Alpha Centauri physics (crosspost)
Janne Salonen
On Sun, 25 Jan 1998, Brian Reynolds wrote:
> My question relates to the Alpha Centauri star system.
>
> Given a sun-like primary star (magnitude 0.33) and a smaller secondary
> (magnitude 1.7) orbiting it w/ perihelion 11 astronomical units and
> aphelion of 35 astronomical units (e.g. approx. Saturn to Pluto)
> completing an orbit about every 80 years.
>
> Now, say we want to imagine a world of roughly Earthlike size and
> temperatures.
>
> * Can such a planet maintain a stable orbit around the primary, and
> would that orbit differ significantly from Earth's in shape and/or
> radius?
I've understood that the real problem with the idea of an earth-like
planet around Alpha Centauri A or B is the difficulty of such a planet
forming at all in a "biozone" around the stars, because of the strong
tidal effects by, say Alpha Centauri B, on matter in an earth-like orbit
around Alpha Centauri A. These tidal waves may have prevented proper
planets forming at all in the orbits we're looking for, in the same way as
Jupiter, which is less than one per cent in mass of Alpha Centauri B,
managed to prevent a "proper" planet forming in the asteroid belt. And
Mars is in fact a planetary torso, probably because of Jupiter.
The likelihood that all there is in Alpha Centauri is a huge jumble of
asteroids, and maybe some gas giant orbiting both stars at a distance. But
maybe we could go and take a look, Alpha Centauri is going to get a little
closer over the next several millennia.
Personally I'm wondering whether the fact that Alpha Centauri is getting
closer might cause an increasing rate of comets entering the inner solar
system. Hale-Bopp may have been just such a comet: it did come from the
same part of the sky as Alpha Centauri.
Best regards
Mr Janne Salonen
Helsinki, Finland
Jedidiah Whitten
Brian Reynolds (brey...@firaxis.com) wrote:
: (I've crossposted this because I'm not sure which group an answer is
: most likely to come from. Apologies if you read more than one group; I
: won't do it again).
:
: My question relates to the Alpha Centauri star system.
:
: Given a sun-like primary star (magnitude 0.33) and a smaller secondary
: (magnitude 1.7) orbiting it w/ perihelion 11 astronomical units and
: aphelion of 35 astronomical units (e.g. approx. Saturn to Pluto)
: completing an orbit about every 80 years.
You're using the apparent magnitudes here, which are irrelevant if you're
talking about the physical properties of the stars. This is just how bright
they appear from Earth. You may want to use absolute magnitudes instead.
: Now, say we want to imagine a world of roughly Earthlike size and
: temperatures.
:
: * Can such a planet maintain a stable orbit around the primary, and
: would that orbit differ significantly from Earth's in shape and/or
: radius?
Yes, a planet at 1 AU would probably have a stable orbit. It may need to be a
little more than 1 AU from Alpha Cent A since that star is brighter than our
Sun, and a little less than 1 AU from Alpha Cent B, which is dimmer, if you
want the planet to receive the same amount of radiation. Both stars could
support an Earthlike planet.
: * Is the secondary a significant factor in terms of solar heat for this
: world? About how much difference in heat?
At minimum separation, the other star in the system would add about 1% to the
total radiation received. It would be significant, but not enough to preclude
the possibility of life, I think.
: * On its closest approaches, how much illumination will the secondary
: provide to the dark side of the planet?
It would be about 1% as bright as the Sun, or magnitude -22 or so at closest
approach. Much brighter than the Moon (-12 when full). The Sun is about -27.
I'm just working these figures out in my head, so they are not very precise.
At farthest separation, the secondary star will be about .02% as bright as the
Sun, which would be magnitude -16 or so.
Another important thing to consider is that although stable orbits exist around
each of the 2 stars, they are probably not stable enough to have allowed
planets to form. The other star would interfere with planet formation in the
same way Jupiter stopped a planet forming between it and Jupiter. So all that
might exist around these stars is an asteroid belt.
--
Jedidiah Whitten (jswh...@ucdavis.edu)
+------------------------------------------+
| University of California, Davis |
| http://wwwcsif.cs.ucdavis.edu/~whitten |
| http://wwp.mirabilis.com/6569964 |
+------------------------------------------+
Steven B. Harris
In <34CC1605...@firaxis.com> Brian Reynolds
<brey...@firaxis.com> writes:
>
>(I've crossposted this because I'm not sure which group an answer is
>most likely to come from. Apologies if you read more than one group; I
>won't do it again).
>
>My question relates to the Alpha Centauri star system.
>
>Given a sun-like primary star (magnitude 0.33) and a smaller secondary
>(magnitude 1.7) orbiting it w/ perihelion 11 astronomical units and
>aphelion of 35 astronomical units (e.g. approx. Saturn to Pluto)
>completing an orbit about every 80 years.
>
>temperatures.
>
>* Can such a planet maintain a stable orbit around the primary, and
>would that orbit differ significantly from Earth's in shape and/or
>radius?
No, you'd have a pretty Earthlike orbit about Alpha Centauri A,
with a distance of 1 AU, since this is a sister star to our own at G2
class.
Most computer problems looking at double stars find that planetary
orbits are stable around one star if the radius is less than about 1/5
of the distance between stars. That's influenced somewhat by the mass
of the stars, but with the much smaller and farther away Alpha Centauri
B, you'd bet essentially no problems. I'm willing to bet anything.
>
>* Is the secondary a significant factor in terms of solar heat for
this
>world? About how much difference in heat?
