it just gives a
measure of how capable an object is _in principle_ of clearing debris
from its orbit over time. If you were to wave a magic wand and make all
of the material in the Solar System vanish except for a one-tonne
boulder in the vicinity of Neptune, that boulder's Stern-Levison
parameter changes not one whit - it doesn't dominate its orbit even
though there's nothing currently 'challenging' it.
The Soter discriminant _does_ take into account the debris that are
actually present, but for that very reason it's unlikely to be of any
use for determining planethood of extrasolar bodies since we can't
detect the other debris in the forseeable future. IMO it's more useful
as an experimental verification of the theory-based Stern-Levison
parameter here in our own solar system.
>>>> My earlier calculations showing that a 0.7-Earth mass _would_ clear an
>>>> orbit in the Kuiper belt aside, have you heard of the Kuiper cliff? It
>>>> was also mentioned earlier in this thread, you can read about it here:
>>>> <http://en.wikipedia.org/wiki/Kuiper_belt#.22Kuiper_cliff.22>. It may be
>>>> a case of orbit-clearing by a yet-undiscovered planet.
>>> "May be the case" is the operative term. It may be that a 0.7 Earth mass
>>> world has not cleared out its orbit, far out in the Kuiper Belt, as Stern
>>> has pointed out. If that turns out to be the case then it will be yet
>>> another example of why the IAU's definition is so flawed.
>> Howso? An object that's orbiting _that_ far out seems pretty distinct from
>> the other objects closer in that are called planets. And we're not going
>> to be detecting things like that any time soon, either. By my calculations
>> in order to reduce a 0.7-Earth mass's orbit-clearing capabilities down to
>> the level of Eris it'll have to have an orbital period of about 10,000,000
>> years. That's a mean orbital radius of 46,400 AU, or 0.73 light-years.
>> Well out into the outer Oort cloud.
>
> Again, this is between you and Stern.
It's been between you and me (and the other thread participants) up
until this point, I don't see where the sudden shift occurred.
Bryan Derksen
> "Bryan Derksen" <bryan....@shaw.ca> wrote in message
> news:iohek.66788$kx.15343@pd7urf3no...
>> K_h wrote:
>>> No, the context does not include oblateness; nearly round means nearly
>>> spherical. Oblate planets like Saturn are not nearly round but they are
>>> nearly spheroidal.
>> Something that's sphereoidal isn't round? This is silly.
>
> The definition of `round' is spherical.
So a wheel isn't round either? Your definition of 'round' is very
idiosyncratic.
>> Secondly, planet formation is a turbulent and dynamic time. It's planet
>> _formation_, after all, the transition from non-planet to planet. So
>> what's wrong with objects gaining and losing their planethood during the
>> process? That's what it's all about.
>
> Well, just think about it. Suppose we defined trees as only objects with
> green leaves. In the fall, when the leaves turn yellow and then orange,
> they are not trees. But next summer they are trees again. Such silly
> definitions do not exist, or are at least very rare, in other scientific
> disciplines (for example botany).
You're right, that is a dumb way to define trees. Just like your example
below of defining buildings as structures that "dominate" surrounding
structures was a dumb way to define buildings. What does it have to do
with planets, though? The orbits of planets don't change much once
planet formation is over, by definition there's nothing left in their
orbits that would disrupt them.
>>> No, see the web sites I referenced. Big dwarf planets in distant orbits
>>> is really a bad idea since "big dwarf" is a contradiction in terms.
>> Are you ignoring me? I gave you numbers using the method that Alan Stern
>> himself co-developed showing that the hypothetical example you gave is as
>> much a planet as Mars is. At this point I call straw man, you're arguing
>> against a problem that doesn't actually exist.
>
> Not at all. "Big dwarf" is a contradiction in terms. Just think about it.
So if we've got a red dwarf star that's right up at the upper edge of
its stellar class, just shy of being K-class, it can't be called a "big
red dwarf"? Is a brown dwarf that's just shy of being able to fuse
hydrogen not a "big brown dwarf"? What's the problem with that?
>> You realize that measuring exoplanetary diameters is generally harder to
>> do with precision compared to mass and orbital period? As far as I'm aware
>> unless you're lucky enough to have the exoplanet's orbit line up in such a
>> way that it transits across the face of its primary there's no way to even
>> try.
>
> So what? My category of three kinds of planets was optional to the
> definitions I provided. So they could be dropped or, if kept, then the
> discovery of certain new planets would remain agnostic until further data
> clarified their diameters. Yes, exoplanetary diameters are harder to do
> with precision but checking to see if their orbits are "clear" is harder
> still.
Not if you do it the Stern-Levison way. You just need the object's mass
and orbital period for that, which is why I mentioned those parameters
specifically.
Doing it the Soter way, yeah, that's completely impractical. So I don't
expect anyone would seriously propose doing it that way.
>>> This is a bad way to define a planet. Try defining a building in such a
>>> way that it is only a building unless it dominates its surroundings.
>> Why? I don't see any connection between the two.
>
> Please consider it.
I did, and I didn't see any connection between the two. That's why I
asked you to explain further.
>>> "Seems?" Definitions should not be justified by how things "seem". The
>>> example I gave, with a star, a brown dwarf, and two jupiter type planets
>>> at L4 and L5 clearly show that the "clearing its orbit of debris" is a
>>> very bad way to define an word like "planet".
>> A hypothetical case with no indication that it's particularly likely to
>> occur. And if it does, so what if the two trojan objects aren't defined as
>> planets? They're behaving quite differently from all the other known
>> planets. They might even warrant a class of their own.
>
> Well, they would qualify as planets under the usual, informal, understanding
> of "planet".
In your opinion. I don't see why they'd have to be in the same category,
and this is such an unusual case compared to known planets that I don't
think relying on an "informal understanding" is going to be useful.
>>> Generally, objects are defined by the properties intrinsic to themselves,
>>> not what is or is not in their neighborhoods.
>> How do you define something as a "moon" if not in relation to what's in
>> its neighborhood? It's perfectly cromulent to define something based on
>> the context it's embedded in, among other characteristics. Lots of things
>> are defined that way.
>
> I have already answered this in the previous postings.
Your definition of moon, from that previous posting, is:
"A moon is a satellite of a planet."
With satellite further defined as:
"A celestial body that has its barycenter continuously inside the body
of another celestial body is a satellite of that celestial body."
Note the 'another celestial body' part of that definition. There needs
to be another nearby object in a dynamic relationship with the moon in
order for your definition to work. That means your definition of moon is
not based solely on properties that are intrinsic to the moon, but
rather depends on what is or is not in its neighborhood. If the planet
it's orbiting were to vanish the object would cease to be a moon.
So what's the big problem with the definition of "planet" having a
similar feature?
K_h
>
> Funny thing is both sides of this particular point are based on Stern's
> arguments. However, the side that says "it can clear its orbit" is from a
> scientific journal article backed with detailed physics and rigorous
> analysis, and the side that says "it can't clear its orbit" is just
> something he said to some MSNBC reporter. I think he needs to support the
> second point a bit better. :)
I cannot speak for Stern, so your disagreement with him should not involve
people like myself. A world with 70% Earth mass will not have cleared the
Kuiper belt and so will be orbiting within the Kuiper belt. Of course, one
can say that any world within the Kuiper belt will have "cleared" its
immediate path around the sun since they seldom collide with each other.
That raises the question as to how much "clearing" is needed before a world
can be said to have "cleared" its orbit. Pluto orbits the sun without
hitting anything so, in a sense, it's orbit is "clear". This is part of the
problem with the IAU's definition, it is not defined how wide the "clear"
zone has to be. With this ambiguity an Earth sized world, in the Kuiper
belt, has not cleared out the Kuiper belt.
K
K_h
> K_h wrote:
>> "Bryan Derksen" <bryan....@shaw.ca> wrote in message
>> news:iohek.66788$kx.15343@pd7urf3no...
>>> K_h wrote:
>>>> No, the context does not include oblateness; nearly round means nearly
>>>> spherical. Oblate planets like Saturn are not nearly round but they
>>>> are nearly spheroidal.
>>> Something that's sphereoidal isn't round? This is silly.
>>
>> The definition of `round' is spherical.
>
> So a wheel isn't round either? Your definition of 'round' is very
> idiosyncratic.
Not at all: http://en.wikipedia.org/wiki/Round. I already answered this
once before. The IAU's definition of a planet states that it is in
hydrostatic equilibrium and is nearly round (i.e. nearly spherical). This
does not cover the case of a very oblate planet that is in hydrostatic
equilibrium since very oblate planets are not nearly spherical.
>>> Secondly, planet formation is a turbulent and dynamic time. It's planet
>>> _formation_, after all, the transition from non-planet to planet. So
>>> what's wrong with objects gaining and losing their planethood during the
>>> process? That's what it's all about.
>>
>> Well, just think about it. Suppose we defined trees as only objects with
>> green leaves. In the fall, when the leaves turn yellow and then orange,
>> they are not trees. But next summer they are trees again. Such silly
>> definitions do not exist, or are at least very rare, in other scientific
>> disciplines (for example botany).
>
> You're right, that is a dumb way to define trees. Just like your example
> below of defining buildings as structures that "dominate" surrounding
> structures was a dumb way to define buildings. What does it have to do
> with planets, though? The orbits of planets don't change much once planet
> formation is over, by definition there's nothing left in their orbits that
> would disrupt them.
The word "Planet", like all nouns, describes a thing; like a person, place,
or thing. It is bad when a defintion defines a thing based on its
surroundings and that is what the IAU's definition does.
>>>> No, see the web sites I referenced. Big dwarf planets in distant
>>>> orbits is really a bad idea since "big dwarf" is a contradiction in
>>>> terms.
>>> Are you ignoring me? I gave you numbers using the method that Alan Stern
>>> himself co-developed showing that the hypothetical example you gave is
>>> as much a planet as Mars is. At this point I call straw man, you're
>>> arguing against a problem that doesn't actually exist.
>>
>> Not at all. "Big dwarf" is a contradiction in terms. Just think about
>> it.
>
> So if we've got a red dwarf star that's right up at the upper edge of its
> stellar class, just shy of being K-class, it can't be called a "big red
> dwarf"? Is a brown dwarf that's just shy of being able to fuse hydrogen
> not a "big brown dwarf"? What's the problem with that?
Much better defintions are possible.
>>> You realize that measuring exoplanetary diameters is generally harder to
>>> do with precision compared to mass and orbital period? As far as I'm
>>> aware unless you're lucky enough to have the exoplanet's orbit line up
>>> in such a way that it transits across the face of its primary there's no
>>> way to even try.
