New planet GJ 1214 b
Alain Fournier
A new planet, GJ 1214 b, has been found some 40 l.y. away
with 2.678 Earth radius and 6.55 Earth mass.
[It was found using a 16-in telescope and a commercially
available camera, kind of something someone could have
done in his backyard, that's cool even if the planet is
hot].
What surprises me is its density, about 1870 kg/m^3, which
is consistent with 75% water and 25% rock. I'm no expert
in this area, but so much water (or other low density
liquid or solid) seems improbable to me. Does anyone
know about how the radius was measured? Is it possible
that it would be a much denser planet with a very large
atmosphere. The atmosphere giving it nearly half its
radius?
Does anyone think that 75% water for such a large planet
is a likely scenario.
Alain Fournier
James Nicoll
Dan Birchall <d...@naoj.org.REMOVE_TO_REPLY> wrote:
>alai...@sympatico.ca (Alain Fournier) wrote:
>> What surprises me is its density, about 1870 kg/m^3, which
>> is consistent with 75% water and 25% rock. I'm no expert
>> in this area, but so much water (or other low density
>> liquid or solid) seems improbable to me.
>
>density is roughly comparable to that of our outer planets -
>Neptune's density is 1760 kg/m^3, Pluto's is 2030 kg/m^3. The
>planet's orbital radius and temperature would probably help shed
>light on what elements would most likely be found there in solid,
>liquid or gaseous forms.
>
The six most common elements in the universe are hydrogen,
helium, oxygen, carbon and neon*. If a world isn't massive/cold enough
to hold onto H2 (and given that neither helium nor neon will form
chemical compounds) the most common chemical compounds should
be made up of combinations of H, O and C. In other words, water
should be pretty common.
* From memory, by mass it's something like 70% H, 24% He, 11% O, 4.5% C
1% Ne.
--
http://www.livejournal.com/users/james_nicoll
http://www.cafepress.com/jdnicoll (For all your "The problem with
defending the English language [...]" T-shirt, cup and tote-bag needs)
Alain Fournier
> In article <slrnhijgq...@my-286.myhome.westell.com>,
> Dan Birchall <d...@naoj.org.REMOVE_TO_REPLY> wrote:
>
>>alai...@sympatico.ca (Alain Fournier) wrote:
>>
>>> What surprises me is its density, about 1870 kg/m^3, which
>>> is consistent with 75% water and 25% rock. I'm no expert
>>> in this area, but so much water (or other low density
>>> liquid or solid) seems improbable to me.
>>
>>I'm not a planetary scientist either, but I googled, and this
>>density is roughly comparable to that of our outer planets -
>>Neptune's density is 1760 kg/m^3, Pluto's is 2030 kg/m^3. The
>>planet's orbital radius and temperature would probably help shed
>>light on what elements would most likely be found there in solid,
>>liquid or gaseous forms.
>>
>
> The six most common elements in the universe are hydrogen,
> helium, oxygen, carbon and neon*. If a world isn't massive/cold enough
> to hold onto H2 (and given that neither helium nor neon will form
> chemical compounds) the most common chemical compounds should
> be made up of combinations of H, O and C. In other words, water
> should be pretty common.
>
>
> * From memory, by mass it's something like 70% H, 24% He, 11% O, 4.5% C
> 1% Ne.
But this planet is near to its star and not very big. Close to the
star water not already on a planet there is likely to be a gas and
blown away. So I have a hard time imagining water accreting to form
a planet there. I would expect that you would need a big rock
core before it can hold on to water. Being so close to the star
the hypothesized 25% rock seems barely enough. Mars has a hard
time holding on to its water and it is much colder, Venus didn't
hold on to much water. I guess the planet could have been formed
in two steps.
1- The 25% rock forms a nucleus (that is a little more than Earth's mass).
2- The 75% water can now accrete to the nucleus.
But I have a hard time imagining the water staying in low stellar orbit
long enough for that to happen, wouldn't it be blown away?