>>
>> So what? My category of three kinds of planets was optional to the
>> definitions I provided. So they could be dropped or, if kept, then the
>> discovery of certain new planets would remain agnostic until further data
>> clarified their diameters. Yes, exoplanetary diameters are harder to do
>> with precision but checking to see if their orbits are "clear" is harder
>> still.
>
> Not if you do it the Stern-Levison way. You just need the object's mass
> and orbital period for that, which is why I mentioned those parameters
> specifically.
>
> Doing it the Soter way, yeah, that's completely impractical. So I don't
> expect anyone would seriously propose doing it that way.
An obvious problem with this is a case where the world's Stern-Levison
method says its a planet but unknown newly injected material, in its orbit,
disqualifies the world from planethood (via IAU). And the IAU's defintion
did not state that "clearing its orbit" refers to the Stern-Levison method.
If you think this method is the proper way to assess planethood then you
should, at least, be in favor of amending the IAU's definition to define
that "clearing its orbit" refers to the Stern-Levison method.
>>>> This is a bad way to define a planet. Try defining a building in such
>>>> a way that it is only a building unless it dominates its surroundings.
>>> Why? I don't see any connection between the two.
>>
>> Please consider it.
>
> I did, and I didn't see any connection between the two. That's why I asked
> you to explain further.
This was already explained at least two times in two earlier posts.
Consider the case of two jupiter mass planets at L4 and L5 in a system with
a 25 mass brown dwarf orbiting a 30 solar mass star at 40AU.
>>>> "Seems?" Definitions should not be justified by how things "seem".
>>>> The example I gave, with a star, a brown dwarf, and two jupiter type
>>>> planets at L4 and L5 clearly show that the "clearing its orbit of
>>>> debris" is a very bad way to define an word like "planet".
>>> A hypothetical case with no indication that it's particularly likely to
>>> occur. And if it does, so what if the two trojan objects aren't defined
>>> as planets? They're behaving quite differently from all the other known
>>> planets. They might even warrant a class of their own.
>>
>> Well, they would qualify as planets under the usual, informal,
>> understanding of "planet".
>
> In your opinion. I don't see why they'd have to be in the same category,
> and this is such an unusual case compared to known planets that I don't
> think relying on an "informal understanding" is going to be useful.
No. Just think about it, it may not be an unusual case at all. A noun
defines an object, that is a person, place, or thing. A jupiter planet is
the same thing if it is the lone object orbiting a star or if it occupies an
L4 or L5 position in the aforementioned system. That is one reason why the
IAU defintion is so bad. The nature of a planet should not depend on
whatever else is, or is not, nearby.
>
>>>> Generally, objects are defined by the properties intrinsic to
>>>> themselves, not what is or is not in their neighborhoods.
>>> How do you define something as a "moon" if not in relation to what's in
>>> its neighborhood? It's perfectly cromulent to define something based on
>>> the context it's embedded in, among other characteristics. Lots of
>>> things are defined that way.
>>
>> I have already answered this in the previous postings.
>
> Your definition of moon, from that previous posting, is:
>
> "A moon is a satellite of a planet."
>
> With satellite further defined as:
>
> "A celestial body that has its barycenter continuously inside the body of
> another celestial body is a satellite of that celestial body."
>
> Note the 'another celestial body' part of that definition. There needs to
> be another nearby object in a dynamic relationship with the moon in order
> for your definition to work. That means your definition of moon is not
> based solely on properties that are intrinsic to the moon, but rather
> depends on what is or is not in its neighborhood. If the planet it's
> orbiting were to vanish the object would cease to be a moon.
>
> So what's the big problem with the definition of "planet" having a similar
> feature?
Some problems with the IAU's definition were listed in my original post.
Also see my remarks above about "trojan" planets. The example definitions I
provided are backwards compatible and jibe with the usual understanding of
what a planet is. Also the example definitions I provided do not require
that a planet orbit a star in order to be a planet.
> So what's the big problem with the definition of "planet" having a similar
> feature?
So, what's the big problem with the definition of a "planet" not having a
similar feature?
K
K_h
I did not do any name calling and neither should you.
> And your point is still utterly absurd and by not backing down on this
> you are demonstrating that you are an idiot. The parenthetical comment
> can be omitted without altering the meaning of the phrase. Absolutely
> nobody is confused about whether Saturn is in Hydrostatic Equilibrium.
> The Dwarf Planet definition has some problems in relationship to smaller
> spheroid bodies this does not affect the definition of Planet as any
> body big enough to be orbitally dominant is going to be in hydrostatic
> equilibrium (largely round due to its own gravity).
No. The IAU's defintion refers to objects that are in hydrostatic
equilibrium and are nearly round (i.e. nearly spherical). Very oblate
objects, that are not nearly spherical, can also be in hydrostatic
equilibrium. The parenthetical comment cannot be omitted without altering
the meaning of the phrase. Again, this was already answered in previous
posts but check again: http://en.wikipedia.org/wiki/Round
>>> A current lack of data can justify a wait and see attitude, we'll worry
>>> about it when it becomes a live issue, at the moment it's a moot point.
>>
>>Never put off to tomorrow what can be done today. As Sky and Telescope
>>magazine has rightly noted, it is only a matter of time before the IAU
>>changes its definition. There is such dissatisfaction with the IAU's
>>definition right now that many astronomers are simply not using it. The
>>aforementioned magazine refuses to recognize the IAU's definition as well
>>as
>>lots of other astronomers.
>
> Don't get ahead of yourself, wait and see what is out there. We have a
> working definition that allows all known systems to be categorised and
> can wait and see how usage is developed.
No, there is no excuse for waiting. The IAU's definition does not cover all
kinds of systems that are bound to be out there. Yet again, that was
already explained in my previous post.
Again, no excuse for waiting.
>>> As things stand the Earth acquire planetary status early in the solar
>>> system's formation, lost planetary status when Theia became so massive
>>> that it became perturbed from either L4 or L5 and regained planetary
>>> status when earth accreted Theia and the Moon formed.
>>
>>Under the IAU's definition, yes. There have been several documentaries on
>>the formation of the moon on the discovery channel. They all refer to
>>Earth
>>and Theia as planets, even though they don't qualify as such under the
>>current IAU definition. Yet another problem for the IAU's definition.
>
> It isn't really applicable in an immature system, actually by the
> Stern-Levinson parameter Earth remained a Planet by Soter's discriminant
> is briefly wasn't. The formative stage is brief and dramatic It isn't
> wholly surprising that definitions developed for a mature system aren't
> really applicable. We can resolve the ambiguities at a later date, when
> we know a bit more about planetary formation.
I disagree. The whole point of nomenclature is to classify, as much as
possible, everything that is out there. No point in lacking nomenclature
for "immature" systems and even mature systems are not covered in the IAU's
definition (see the aforementioned examples of "trojan" planets).
>>> If it caused the Kuiper wall it is a planet as it has cleared its
>>> neighbourhood creating the Kuiper cliff. You are talking about a
>>> somewhat
>>> smaller body which had not cleared its neighbourhood, that would not be
>>> a
>>> planet.
>>
>>It should be called a planet. Pluto, for example, has polar caps, an
>>atmosphere, seasons, three moons, and interesting geological activity at
>>its
>>surface.
>
> Being interesting does not make it a planet. Comets also have
> atmospheres sometimes very big atmospheres. Pluto looks to be a
> gargantuan comet.
Again, the nomenclature should cover everything that is bound to be out
there. Pluto looks nothing like a "gargantuan comet".
>>>>> Asteroids are already Minor Planets, a term which denotes an entirely
>>>>> disjoint set from Planet. As the new term Dwarf Planet does.
>>>>
>>>>This only underscores the need to get better definitions for things
>>>>like "Minor Planets" and "Asteroids". A minor planet is not a
>>>>planet? Silly. A great way to define asteroids is as objects small
>>>>enough so that hydrostatic equilibrium does not apply.
>>>
>>> You may think it silly but it has been the case for more than a century
>>> and a half that Minor Planet is a term for objects too small to be
>>> Planets
>>> and which aren't comets. It is a synonym for Planetoid and Asteroid.
>>> This
>>> has not caused any confusion. They are listed and named in the Minor
>>> Planets Catalogue.
>>
>>"A minor planet is not a planet" is a contradiction in terms. Yes, for
>>the
>>last century there has been some very bad terminology.
>
> No it isn't, it is perfectly normal English. Minor Planets aren't
> Planets, Beach Volleyball [1] isn't Volleyball [2], Koala Bears aren't
> Bears. A modifier and a noun denoting an entirely separate class from
> the unmodified noun is hardly unusual or remotely ungrammatical.
Yes, "Koala Bears" is an example of bad terminology because it is
taxonomically incorrect (as the link points out). A few bad examples like
these do not justify such bad behavior by the IAU.
http://en.wikipedia.org/wiki/Koala
No, it is very supportive of my case. Many definitions in science have
changed numerous times. The IAU's definition is no exception; it too will
be changed because it is so bad and so much has yet to be learned about
other solar systems. BTW, the current definition of a star does not
explicitly make reference to event horizons.
http://www.answers.com/topic/star
K
Michael Ash
> Not at all: http://en.wikipedia.org/wiki/Round. I already answered this
> once before. The IAU's definition of a planet states that it is in
> hydrostatic equilibrium and is nearly round (i.e. nearly spherical).
I have to wonder if you understand the purpose of a parenthetical. In
general, parentheticals are for clarification only, and the greater
meaning of the sentence should not change in any way if it is omitted. The
"nearly round" part of the definition is in parentheses and thus should be
considered to be merely a remark to clarify what is considered significant
about "hydrostatic equilibrium", and not a further restriction of it.
Brett Paul Dunbar
I wasn't name calling I was given my considered opinion based on this
discussion. Your comprehension of perfectly simple English is remarkably
poor and you are engaging in stupid complaints about perfectly
grammatical standard English.
I get the distinct impression that you are a grammatical prescriptivist.
Your pattern of bizarre objections to perfectly normal established
linguistic usage and inconsistent quibbling to justify a position based
on personal preference is quite reminiscent of grammatical
prescriptivism.
>
>> And your point is still utterly absurd and by not backing down on this
>> you are demonstrating that you are an idiot. The parenthetical comment
>> can be omitted without altering the meaning of the phrase. Absolutely
>> nobody is confused about whether Saturn is in Hydrostatic Equilibrium.
>> The Dwarf Planet definition has some problems in relationship to smaller
>> spheroid bodies this does not affect the definition of Planet as any
>> body big enough to be orbitally dominant is going to be in hydrostatic
>> equilibrium (largely round due to its own gravity).
>
>No. The IAU's defintion refers to objects that are in hydrostatic
>equilibrium and are nearly round (i.e. nearly spherical). Very oblate
>objects, that are not nearly spherical, can also be in hydrostatic
>equilibrium. The parenthetical comment cannot be omitted without altering
>the meaning of the phrase. Again, this was already answered in previous
>posts but check again: http://en.wikipedia.org/wiki/Round
You do not understand what a parenthetical comment is. The whole point
of a parenthetical comment is that IT CAN BE OMITTED WITHOUT ALTERING
THE MEANING OF THE SENTENCE. It is NOT a restrictive clause. It appears
to be intended to explain to a laymen roughly what hydrostatic
equilibrium means, as hydrostatic equilibrium is a piece of technical
vocabulary that a non-specialist is unlikely to have encountered.
>No, there is no excuse for waiting. The IAU's definition does not cover all
>kinds of systems that are bound to be out there. Yet again, that was
>already explained in my previous post.
There is a definition that covers all known cases. It is intended as a
working definition and will be extended as more stuff is observed. We
don't have any great need to classify objects we can't see and are
unlikely to see for quite a while yet. Although we have provisionally
decided that an object too small to fuse Deuterium formed in a star like
manner is a Sup-Brown Dwarf not a planet, this would mean a Jupiter
sized body which wasn't a planet.
I should note that it was only with the discovery of several other
Kuiper belt bodies that there was any great drive to define the class of
bodies to which Pluto belonged.
>
>>>>>> All known exoplanets of main sequence stars fall well above the
>>>>>> gap in Figure 1 and would be classified as planets by the
>>>>>> criterion of dynamical dominance.
>>>>>
>>>>>A moot point since gas giants will be discovered first. Dynamic
>>>>>dominance is a vague and ambiguous term and should not be used to
>>>>>define a planet. Imagine a solar system in formation and a growing
>>>>>planet that has to periodically "clear out" its neighborhood due to
>>>>>constantly influxing material. Under the IAU's bad definition, it
>>>>>is alternately a planet at sometimes and not at others. So the
>>>>>IAU's definition is really quite bad.
>>>>
>>>> The IAU definition currently applies only to one mature system, it
>>>> easily be extended to other mature systems. The formation phase is
rather short but the definition cannot easily be applied to them. You
>>>>could explicitly include the phrase "in a mature system" in the
>>>>definition and classify protoplanetary systems separately. Or you
>>>>could include "has cleared, or is in the process of clearing, its
>>>>
>>>
>>>Yes, you rightly point out other flaws in the IAU's definition; it
>>>does not distinguish between mature and immature planetary systems.
>>>The example definitions I suggested avoid this problem totally since
>>>they can be applied at any time to any world.
>>
>> At the moment it only apples to a single system, which happens to be
>> mature. It apples to no immature systems and we have, so far, not
>> detected any immature systems so the working definition for exponents
>> has not had to deal with any as yet.
>
>Again, no excuse for waiting.
The discovery of the first exoplanets demonstrated that planetary
formation didn't typically proceed the way we had thought they
proceeded. Any scheme of nomenclature we had devised beforehand would
have been utterly inadequate to classify what was actually out there.
This has resulted in the definition of several new classes of planet,
such as Hot Jupiters. Any definition introduced now would probably
poorly reflect what actually exists.
>>>> If it caused the Kuiper wall it is a planet as it has cleared its
>>>>neighbourhood creating the Kuiper cliff. You are talking about a
>>>>somewhat smaller body which had not cleared its neighbourhood, that
>>>>would not be a planet.
>>>
>>>It should be called a planet. Pluto, for example, has polar caps, an
>>>atmosphere, seasons, three moons, and interesting geological activity
>>>at its surface.
>>
>> Being interesting does not make it a planet. Comets also have
>> atmospheres sometimes very big atmospheres. Pluto looks to be a
>> gargantuan comet.
>
>Again, the nomenclature should cover everything that is bound to be out
>there. Pluto looks nothing like a "gargantuan comet".
>
Actually it does, it appears to have been formed from the same bit of
the sun's circumstellar disk as the short period comets and seems to be
made of the same stuff. Which, on compositional grounds, make it a giant
comet. If it were on perturbed onto an orbit which took it into the
inner solar system it would look like a comet forming a coma and tail as
its ices vaporised. Several other outer solar system bodies have
previously displayed comet like behaviour, Chiron has displayed a coma
and is one of a number of bodies designated as both a comet and an
asteroid.
>>>>>> Asteroids are already Minor Planets, a term which denotes an entirely
>>>>>> disjoint set from Planet. As the new term Dwarf Planet does.
>>>>>
>>>>>This only underscores the need to get better definitions for things
>>>>>like "Minor Planets" and "Asteroids". A minor planet is not a
>>>>>planet? Silly. A great way to define asteroids is as objects small
>>>>>enough so that hydrostatic equilibrium does not apply.
>>>>
>>>> You may think it silly but it has been the case for more than a
>>>>century and a half that Minor Planet is a term for objects too
>>>>small to be Planets and which aren't comets. It is a synonym for
>>>>Planetoid and Asteroid. This has not caused any confusion. They
>>>>are listed and named in the Minor Planets Catalogue.
>>>
>>>"A minor planet is not a planet" is a contradiction in terms. Yes,
>>>for the last century there has been some very bad terminology.
>>
>> No it isn't, it is perfectly normal English. Minor Planets aren't
>> Planets, Beach Volleyball [1] isn't Volleyball [2], Koala Bears aren't
>> Bears. A modifier and a noun denoting an entirely separate class from
>> the unmodified noun is hardly unusual or remotely ungrammatical.
>
>Yes, "Koala Bears" is an example of bad terminology because it is
>taxonomically incorrect (as the link points out). A few bad examples like
>these do not justify such bad behavior by the IAU.
>
I note that you chose only to comment on one case where the terminology
has been changed to reflect a better understanding of the facts. In many
other cases, Moles, Golden Moles and Marsupial Moles or Crabs, Hermit
Crabs and Porcelain Crabs or Chimps and Pygmy Chimps or Hippopotamuses
and Pygmy Hippopotamuses, the noun with the modifier is not a subset of
the noun without the modifier. It is perfectly normal English.
>
>
>>>> What is wrong with developing the definitions based on
>>>>observations? If you over specify prior to observation you may
>>>>find that your definition is a poor match for what exists. This
>>>>sort of thing has been a persistent problem in biology, many folk
>>>>classifications bear little relation to the underlying nature of
>>>>the species concerned, as they were compiled in ignorance of the
>>>>underlying evolutionary relationships.
>>>
>>>This is why the definitions should be such that they are observation
>>>independent and will not change with new discoveries. The more
>>>science learns about stars does not cause a change in the definition
>>>of what a star is and this should be the case with any resonable
>>>definition of a planet. That is the case with the examples I provided.
You then end up with definitions that do not match well with what exists
and then you either re-define the terms to match usage or abandon the
term and use a new term with a more appropriate meaning.
>>>
>>
>> We know what a Star is, and have now based the definition on the
>> fundamental physical characteristics. It is however nothing like the
>> original definition. The oldest definition was "lights in the sky other
>> than the Sun", then "fixed lights in the Sky" then "big hot discrete
>> things" then we eventually worked out what made them glow and switched
>> to roughly "discrete bodies big enough to fuse hydrogen or which have in
>> the past fused hydrogen" it has since been revised to exclude black
>> holes so is now "discrete bodies big enough to fuse hydrogen or which
>> have in the past fused hydrogen which are not within an event horizon".
>> That is hardly supportive of your case, while the membership of the
>> class has been reasonably stable (of known bodies only the Moon and some
>> Planets have been excluded and the Sun added) the definition has changed
>> massively.
>
>No, it is very supportive of my case. Many definitions in science have
>changed numerous times. The IAU's definition is no exception; it too will
>be changed because it is so bad and so much has yet to be learned about
>other solar systems. BTW, the current definition of a star does not
>explicitly make reference to event horizons.
>http://www.answers.com/topic/star
>
The term star is used to include White Dwarfs and Neutron Stars, both
discrete stellar remnants, Black Holes, even when they are stellar
remnants, are not counted as stars.
Doesn't the fact the current definition of so many thing is the product
of a long period of revision indicate that we shouldn't adopt an overly
prescriptive approach to classifying things we have never seen?
Bryan Derksen
> "Bryan Derksen" <bryan....@shaw.ca> wrote in message
> news:IE5fk.5767$nD.3059@pd7urf1no...
>> K_h wrote:
>>> "Bryan Derksen" <bryan....@shaw.ca> wrote in message
>>> If Stern is correct then your argument on this point is invalid.
>> Funny thing is both sides of this particular point are based on Stern's
>> arguments. However, the side that says "it can clear its orbit" is from a
>> scientific journal article backed with detailed physics and rigorous
>> analysis, and the side that says "it can't clear its orbit" is just
>> something he said to some MSNBC reporter. I think he needs to support the
>> second point a bit better. :)
>
> I cannot speak for Stern, so your disagreement with him should not involve
> people like myself.
You've been pretty free with "speaking for Stern" to this point so long
as he was saying things that supported your position. For example,
you're simply parroting the following statement Stern made in that MSNBC
article, apparently without really understanding it:
> A world with 70% Earth mass will not have cleared the
> Kuiper belt and so will be orbiting within the Kuiper belt.
This is false. Stern's own methods show it to be false. Have you even
opened up the paper my calculation was based on yet? Show me where I
made an error and maybe you'll have something to base an argument on,
otherwise it's just meaningless repetition.
> Of course, one
> can say that any world within the Kuiper belt will have "cleared" its
> immediate path around the sun since they seldom collide with each other.
> That raises the question as to how much "clearing" is needed before a world
> can be said to have "cleared" its orbit. Pluto orbits the sun without
> hitting anything so, in a sense, it's orbit is "clear". This is part of the
> problem with the IAU's definition, it is not defined how wide the "clear"
> zone has to be. With this ambiguity an Earth sized world, in the Kuiper
> belt, has not cleared out the Kuiper belt.
This is all stuff that I've been pointing out ways of quantifying right
from the start of this subthread. And when you chart the numbers for
various solar system objects you'll find that there is no ambiguity;
there's a huge gap between planets and non-planets as defined by the IAU.