Alain Fournier
James Nicoll
My impression is that worlds like these form beyond the frost line and
then migrate. In our solar system, there's reason to think Uranus and
Neptune have migrated outwards but in other systems, large worlds can be
seen in orbits where it appears unlikely they could have formed. Presumably
they migrated in.
A popular account of this process can be found here:
http://en.wikipedia.org/wiki/Planetary_migration
Jonathan Thornburg [remove -animal to reply]
> A new planet, GJ 1214 b, has been found some 40 l.y. away
> with 2.678 Earth radius and 6.55 Earth mass.
The key technical papers are
Charbonneau et al,
"A super-Earth transiting a nearby low-mass star"
http://arxiv.org/abs/0912.3229
and
Rogers & Seager
"Three Possible Origins for the Gas Layer on GJ 1214b"
http://arxiv.org/abs/0912.3243
--
-- "Jonathan Thornburg [remove -animal to reply]" <jth...@astro.indiana-zebra.edu>
Dept of Astronomy, Indiana University, Bloomington, Indiana, USA
"Space travel is utter bilge" -- common misquote of UK Astronomer Royal
Richard Woolley's remarks of 1956
"All this writing about space travel is utter bilge. To go to the
moon would cost as much as a major war." -- what he actually said
Steve Willner
Alain Fournier <alai...@sympatico.ca> writes:
> But this planet is near to its star and not very big. Close to the
> star water not already on a planet there is likely to be a gas and
> blown away. So I have a hard time imagining water accreting to form
> a planet there.
There's a press release with a few more details at
http://www.cfa.harvard.edu/news/2009/pr200924.html
and of course the full article is in this week's _Nature_.
The article doesn't actually say, but the planet diameter ought to
come from the transit timing and independently from the transit
depth. In any case, the ratio of planet to star diameter has an
error bar of less than one percent according to the article. The
error bar on planet density (which includes a variety of other
uncertainties) is about 21%.
The authors consider the time for atmospheric escape and estimate a
maximum of 700 Myr, whereas the age of the system is likely 3-10 Gyr.
The authors suggest that either the planet may have had much more
massive atmosphere in the past (most of which has now been lost) or
that the current atmosphere has recently been outgassed from the
planet's core. It wouldn't surprise me if there are other
possibilities.
The authors say nothing about formation, but we can speculate that
the planet may have formed relatively far from the star and migrated
inwards. It still seems to me that preserving a volatile atmosphere
for such a long time is difficult to explain.
--
Help keep our newsgroup healthy; please don't feed the trolls.
Steve Willner Phone 617-495-7123 swil...@cfa.harvard.edu
Cambridge, MA 02138 USA
Alain Fournier
Thanks to James Nicoll, Jonathan Thornburg and Steve Willner for their
interesting replies.
I went to read the article
http://arxiv.org/abs/0912.3229
provided by Jonathan, I had search for the article earlier but got to
a site where one has to pay to have access, I thought I would have to
wait until I went to work (where we have subscriptions to a great variety
of scientific journals) to see the article, but thanks to Jonathan's link
I got to see it today.
They state both the 75% water 25% rock hypothesis I saw in the press release
and the much denser planet with a large atmosphere as I was hypothesizing in
my original post.
Alain Fournier
Alain Fournier
So this is a planet which can be detected with amateur's
instruments. (16-in telescope and off the shelf commercial
camera) But it doesn't seem to be known whether it is a
mostly gas planet with a small heavy core or a 25% rock
75% water planet.
How could we distinguish between these two possibilities?
Alain Fournier
Steve Willner
Alain Fournier <alai...@sympatico.ca> writes:
> it doesn't seem to be known whether it is a
> mostly gas planet with a small heavy core or a 25% rock
> 75% water planet.
>
> How could we distinguish between these two possibilities?
Observe transits at a wavelength where the atmosphere is expected to
be transparent?
It wouldn't surprise me if the authors have already requested
Director's Discretionary Time to observe with _Spitzer_, but no
programs are in the database as of now. Proposals for General
Observer time will be due in April. However, depending on the
atmospheric composition, I'm not sure it will be transparent at
3.6 microns, much less at 4.5 microns.