Are you actually interested in learning about any of that, though? You
seem to be more intent on arguing for argument's sake, refusing to yield
any objection to the IAU's definition no matter how absurd a position it
devolves into (e.g. Saturn not being "round"). Since the Stern-Levison
paper potentially addresses your concern about this aspect of the IAU's
definition I expect you're likely to just continue ignoring it.
K_h
> K_h wrote:
>> "Bryan Derksen" <bryan....@shaw.ca> wrote in message
>> news:IE5fk.5767$nD.3059@pd7urf1no...
>>> K_h wrote:
>>>> "Bryan Derksen" <bryan....@shaw.ca> wrote in message
>>>> If Stern is correct then your argument on this point is invalid.
>>> Funny thing is both sides of this particular point are based on Stern's
>>> arguments. However, the side that says "it can clear its orbit" is from
>>> a scientific journal article backed with detailed physics and rigorous
>>> analysis, and the side that says "it can't clear its orbit" is just
>>> something he said to some MSNBC reporter. I think he needs to support
>>> the second point a bit better. :)
>>
>> I cannot speak for Stern, so your disagreement with him should not
>> involve people like myself.
>
> You've been pretty free with "speaking for Stern" to this point so long as
> he was saying things that supported your position. For example, you're
> simply parroting the following statement Stern made in that MSNBC article,
> apparently without really understanding it:
>
>> A world with 70% Earth mass will not have cleared the Kuiper belt and so
>> will be orbiting within the Kuiper belt.
>
> This is false. Stern's own methods show it to be false. Have you even
> opened up the paper my calculation was based on yet? Show me where I made
> an error and maybe you'll have something to base an argument on, otherwise
> it's just meaningless repetition.
From what I have read on this subject, there could be a world 70% as massive
as the Earth orbiting within the Kuiper belt. Such a world has obviously
not cleared out the entire Kuiper belt but it may have largely cleared out a
region within the Kuiper belt. So the question arises, how much "clearing"
is needed before such a world qualifies as a planet? For example, how wide
would such a clearing need to be? More generally, what is the cut-off value
of the Stern-Levison value for planethood? Obviously, such a cut-off value
needs to be part of the definition of a "planet" under the IAU's incomplete
definition. Worlds with small Stern-Levison values might clear out their
orbits over very long time scales so what is a "mature" planetary system and
what is an "immature" planetary system?
>> Of course, one can say that any world within the Kuiper belt will have
>> "cleared" its immediate path around the sun since they seldom collide
>> with each other. That raises the question as to how much "clearing" is
>> needed before a world can be said to have "cleared" its orbit. Pluto
>> orbits the sun without hitting anything so, in a sense, it's orbit is
>> "clear". This is part of the problem with the IAU's definition, it is
>> not defined how wide the "clear" zone has to be. With this ambiguity an
>> Earth sized world, in the Kuiper belt, has not cleared out the Kuiper
>> belt.
>
> This is all stuff that I've been pointing out ways of quantifying right
> from the start of this subthread. And when you chart the numbers for
> various solar system objects you'll find that there is no ambiguity;
> there's a huge gap between planets and non-planets as defined by the IAU.
Well, fine but see my above questions. All the extra stuff that you have
been pointing out need to be added to the IAU's definition (at least as a
footnote) so that it is clear what "clearing its orbit" means.
Unfortunately, the IAU's definition does not explain any of this; hence its
ambiguity.
> Are you actually interested in learning about any of that, though? You
> seem to be more intent on arguing for argument's sake, refusing to yield
> any objection to the IAU's definition no matter how absurd a position it
> devolves into (e.g. Saturn not being "round"). Since the Stern-Levison
> paper potentially addresses your concern about this aspect of the IAU's
> definition I expect you're likely to just continue ignoring it.
I have not ignored any of it; the IAU's definition does not reference any of
it. None of my objections are absurd, Saturn is oblate enough that it is
unclear, from the IAU's definition, if it qualifies as "nearly round" since
round is a synonym for spherical and, as I have explained several times now,
Saturn is oblate enough that it isn't "nearly round". It seems like you are
ignoring all these issues I have been pointing out.
K
K_h
"Michael Ash" <mi...@mikeash.com> wrote in message
news:12161676...@web1.segnet.com...
> K_h <KHo...@sx729.com> wrote:
>> Not at all: http://en.wikipedia.org/wiki/Round. I already answered this
>> once before. The IAU's definition of a planet states that it is in
>> hydrostatic equilibrium and is nearly round (i.e. nearly spherical).
>
> I have to wonder if you understand the purpose of a parenthetical. In
> general, parentheticals are for clarification only, and the greater
> meaning of the sentence should not change in any way if it is omitted. The
> "nearly round" part of the definition is in parentheses and thus should be
> considered to be merely a remark to clarify what is considered significant
> about "hydrostatic equilibrium", and not a further restriction of it.
The IAU's defintion uses the parenthetical as a qualifier (adjective) saying
that only those "nearly round" worlds that are in hydrostaic equilibrium
qualify as planets. Unfortunately, this excludes very oblate worlds that
are in hydrostatic equilibrium. So, for the IAU's definition, omitting the
parenthetical changes its meaning by generalizing to all worlds in
hydrostatic equilibrium.
http://www.thefreedictionary.com/parenthetical
http://ol.scc.spokane.edu/jstrever/tw/Resources/define.htm
k
K_h
Yes, you were name calling since you claimed that I was an "idiot". And
your above insults ignore the issues I have been pointing out. Well, this
is usenet so I guess I should expect the usual denigration that posters give
each other.
>>> And your point is still utterly absurd and by not backing down on this
>>> you are demonstrating that you are an idiot. The parenthetical comment
>>> can be omitted without altering the meaning of the phrase. Absolutely
>>> nobody is confused about whether Saturn is in Hydrostatic Equilibrium.
>>> The Dwarf Planet definition has some problems in relationship to smaller
>>> spheroid bodies this does not affect the definition of Planet as any
>>> body big enough to be orbitally dominant is going to be in hydrostatic
>>> equilibrium (largely round due to its own gravity).
>>
>>No. The IAU's defintion refers to objects that are in hydrostatic
>>equilibrium and are nearly round (i.e. nearly spherical). Very oblate
>>objects, that are not nearly spherical, can also be in hydrostatic
>>equilibrium. The parenthetical comment cannot be omitted without altering
>>the meaning of the phrase. Again, this was already answered in previous
>>posts but check again: http://en.wikipedia.org/wiki/Round
>
> You do not understand what a parenthetical comment is. The whole point of
> a parenthetical comment is that IT CAN BE OMITTED WITHOUT ALTERING THE
> MEANING OF THE SENTENCE. It is NOT a restrictive clause. It appears to be
> intended to explain to a laymen roughly what hydrostatic equilibrium
> means, as hydrostatic equilibrium is a piece of technical vocabulary that
> a non-specialist is unlikely to have encountered.
It is a restrictive clause since it is acting as a modifier to the term.
Yes, there can be cases where a parenthetical can be omitted without
altering any meaning. For this discussion, if the parenthetical were
"(nearly spherical or nearly spheroidal)" then it could be omitted without
any change in meaning.
http://www.thefreedictionary.com/parenthetical
http://ol.scc.spokane.edu/jstrever/tw/Resources/define.htm
>>No, there is no excuse for waiting. The IAU's definition does not cover
>>all
>>kinds of systems that are bound to be out there. Yet again, that was
>>already explained in my previous post.
>
> There is a definition that covers all known cases. It is intended as a
> working definition and will be extended as more stuff is observed. We
> don't have any great need to classify objects we can't see and are
> unlikely to see for quite a while yet. Although we have provisionally
> decided that an object too small to fuse Deuterium formed in a star like
> manner is a Sup-Brown Dwarf not a planet, this would mean a Jupiter sized
> body which wasn't a planet.
I disagree, there is no excuse for not fixing the definition now. Saying
that it can be fixed only after objects, which we *think* won't be
discovered for quite a while, just postpones the inevitable.
> I should note that it was only with the discovery of several other Kuiper
> belt bodies that there was any great drive to define the class of bodies
> to which Pluto belonged.
The definition of a planet should not have had to wait for this.
The example definitions I provided are generic enough that they would not
have this problem. Lots of other possible generic ways of defining these
terms, that would not require future revisions, could also be found.
>>>>> If it caused the Kuiper wall it is a planet as it has cleared its
>>>>> neighbourhood creating the Kuiper cliff. You are talking about a
>>>>> somewhat smaller body which had not cleared its neighbourhood, that
>>>>> would not be a planet.
>>>>
>>>>It should be called a planet. Pluto, for example, has polar caps, an
>>>>atmosphere, seasons, three moons, and interesting geological activity at
>>>>its surface.
>>>
>>> Being interesting does not make it a planet. Comets also have
>>> atmospheres sometimes very big atmospheres. Pluto looks to be a
>>> gargantuan comet.
>>
>>Again, the nomenclature should cover everything that is bound to be out
>>there. Pluto looks nothing like a "gargantuan comet".
>>
>
> Actually it does, it appears to have been formed from the same bit of the
> sun's circumstellar disk as the short period comets and seems to be made
> of the same stuff. Which, on compositional grounds, make it a giant comet.
> If it were on perturbed onto an orbit which took it into the inner solar
> system it would look like a comet forming a coma and tail as its ices
> vaporised. Several other outer solar system bodies have previously
> displayed comet like behaviour, Chiron has displayed a coma and is one of
> a number of bodies designated as both a comet and an asteroid.
Not at all. Suppose there is an Earth sized world, made out of largely ice
and a little rock, that has "clear out its orbit" out in the Kuiper belt.
It too would form a tail, if it got close to the sun, but it would qualify
as a planet under the IAU's definition. So your compositional argument
makes no sense.
Those are still exceptions to the rule and is not the best of English.
>>>>> What is wrong with developing the definitions based on observations?
>>>>> If you over specify prior to observation you may find that your
>>>>> definition is a poor match for what exists. This sort of thing has
>>>>> been a persistent problem in biology, many folk classifications bear
>>>>> little relation to the underlying nature of the species concerned,
>>>>> as they were compiled in ignorance of the underlying evolutionary
>>>>> relationships.
>>>>
>>>>This is why the definitions should be such that they are observation
>>>>independent and will not change with new discoveries. The more science
>>>>learns about stars does not cause a change in the definition of what a
>>>>star is and this should be the case with any resonable definition of a
>>>>planet. That is the case with the examples I provided.
>
> You then end up with definitions that do not match well with what exists
> and then you either re-define the terms to match usage or abandon the term
> and use a new term with a more appropriate meaning.
Unclear what your point is. Generic definitions of planets are possible
that will not change with new discoveries. I believe Mark Sykes has
suggested defining a planet as an object in hydrostatic equilibrium that is
not a star. Simple definitions like these are broad enough that they will
cover everything that is out there.
>>> We know what a Star is, and have now based the definition on the
>>> fundamental physical characteristics. It is however nothing like the
>>> original definition. The oldest definition was "lights in the sky other
>>> than the Sun", then "fixed lights in the Sky" then "big hot discrete
>>> things" then we eventually worked out what made them glow and switched
>>> to roughly "discrete bodies big enough to fuse hydrogen or which have in
>>> the past fused hydrogen" it has since been revised to exclude black
>>> holes so is now "discrete bodies big enough to fuse hydrogen or which
>>> have in the past fused hydrogen which are not within an event horizon".
>>> That is hardly supportive of your case, while the membership of the
>>> class has been reasonably stable (of known bodies only the Moon and some
>>> Planets have been excluded and the Sun added) the definition has changed
>>> massively.
>>
>>No, it is very supportive of my case. Many definitions in science have
>>changed numerous times. The IAU's definition is no exception; it too will
>>be changed because it is so bad and so much has yet to be learned about
>>other solar systems. BTW, the current definition of a star does not
>>explicitly make reference to event horizons.
>>http://www.answers.com/topic/star
>>
>
> The term star is used to include White Dwarfs and Neutron Stars, both
> discrete stellar remnants, Black Holes, even when they are stellar
> remnants, are not counted as stars.
Yet another problem with the IAU's definition, since it requires a planet to
be something that is orbiting a star. The example definitions I provided do
not require that a planet orbit a star. Suppose there is a Jupiter like
world orbiting a black hole and further suppose it has even "cleared out its
neighborhood". Under the IAU's definition it is not a planet. According to
the IAU, then, what the heck is it?
> Doesn't the fact the current definition of so many thing is the product of
> a long period of revision indicate that we shouldn't adopt an overly
> prescriptive approach to classifying things we have never seen?
No.
k
Michael Ash
Note that it says qualifying or explanatory. Explanatory is the standard
usage. Why do you say that they use it in a qualifying manner? I see no
evidence of this. On the other hand there are two highly convincing pieces
of evidence that it is being used in an explanatory manner:
1) Explanatory is more common.
2) The people who wrote the definition consider Saturn to be a planet.
Taken as an explanatory parenthetical, there is no conflict between the
definition as written and the consideration of Saturn as a planet.
Insisting that this parenthetical is qualifying and that Saturn therefore
does not fit is, to put it bluntly, artificial and asinine.
Brett Paul Dunbar
<KHo...@SX729.com> writes
>
>"Brett Paul Dunbar" <br...@nospam.demon.co.uk> wrote in message
>news:4htBfzPO...@dimetrodon.demon.co.uk...
>> In message <jridnfxvzcbapuDV...@comcast.com>, K_h
>> <KHo...@SX729.com> writes
>>>
>>>"Brett Paul Dunbar" <br...@nospam.demon.co.uk> wrote in message
>>>news:2OPy0vGF...@dimetrodon.demon.co.uk...
>>>> In message <ALOdnR7-EYk6dObV...@comcast.com>, K_h
>>>> <KHo...@SX729.com> writes
>>>>>
>>>>>"Brett Paul Dunbar" <br...@nospam.demon.co.uk> wrote in message
>>>>>news:zCTz3UGz...@dimetrodon.demon.co.uk...
>>>>>> In message <sdudnY3Eef4CC-TV...@comcast.com>, K_h
>>>>>> <KHo...@SX729.com> writes
>>>>>>>
>>>>>>>"Brett Paul Dunbar" <br...@nospam.demon.co.uk> wrote in message
>>>>>>>news:8ZqAhGNb$9dI...@dimetrodon.demon.co.uk...
>>>>>>>> In message <NeudnfOCrshjWOrV...@comcast.com>, K_h
>>>>>>>> <KHo...@SX729.com> writes
>>>>>>>>>
>>>>>>>>>"Bryan Derksen" <bryan....@shaw.ca> wrote in message
>>>>>>>>>news:GCvdk.69025$Jx.48462@pd7urf1no...
>>>>>>>>>> K_h wrote:
>>>>>>>>>>> "Brett Paul Dunbar" <br...@nospam.demon.co.uk> wrote in message
>>>>>>>>>>> news:FV$OyfDNg...@dimetrodon.demon.co.uk...
>>>>>>>>>>>> In message <PYKdnToYLvdOlenV...@comcast.com>, K_h
>>>>>>>>>>>> <KHo...@SX729.com> writes
>> a parenthetical comment is that IT CAN BE OMITTED WITHOUT ALTERING THE
>> MEANING OF THE SENTENCE. It is NOT a restrictive clause. It appears to be
>> intended to explain to a laymen roughly what hydrostatic equilibrium
>> means, as hydrostatic equilibrium is a piece of technical vocabulary that
>> a non-specialist is unlikely to have encountered.
>
>It is a restrictive clause since it is acting as a modifier to the term.
>Yes, there can be cases where a parenthetical can be omitted without
>altering any meaning. For this discussion, if the parenthetical were
>"(nearly spherical or nearly spheroidal)" then it could be omitted without
>any change in meaning.
You are utterly, utterly wrong. The parenthetical clause is obviously
explanatory, the fact that the explanatory footnote also explicitly
lists Saturn as a planet should give you a clue.
>>>No, there is no excuse for waiting. The IAU's definition does not
>>>cover all kinds of systems that are bound to be out there. Yet
>>>again, that was already explained in my previous post.
>>
>> There is a definition that covers all known cases. It is intended as a
>> working definition and will be extended as more stuff is observed. We
>> don't have any great need to classify objects we can't see and are
>> unlikely to see for quite a while yet. Although we have provisionally
>> decided that an object too small to fuse Deuterium formed in a star like
>> manner is a Sup-Brown Dwarf not a planet, this would mean a Jupiter sized
>> body which wasn't a planet.
>
>I disagree, there is no excuse for not fixing the definition now. Saying
>that it can be fixed only after objects, which we *think* won't be
>discovered for quite a while, just postpones the inevitable.
>
So?
We would like to get it right when we do come up with a more permanent
definition. There is no great cause for urgency.
>> I should note that it was only with the discovery of several other Kuiper
>> belt bodies that there was any great drive to define the class of bodies
>> to which Pluto belonged.
>
>The definition of a planet should not have had to wait for this.
>
No it probably shouldn't have done. But while only Pluto was known there
wasn't any other class in which to put it. It didn't really fit in with
the planets but there wasn't anything else like either. It had been
mis-classified as a planet based on an initial gross overestimate of its
mass. It remained there due to inertia and not really having anywhere
better to class the lone anomaly. With the discovery of a number of
other Trans Neptunian Objects we had a better place to classify Pluto
and eventually the IAU got round to fixing the definition of planet.
>> The discovery of the first exoplanets demonstrated that planetary
>> formation didn't typically proceed the way we had thought they proceeded.
>> Any scheme of nomenclature we had devised beforehand would have been
>> utterly inadequate to classify what was actually out there. This has
>> resulted in the definition of several new classes of planet, such as Hot
>> Jupiters. Any definition introduced now would probably poorly reflect what
>> actually exists.
>
>The example definitions I provided are generic enough that they would not
>have this problem. Lots of other possible generic ways of defining these
>terms, that would not require future revisions, could also be found.
Unlike most of your suggestions the IAU definition can be applied to
exoplanetary systems using observations we have the technical capacity
to make. Measuring diameter requires a transit which is often
unavailable. Whereas the orbital characteristics, including minimum
mass, are available in most cases allowing the calculation of the
Stern-Levinson parameter.
The current working definition is usable.
It does make sense. If you define a comet by what it is made of then
Pluto is a comet. This is based purely on the intrinsic qualities of
Pluto. Pluto is apparently a product of the accretion of a very large
quantity of cometary material, hence it is a gargantuan comet. What
doesn't make sense is to claim that Pluto looks nothing like a comet.
The Kuiper Wall planet would fit in both the class planet and the class
comet.
>> I note that you chose only to comment on one case where the terminology
>> has been changed to reflect a better understanding of the facts. In many
>> other cases, Moles, Golden Moles and Marsupial Moles or Crabs, Hermit
>> Crabs and Porcelain Crabs or Chimps and Pygmy Chimps or Hippopotamuses and
>> Pygmy Hippopotamuses, the noun with the modifier is not a subset of the
>> noun without the modifier. It is perfectly normal English.
>
>Those are still exceptions to the rule and is not the best of English.
There are so many exceptions that it isn't really possible to say that
there is a rule. Both the inclusive and the exclusive cases are
perfectly normal English.
>
>
>>>>>> What is wrong with developing the definitions based on observations?
>>>>>> If you over specify prior to observation you may find that your
>>>>>> definition is a poor match for what exists. This sort of thing has
>>>>>> been a persistent problem in biology, many folk classifications bear
>>>>>> little relation to the underlying nature of the species concerned,
>>>>>> as they were compiled in ignorance of the underlying evolutionary
>>>>>> relationships.
>>>>>
>>>>>This is why the definitions should be such that they are observation
>>>>>independent and will not change with new discoveries. The more science
>>>>>learns about stars does not cause a change in the definition of what a
>>>>>star is and this should be the case with any resonable definition of a
>>>>>planet. That is the case with the examples I provided.
>>
>> You then end up with definitions that do not match well with what exists
>> and then you either re-define the terms to match usage or abandon the term
>> and use a new term with a more appropriate meaning.
>
>Unclear what your point is. Generic definitions of planets are possible
>that will not change with new discoveries. I believe Mark Sykes has
>suggested defining a planet as an object in hydrostatic equilibrium that is
>not a star. Simple definitions like these are broad enough that they will
>cover everything that is out there.
>
Bad definition. It includes a large number of bodies that nobody ever
calls Planets, like Ganymede, it thus fails to follow established usage.
There might be a reason for defining a class of sub-stellar objects in
hydrostatic equilibrium. It should not be labelled planet.
The IAU definition both defines a natural class, objects that have
cleared their orbits, and applies a label whose traditional usage
matches well to that class.
I had stopped counting Pluto as a planet well before the IAU vote. It
had been a lone anomaly that didn't really fit for a long time. With the
discovery of other Trans Neptunian Objects there were other bodies of a
similar nature so it made more sense to treat Pluto as one of them
rather than group it with a bunch of very different bodies.
>> The term star is used to include White Dwarfs and Neutron Stars, both
>> discrete stellar remnants, Black Holes, even when they are stellar
>> remnants, are not counted as stars.
>
>Yet another problem with the IAU's definition, since it requires a planet to
>be something that is orbiting a star. The example definitions I provided do
>not require that a planet orbit a star. Suppose there is a Jupiter like
>world orbiting a black hole and further suppose it has even "cleared out its
>neighborhood". Under the IAU's definition it is not a planet. According to
>the IAU, then, what the heck is it?
It is orbiting a stellar remnant, therefore it would be a planet. The
IAU working definition explicitly states this.
<http://www.dtm.ciw.edu/boss/definition.html>
1 Objects with true masses below the limiting mass for
thermonuclear fusion of deuterium (currently calculated to be 13
Jupiter masses for objects of solar metallicity) that orbit
stars or stellar remnants are "planets" (no matter how they
formed). The minimum mass/size required for an extrasolar object
to be considered a planet should be the same as that used in our
Solar System.
2 Substellar objects with true masses above the limiting mass for
thermonuclear fusion of deuterium are "brown dwarfs", no matter
how they formed nor where they are located.
3 Free-floating objects in young star clusters with masses below
the limiting mass for thermonuclear fusion of deuterium are not
"planets", but are "sub-brown dwarfs" (or whatever name is most
appropriate).
Bryan Derksen
>>> will be orbiting within the Kuiper belt.
>> This is false. Stern's own methods show it to be false. Have you even
>> opened up the paper my calculation was based on yet? Show me where I made
>> an error and maybe you'll have something to base an argument on, otherwise
>> it's just meaningless repetition.
>
> From what I have read on this subject, there could be a world 70% as massive
> as the Earth orbiting within the Kuiper belt. Such a world has obviously
> not cleared out the entire Kuiper belt but it may have largely cleared out a
> region within the Kuiper belt. So the question arises, how much "clearing"
> is needed before such a world qualifies as a planet?
So I take it you've not read that paper, then. It goes into detail about
what "clearing" means and what degree of clearing is significant.
Soter's paper is also good at this,
http://arxiv.org/abs/astro-ph/0608359 in case the link hadn't been
posted elsewhere in this thread yet.
> For example, how wide
> would such a clearing need to be? More generally, what is the cut-off value
> of the Stern-Levison value for planethood? Obviously, such a cut-off value
> needs to be part of the definition of a "planet" under the IAU's incomplete
> definition. Worlds with small Stern-Levison values might clear out their
> orbits over very long time scales so what is a "mature" planetary system and
> what is an "immature" planetary system?
There isn't a definite threshold in the IAU's definition because it's
not currently needed to be unambiguous for our solar system, which is
the only domain the definition currently covers. The Stern-Levison paper
proposes a cutoff value, if you're interested in a good idea for one go
and read it.
Based on how planets are generally accepted to form, though, I doubt
borderline cases are going to be particularly common in the universe.
>> Are you actually interested in learning about any of that, though? You
>> seem to be more intent on arguing for argument's sake, refusing to yield
>> any objection to the IAU's definition no matter how absurd a position it
>> devolves into (e.g. Saturn not being "round"). Since the Stern-Levison
>> paper potentially addresses your concern about this aspect of the IAU's
>> definition I expect you're likely to just continue ignoring it.
>
> I have not ignored any of it; the IAU's definition does not reference any of
> it.
Not explicitly, but it certainly seems to be what they had in mind. Find
me a better definition of "clearing an orbit" (or even an almost-as-good
one) and maybe there'll be something worth debating.
> None of my objections are absurd, Saturn is oblate enough that it is
> unclear, from the IAU's definition, if it qualifies as "nearly round" since
> round is a synonym for spherical and, as I have explained several times now,
> Saturn is oblate enough that it isn't "nearly round". It seems like you are
> ignoring all these issues I have been pointing out.
I've addressed your ridiculous "spheroids aren't round" quibble to the
point of exhaustion, as have several other participants in this thread.
Perhaps it will help resolve your sense of ambiguity to note that the
IAU explicitly listed Saturn as a planet in a footnote of the
definition, which means that it is indeed "round" enough. So whatever
strange definition of "roundness" you've got needs to be adjusted to
account for that.
K_h
> K_h wrote:
>> "Bryan Derksen" <bryan....@shaw.ca> wrote in message
>>>> A world with 70% Earth mass will not have cleared the Kuiper belt and
>>>> so will be orbiting within the Kuiper belt.
>>> This is false. Stern's own methods show it to be false. Have you even
>>> opened up the paper my calculation was based on yet? Show me where I
>>> made an error and maybe you'll have something to base an argument on,
>>> otherwise it's just meaningless repetition.
>>
>> From what I have read on this subject, there could be a world 70% as
>> massive as the Earth orbiting within the Kuiper belt. Such a world has
>> obviously not cleared out the entire Kuiper belt but it may have largely
>> cleared out a region within the Kuiper belt. So the question arises, how
>> much "clearing" is needed before such a world qualifies as a planet?
>
> So I take it you've not read that paper, then. It goes into detail about
> what "clearing" means and what degree of clearing is significant. Soter's
> paper is also good at this, http://arxiv.org/abs/astro-ph/0608359 in case
> the link hadn't been posted elsewhere in this thread yet.
The paper defines a planet as having a "planetary discriminant" greater than
100. Well, the IAU definition does not contain anything about "planetary
discriminants" and all those other highly technical numbers Soter's paper
uses to define planets. A good definition should be short and extending the
IAU's definition to include all Soter's gunk gives us a very unwieldly
definition. But the problem remains: the IAU's definition does not include,
nor does it point to, Soter's stuff. By the way, the case I mentioned
earlier (for trojan planets) will not have a "planetary discriminant"
greater than 100.
>> For example, how wide would such a clearing need to be? More generally,
>> what is the cut-off value of the Stern-Levison value for planethood?
>> Obviously, such a cut-off value needs to be part of the definition of a
>> "planet" under the IAU's incomplete definition. Worlds with small
>> Stern-Levison values might clear out their orbits over very long time
>> scales so what is a "mature" planetary system and what is an "immature"
>> planetary system?
>
> There isn't a definite threshold in the IAU's definition because it's not
> currently needed to be unambiguous for our solar system, which is the only
> domain the definition currently covers. The Stern-Levison paper proposes a
> cutoff value, if you're interested in a good idea for one go and read it.
Again, the definition of a planet should be solar system independent as well
as clear and comprehensive.
> Based on how planets are generally accepted to form, though, I doubt
> borderline cases are going to be particularly common in the universe.
Too soon to tell; the definition should broad enough to cover these cases.
>>> Are you actually interested in learning about any of that, though? You
>>> seem to be more intent on arguing for argument's sake, refusing to yield
>>> any objection to the IAU's definition no matter how absurd a position it
>>> devolves into (e.g. Saturn not being "round"). Since the Stern-Levison
>>> paper potentially addresses your concern about this aspect of the IAU's
>>> definition I expect you're likely to just continue ignoring it.
>>
>> I have not ignored any of it; the IAU's definition does not reference any
>> of it.
>
> Not explicitly, but it certainly seems to be what they had in mind. Find
> me a better definition of "clearing an orbit" (or even an almost-as-good
> one) and maybe there'll be something worth debating.
A good definition should not depend on, nor rely on, what is "seems" sombody
had in mind. That's the point that Stern, Sykes, lots of other astronomers,
and myself are making: there is no good way to define "clearing an orbit"
which is why it is a bad criteria for planethood.
>> None of my objections are absurd, Saturn is oblate enough that it is
>> unclear, from the IAU's definition, if it qualifies as "nearly round"
>> since round is a synonym for spherical and, as I have explained several
>> times now, Saturn is oblate enough that it isn't "nearly round". It
>> seems like you are ignoring all these issues I have been pointing out.
>
> I've addressed your ridiculous "spheroids aren't round" quibble to the
> point of exhaustion, as have several other participants in this thread.
> Perhaps it will help resolve your sense of ambiguity to note that the IAU
> explicitly listed Saturn as a planet in a footnote of the definition,
> which means that it is indeed "round" enough. So whatever strange
> definition of "roundness" you've got needs to be adjusted to account for
> that.
Round means spherical, as I have pointed out numerous times on this thread
(see the links on `round' that I provided). Nearly round means nearly
spherical and nearly spherical cannot cover the case of very oblate.
k
K_h
It is clearly qualifying when used in the IAU's definition. Consider a
definition for a British freedom fighter ::
A man or woman (british) that fights for freedom.
The parenthetical modifies "man or woman" because it says what type of man
or woman is being referenced. Now look at section (b) of the IAU's
definition:
(b) has sufficient mass for its self-gravity to overcome rigid body
forces so that it assumes a hydrostatic equilibrium (nearly round) shape.
In this case "nearly round" is a qualifying requirement for the shape of the
body in hydrostatic equilibrium. Parentheticals could also be specified by
commas in which case section (b) of the IAU's definition would read:
(b) has sufficient mass for its self-gravity to overcome rigid body
forces so that it assumes a hydrostatic equilibrium, nearly round, shape.
So there is clearly a conflict between the IAU's definition of a planet and
its listing of Saturn as a planet (unless the IAU says that 12% oblateness
is "nearly round"). Please note how I used the parenthetical in that last
sentence.
k
K_h
No, I am correct. Consider a definition for a British freedom fighter:
A man or woman (british) that fights for freedom.
The parenthetical modifies "man or woman" because it says what type of man
or woman is being referenced. Now look at section (b) of the IAU's
definition:
(b) has sufficient mass for its self-gravity to overcome rigid body
forces so that it assumes a hydrostatic equilibrium (nearly round) shape.
In this case "nearly round" is a qualifying requirement for the shape of the
body in hydrostatic equilibrium.
>>>>No, there is no excuse for waiting. The IAU's definition does not
>>>>cover all kinds of systems that are bound to be out there. Yet
>>>>again, that was already explained in my previous post.
>>>
>>> There is a definition that covers all known cases. It is intended as a
>>> working definition and will be extended as more stuff is observed. We
>>> don't have any great need to classify objects we can't see and are
>>> unlikely to see for quite a while yet. Although we have provisionally
>>> decided that an object too small to fuse Deuterium formed in a star like
>>> manner is a Sup-Brown Dwarf not a planet, this would mean a Jupiter
>>> sized
>>> body which wasn't a planet.
>>
>>I disagree, there is no excuse for not fixing the definition now. Saying
>>that it can be fixed only after objects, which we *think* won't be
>>discovered for quite a while, just postpones the inevitable.
>>
>
> So?
>
> We would like to get it right when we do come up with a more permanent
> definition. There is no great cause for urgency.
I disagree, there is no excuse for not fixing it now.
>>> The discovery of the first exoplanets demonstrated that planetary
>>> formation didn't typically proceed the way we had thought they
>>> proceeded.
>>> Any scheme of nomenclature we had devised beforehand would have been
>>> utterly inadequate to classify what was actually out there. This has
>>> resulted in the definition of several new classes of planet, such as Hot
>>> Jupiters. Any definition introduced now would probably poorly reflect
>>> what
>>> actually exists.
>>
>>The example definitions I provided are generic enough that they would not
>>have this problem. Lots of other possible generic ways of defining these
>>terms, that would not require future revisions, could also be found.
>
> Unlike most of your suggestions the IAU definition can be applied to
> exoplanetary systems using observations we have the technical capacity
> to make. Measuring diameter requires a transit which is often
> unavailable. Whereas the orbital characteristics, including minimum
> mass, are available in most cases allowing the calculation of the
> Stern-Levinson parameter.
>
> The current working definition is usable.
I have already explained why this is not the case with many examples.
Again, the IAU definition makes no mention of the Stern-Levinson parameter.
Even if you want to make the case that the current definition should remain,
you have (at least implicitly) argued that it should be modified to include
the Stern-Levinson parameter. You are arguing that the IAU's definition
should be changed to:
"(1) A planet [1] is a celestial body that (a) is in orbit around the Sun,
(b) has sufficient mass for its self-gravity to overcome rigid body forces
so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c)
has cleared the neighbourhood around its orbit according to the
Stern-Levinson parameter[2]."
where [2] would be a footnote specifying numerical values that you (and
others) agree is reasonable. And you might also argue that it could be
generalized so that section (a) replaces "the Sun" with "a Star". Then the
definition, at least, has been clarified somewhat.
But there is no simple way to define what a comet is made out of. For
example there are a continuum of possible ratios for things like ices,
rocks, metals, etc in what we could define as a "comet". How much iron must
be in a comet before it loses its cometary status? Ratio ranges for all the
materials must be given in any definition of a comet and such definitions
quickly become unwieldly.
>>> I note that you chose only to comment on one case where the terminology
>>> has been changed to reflect a better understanding of the facts. In many
>>> other cases, Moles, Golden Moles and Marsupial Moles or Crabs, Hermit
>>> Crabs and Porcelain Crabs or Chimps and Pygmy Chimps or Hippopotamuses
>>> and
>>> Pygmy Hippopotamuses, the noun with the modifier is not a subset of the
>>> noun without the modifier. It is perfectly normal English.
>>
>>Those are still exceptions to the rule and is not the best of English.
>
> There are so many exceptions that it isn't really possible to say that
> there is a rule. Both the inclusive and the exclusive cases are
> perfectly normal English.
No excuse for not fixing the IAU definition since most of the time
adjectives modify nouns.
I did not fully characterize Sykes definition, obviously it would need to
distinguish between moons and planets. If I recall, he defined a planet as
a celestial body, in hydrostatic equilibrium, that is in orbit around a star
and that would exclude Ganymede (which orbits Jupiter).
> There might be a reason for defining a class of sub-stellar objects in
> hydrostatic equilibrium. It should not be labelled planet.
>
> The IAU definition both defines a natural class, objects that have
> cleared their orbits, and applies a label whose traditional usage
> matches well to that class.
No. I listed some of the most glaring problems of the IAU's definition in
my original post.
But the problem is with the two definitions. One definition for the solar
system, and one for everywhere else, makes no sense. We don't have one
definition of a tree, for trees in America, and another definition for a
tree everywhere else. So the IAU's definition of a planet (for the solar
system) does have the problem I mentioned when we try to combine it with the
definition of a planet (everywhere else). That is the point, those
definitions are inconsistent with each other.
k
Bryan Derksen
>> K_h wrote:
>>> From what I have read on this subject, there could be a world 70% as
>>> massive as the Earth orbiting within the Kuiper belt. Such a world has
>>> obviously not cleared out the entire Kuiper belt but it may have largely
>>> cleared out a region within the Kuiper belt. So the question arises, how
>>> much "clearing" is needed before such a world qualifies as a planet?
>>
>> So I take it you've not read that paper, then. It goes into detail about
>> what "clearing" means and what degree of clearing is significant. Soter's
>> paper is also good at this, http://arxiv.org/abs/astro-ph/0608359 in case
>> the link hadn't been posted elsewhere in this thread yet.
>
> The paper defines a planet as having a "planetary discriminant" greater than
> 100.
That's the Soter paper you're referring to. The Stern-Levison paper was
linked earlier in the thread, it proposes a Stern-Levison parameter of 1
as the threshold. But anyway, it's good to see you're actually reading
this stuff now. I take it you've seen that there are indeed ways to
rigorously define what 'clearing the neighborhood' means.
> Well, the IAU definition does not contain anything about "planetary
> discriminants" and all those other highly technical numbers Soter's paper
> uses to define planets. A good definition should be short and extending the
> IAU's definition to include all Soter's gunk gives us a very unwieldly
> definition.
You're complaining that the IAU's definition doesn't specify what it
means by clearing the neighborhood, but if they'd included a detailed
explanation you'd consider it unweildy "gunk"?
> But the problem remains: the IAU's definition does not include,
> nor does it point to, Soter's stuff.
No, but it does include the concept of clearing an orbital neighborhood,
which is what Stern, Levison, and Soter were discussing in their papers.
I have yet to see any references to other work in this field that the
IAU might possibly have been referring to.
> By the way, the case I mentioned
> earlier (for trojan planets) will not have a "planetary discriminant"
> greater than 100.
Yes, and my response is still "So?" 'Trojan planets' are an unusual case
that could well warrant a separate classification, if we happen to
actually find some at some point. They do not obviously belong in the
same class as conventional planets.
>> There isn't a definite threshold in the IAU's definition because it's not
>> currently needed to be unambiguous for our solar system, which is the only
>> domain the definition currently covers. The Stern-Levison paper proposes a
>> cutoff value, if you're interested in a good idea for one go and read it.
>
> Again, the definition of a planet should be solar system independent as well
> as clear and comprehensive.
And IMO this new definition by the IAU is a strong step in that
direction. The old definition was very far from either of those goals.
We're just repeating our basic opinions here, it isn't advancing the
discussion any.
>> Based on how planets are generally accepted to form, though, I doubt
>> borderline cases are going to be particularly common in the universe.
>
> Too soon to tell; the definition should broad enough to cover these cases.
If you think we _don't_ have a pretty good idea of how planets form, why
is it a good idea to settle our definition of what a planet is _now_?
That would make it likely we'd wind up with more misclassifications just
like Pluto's.
Once again, we're repeating this particular point without adding
anything new to it.
> A good definition should not depend on, nor rely on, what is "seems" sombody
> had in mind. That's the point that Stern, Sykes, lots of other astronomers,
> and myself are making: there is no good way to define "clearing an orbit"
> which is why it is a bad criteria for planethood.
You've yet to point out the flaws in the Stern-Levison or Soter methods.
Just asserting over and over that they're "bad" is unconvincing.
> Round means spherical, as I have pointed out numerous times on this thread
> (see the links on `round' that I provided). Nearly round means nearly
> spherical and nearly spherical cannot cover the case of very oblate.
So I ask again, are a car's wheels round?
As Brett has been trying to explain to you, the parenthetical 'nearly
round' bit is intended to help explain the preceding 'hydrostatic
equilibrium' part for lay audiences. The hydrostatic equilibrium part is
the important part. If your concept of 'round' in this case conflicts
with the meaning of hydrostatic equilibrium, your concept of 'round' is
wrong. Not much else to this one.
Brett Paul Dunbar
<KHo...@SX729.com> writes
>
>"Michael Ash" <mi...@mikeash.com> wrote in message
>news:12162696...@web1.segnet.com...
>> K_h <KHo...@sx729.com> wrote:
>>>
>> Note that it says qualifying or explanatory. Explanatory is the standard
>> usage. Why do you say that they use it in a qualifying manner? I see no
>> evidence of this. On the other hand there are two highly convincing pieces
>> of evidence that it is being used in an explanatory manner:
>>
>> 1) Explanatory is more common.
>> 2) The people who wrote the definition consider Saturn to be a planet.
>>
>> Taken as an explanatory parenthetical, there is no conflict between the
>> definition as written and the consideration of Saturn as a planet.
>> Insisting that this parenthetical is qualifying and that Saturn therefore
>> does not fit is, to put it bluntly, artificial and asinine.
>
>It is clearly qualifying when used in the IAU's definition. Consider a
>definition for a British freedom fighter ::
>
> A man or woman (british) that fights for freedom.
>
>The parenthetical modifies "man or woman" because it says what type of man
>or woman is being referenced. Now look at section (b) of the IAU's
>definition:
That is ungrammatical as written. The brackets are misplaced as omitting
the parenthesised statement alters the sense of the statement. Square
brackets might be appropriate in that context; round brackets are wrong.
A man or woman [British] that fights for freedom.
Could be appropriate, if you were quoting from a longer statement and
that was implied in the original context. Square brackets are used for
editorial comments and inserting words required due to trimming context.
>
> (b) has sufficient mass for its self-gravity to overcome rigid body
>forces so that it assumes a hydrostatic equilibrium (nearly round) shape.
>
>In this case "nearly round" is a qualifying requirement for the shape of the
>body in hydrostatic equilibrium.
No it is clearly explanatory. You are talking utter bollocks as is
entirely clear from the context.
In any event a body in hydrostatic equilibrium is nearly round, by some
not unusual senses of the word round. Saturn is round if you measure
round the equator and round can include spheroid bodies anyway. Round
can include circles and wheels, both of which are far further from
spherical than Saturn.
>
> (b) has sufficient mass for its self-gravity to overcome rigid body
>forces so that it assumes a hydrostatic equilibrium, nearly round, shape.
>
>So there is clearly a conflict between the IAU's definition of a planet and
>its listing of Saturn as a planet (unless the IAU says that 12% oblateness
>is "nearly round"). Please note how I used the parenthetical in that last
>sentence.
You really are an idiot aren't you?
You used the parenthetical for a comment that could be omitted. It is
clarifying statement not a restrictive one, specifying what your
(manifestly ill-founded) objection to classifying Saturn as a planet
under the IAU definition is. The sentence already states that you have
an objection.
The IAU, by implication, does count Saturn as nearly round as it lists
it in the footnotes as a planet.
Brett Paul Dunbar
>
>"Brett Paul Dunbar" <br...@nospam.demon.co.uk> wrote in message
>news:M0nc6VDl...@dimetrodon.demon.co.uk...
>> In message <pcOdna4DibNJH-PV...@comcast.com>, K_h
>> <KHo...@SX729.com> writes
>>>
>>>"Brett Paul Dunbar" <br...@nospam.demon.co.uk> wrote in message
>>>news:4htBfzPO...@dimetrodon.demon.co.uk...
>>>> In message <jridnfxvzcbapuDV...@comcast.com>, K_h
>>>> <KHo...@SX729.com> writes
>>>>>
>>>>>"Brett Paul Dunbar" <br...@nospam.demon.co.uk> wrote in message
>>>>>news:2OPy0vGF...@dimetrodon.demon.co.uk...
>>>>>> In message <ALOdnR7-EYk6dObV...@comcast.com>, K_h
>>>>>> <KHo...@SX729.com> writes
>>>>>>>It should be called a planet. Pluto, for example, has polar
>>>>>>>caps, an atmosphere, seasons, three moons, and interesting
>>>>>>>geological activity at its surface.
>>>>>>
>>>>>> Being interesting does not make it a planet. Comets also have
>>>>>> atmospheres sometimes very big atmospheres. Pluto looks to be a
>>>>>> gargantuan comet.
>>>>>
>>>>>Again, the nomenclature should cover everything that is bound to be out
>>>>>there. Pluto looks nothing like a "gargantuan comet".
>>>>>
>>>>
>>>> Actually it does, it appears to have been formed from the same bit
So far it hasn't posed much of a problem, primarily icy bodies from the
outer solar system are comets while rocky or metallic inner system
bodies are asteroids. A few are officially classified as both, Chiron
for example.
The class of comet was originally defined based on their behaviour in
the inner solar system, forming comas and tails &c. without knowledge of
structure, origin or composition. The behaviour happens to derive from
the composition so the composition is a key part of the definition.
Asteroids are basically debris which doesn't show cometary behaviour.
Some asteroids are dead comets as they have lost enough ice that they
have a thick non-volatile crust and no longer erupt under heating. This
doesn't happen at any specific ratio, for example a comet might have a
very thick impermeable crust over most of its surface with a single
active spot and be a comet while a different body might have a much
thinner impermeable crust with no active spots and be a cometary remnant
type asteroid.
Pluto was, we think, formed from the same stuff that the short period
comets were, which means that its composition falls into the cometary
range.
>>>> I note that you chose only to comment on one case where the
>>>>terminology has been changed to reflect a better understanding of
>>>>the facts. In many other cases, Moles, Golden Moles and Marsupial
>>>>Moles or Crabs, Hermit Crabs and Porcelain Crabs or Chimps and
>>>>Pygmy Chimps or Hippopotamuses and Pygmy Hippopotamuses, the noun
>>>>with the modifier is not a subset of the noun without the modifier.
>>>>It is perfectly normal English.
>>>
>>>Those are still exceptions to the rule and is not the best of English.
>>
>> There are so many exceptions that it isn't really possible to say that
>> there is a rule. Both the inclusive and the exclusive cases are
>> perfectly normal English.
>
>No excuse for not fixing the IAU definition since most of the time
>adjectives modify nouns.
There isn't any great prevalence of one or the other, not that it
matters materially what label you apply. Planet as currently defined is
a clear natural category. Labelling is a separate matter. Dwarf Planets
could be renamed, You could call them planetoids for example, or medium
solar system bodies, or asteroids.
>>>Unclear what your point is. Generic definitions of planets are
>>>possible that will not change with new discoveries. I believe Mark
>>>Sykes has suggested defining a planet as an object in hydrostatic
>>>equilibrium that is not a star. Simple definitions like these are
>>>broad enough that they will cover everything that is out there.
>>>
>>
>> Bad definition. It includes a large number of bodies that nobody ever
>> calls Planets, like Ganymede, it thus fails to follow established usage.
>
>I did not fully characterize Sykes definition, obviously it would need to
>distinguish between moons and planets. If I recall, he defined a planet as
>a celestial body, in hydrostatic equilibrium, that is in orbit around a star
>and that would exclude Ganymede (which orbits Jupiter).
>
>
All he is doing is making Dwarf Planet a sub class of Planet, this puts
an excessive number of objects in the class of planet and replaces an
unambiguous lower limit with a much harder to detect criterion.
Ceres is not a planet, this was settled 150 years ago. It was intended
that the term "Minor Planet" be a subset of planet along with "Major
Planet". Usage quickly developed so that "Major Planet" became simply
Planet and therefore planet and minor planet were mutually exclusive
categories.
>> There might be a reason for defining a class of sub-stellar objects in
>> hydrostatic equilibrium. It should not be labelled planet.
>>
>> The IAU definition both defines a natural class, objects that have
>> cleared their orbits, and applies a label whose traditional usage
>> matches well to that class.
>
>No. I listed some of the most glaring problems of the IAU's definition in
>my original post.
Your objections were either ridiculous grammatical quibbles, or just
plain wrong. Orbit clearing is a dynamic process that produces a clear
gap, it is a natural category whatever you choose to call the members of
the category. It happens that the normal usage of the term Planet
matches pretty well.
Paragraph 1 indicates that the solar system definition should usually be
used, with appropriate modifications.
Paragraph 2 imposes an upper limit on planet size, which isn't needed in
the solar system as Jupiter is about on thirteenth the mass needed to
fuse Deuterium. Incidentally there is another potential lower limit for
Brown Dwarfs, a body becomes fully convecting at about 7 Jupiter masses
and there is some support for making that the lower limit of Brown Dwarf
status.
Paragraph 3 deals with a class of objects we haven't seen yet but do
expect to detect in the not too distant future. They are theorised to
form in a rather different manner to solar system planets and have
currently been excluded from the class of planet.
The working definition is the solar system definition; with a couple of
extra specifications to deal with types of bodies absent from the solar
system.
Michael Ash
Look, this is quite simple. There are two choices here:
1) The parenthetical is explanatory.
2) The parenthetical is qualifying.
Both choices, without further context, are equally valid, right?
Now then, add in the fact that, at least according to you, #2 conflicts
with the definition of Saturn as a planet.
The reaction to this by any *sane* person would be to say that, given that
#2 produces a conflict, the correct interpretation must be #1. Could you
please explain why you do not do this?
K_h
Yes, but by that I do not mean to demean their work. My only point is that
including it, or referencing it, makes the definition bulky and cumbersome.
Okay, it looks like we have reached a point here where we disagree that
brevity and simplicity are necessary for a good definition. I think you and
I will simply have to agree to disagree on this point.
>> By the way, the case I mentioned earlier (for trojan planets) will not
>> have a "planetary discriminant" greater than 100.
>
> Yes, and my response is still "So?" 'Trojan planets' are an unusual case
> that could well warrant a separate classification, if we happen to
> actually find some at some point. They do not obviously belong in the same
> class as conventional planets.
Hmmm... We have reached a point here where we will simply have to agree to
disagree. It is obvious to me that they do belong in the same class as
conventional planets simply because a trojan planet could be very nearly
identical to Earth. You feel that a world nearly identical to Earth, as a
trojan planet, is "does not obviously belong in the same class as
conventional planets". So if the Earth were somehow magically transported
to another solar system, and in a trojan planet orbit, it would no longer
obviously be a planet. You are entitled to your opinion and so I won't
press this point any more.
>>> Based on how planets are generally accepted to form, though, I doubt
>>> borderline cases are going to be particularly common in the universe.
>>
>> Too soon to tell; the definition should broad enough to cover these
>> cases.
>
> If you think we _don't_ have a pretty good idea of how planets form, why
> is it a good idea to settle our definition of what a planet is _now_? That
> would make it likely we'd wind up with more misclassifications just like
> Pluto's.
I think we are just learning about how planets form and that there are a
large number of very diverse ways that planets can form. I explain it as
follows. There are many ways that one write a letter. It can be done using
pen and paper or it can be done using a word processor or it can be done by
dictation, etc. The definition of a letter does not depend on all the
constantly new and differing ways that creating a letter can be done. It is
my view that the definition of a planet should be like the definition of a
letter: the definition of the noun does not depend on the numerous ways it
can be formed. Obviously, you feel that the definition should depend on the
formation process and the problem I, and many others, have is that this
approach will end up with the definition being very complex after the
process of planetary formation is much better understood. Of course it is a
judgment call and so I won't try further to get you to see the virtues of a
different approach.
>> Round means spherical, as I have pointed out numerous times on this
>> thread (see the links on `round' that I provided). Nearly round means
>> nearly spherical and nearly spherical cannot cover the case of very
>> oblate.
>
> So I ask again, are a car's wheels round?
I don't remember you asking this before but, yes, they are round. The link
I pointed to defined round as circular or spherical.
> As Brett has been trying to explain to you, the parenthetical 'nearly
> round' bit is intended to help explain the preceding 'hydrostatic
> equilibrium' part for lay audiences. The hydrostatic equilibrium part is
> the important part. If your concept of 'round' in this case conflicts with
> the meaning of hydrostatic equilibrium, your concept of 'round' is wrong.
> Not much else to this one.
I fully realize what it was intended to do, my point is that it failed to do
it. Yes, the hydrostatic equilibrium part is the important part and if the
parenthetical were removed there would be no problem. But look again at the
phrase used "(nearly round) shape". It is clearly restricting the object to
be nearly spherical in shape (from webster's definition of round). My point
was that it isn't necessarily clear that Saturn qualifies as "nearly round"
but an argument can be made that it does (since 12% may be considered small
enough that it is "nearly"...) But in other solar systems there may be very
oblate worlds and a good generic definition of planets should apply to them
as well as planets in our solar system.
k
K_h
The original point I made in my first post was that it is not clear if
Saturn qualifies as a planet from the definition therefore the definition
should be cleared up. I agree that sane people will see that interpretation
1 should be chosen and my whole point is that the IAU's wording should be
changed to eliminate that ambiguity.
Look again at the phrase used "(nearly round) shape". It is clearly
restricting the object to be nearly spherical in shape (from webster's
definition of round). My point was that it isn't necessarily clear that
Saturn qualifies as "nearly round" but an argument can be made that it does
(since 12% may be considered small enough that it is "nearly"...) But in
other solar systems there may be very oblate worlds and a good generic
definition of planets should apply to them as well as planets in our solar
system. To reach that goal, the "nearly round" phrase should be omitted.
k