Nah. The ratio of luminosities for Alpha Centauri A and B is
about 1:4, and even at close approach you have 11 AU squared = 1/121 *
4 = about 1/500th. How can increasing the Sun's power by 1/500 make
any significant difference?
>
>* On its closest approaches, how much illumination will the secondary
>provide to the dark side of the planet?
Well, 1/500th of it. I don't know how that compares with a full
moon. Probably close.
>
>Thank you very much for your help. I am neither a physicist nor an
>astronomer, but rather a computer game designer. :-)
>
>If you can, please send me an e-mail of your reply as well as replying
>to the group.
>
>Brian Reynolds
>Firaxis Games
Jedidiah Whitten
Janne Salonen (jesa...@alpha.hut.fi) wrote:
: The likelihood that all there is in Alpha Centauri is a huge jumble of
: asteroids, and maybe some gas giant orbiting both stars at a distance. But
It would have to be quite a large distance. Just as planets orbiting a
single member of a double star system must be within 1/5th the minimum
separation between the two stars to have a stable orbit, a planet orbiting
both stars at once needs to be at least 5 times their maximum separation.
This is a rough estimate, but it implies that a gas giant would have to be
at least 400 AU from the two stars. I don't think there would be enough
material at that distance to form anything more than comets and
Kuiper-belt planetoids.
Steve Schaper
Janne Salonen (jesa...@alpha.hut.fi) wrote:
: Mars is in fact a planetary torso, probably because of Jupiter.
: asteroids, and maybe some gas giant orbiting both stars at a distance. But
: closer over the next several millennia.
I've read abstracts that seem to indicate that formation is not a problem
under those conditions. We have 16 Cygni B 1 as an existing instance.
--Steve
Frank Crary
In article <6ak0h4$a...@dfw-ixnews7.ix.netcom.com>,
Steven B. Harris <sbha...@ix.netcom.com> wrote:
>>* Is the secondary a significant factor in terms of solar heat for
>this
>>world? About how much difference in heat?
> Nah. The ratio of luminosities for Alpha Centauri A and B is
>about 1:4, and even at close approach you have 11 AU squared = 1/121 *
>4 = about 1/500th. How can increasing the Sun's power by 1/500 make
>any significant difference?
You might be surprised. Computer models of the Earth's climate suggest
that a 0.3% change in solar output would cause climate change. This
tends to come up when man-made versus natural climate change is debated.
(Solar variability seems to be under 0.3%, but it's close enough and
the models are uncertain enough, that the matter gets a fair amount of
attention.) I'd say a 0.2% change in heating due to Alpha Cent B
could cause some interesting weather/climate patters. But it certainly
isn't enough to make such a planet uninhabitable.
Frank Crary
CU Boulder
Frank Crary
In article <6ann4i$r5v$2...@news1.inlink.com>,
16 Cyg B may not be a good example. If memory serves, the distance
between the two stars is orders of magnitude greater than the
semimajor axis of the planet. In that case, the other star would
have very little effect on planetary formation. For Alpha Cent,
the perapsis distance of the two stars would be around 10 AU, which
could be close enough to prevent planetary formation near 1 AU.
Frank Crary
CU Boulder
radiospace
On 28 Jan 1998 01:42:38 GMT, fcr...@rintintin.Colorado.EDU (Frank
Crary) scribbled with the virtual quill:
If Alpha Centauri B has an interesting but not radical effect on this
fictional planet's weather and climate, and it has an 80-year orbit
around AC-A, does this mean that our little planet would have an
80-year cycle of climate change? (Warmer temperatures, more rainfall?
Bigger storms?).
What if the situation was reversed and the fictional planet is
orbiting Alpha Cent. B? What would be the luminence and climate
effect from Alpha Cent. A? (Clearly it would be greater {4
times??]since it is brighter, but what of the effect of the planet
orbiting about 1.7 AU closer/further away as it moved around AC-B, in
addition to moving closer/further away in tandem with AC-B's orbit
around AC-A?). If this isn't too confusing (sorry) it would be
interesting to speculate what the weather and climate might be like on
such a planet. (I'm guessing significant yearly variation due to the
1.7 AU difference and a possibly traumatic 80 year cycle?)
Patrick
GCFisheris
Brian Reynolds <brey...@firaxis.com> wrote:
>My question relates to the Alpha Centauri star system.
>
>Given a sun-like primary star (magnitude 0.33) and a smaller secondary
>(magnitude 1.7) orbiting it w/ perihelion 11 astronomical units and
>aphelion of 35 astronomical units (e.g. approx. Saturn to Pluto)
>completing an orbit about every 80 years.
>
>Now, say we want to imagine a world of roughly Earthlike size and
>temperatures.
A comment then a question.
This discussion about the Alpha-Centauri system seems to be pointing to the
idea that while stable orbits exist around these stars, tidal forces may have
prevented planet formation.
This strikes me as very good news. By the time human civilization has advanced
to the point where we are able to mount a colonization effort to another star
system we may not be a predominately planet-living people. Most of humanity
may be living in space habitats on asteroids or built from asteroidial
material. Colonists arriving at Alpha-Centauri would find:
- Two sources of solar energy, one very much like Sol.
- An unlimited supply of building material, none of it at
the bottom of a deep gravity well.
I have always had a prejudice for planets, but imagine if there were an
Earth-like planet around one of these stars with indigenous life. This would
be momentous, but from the standpoint of colonization (of the planet), a moral
and technological problem.
Now the question. I recall from earlier discussions that a major problem with
interstellar spaceflight is the need to slow down when you arrive at the
destination star. In a one-star system you are accelerated towards the star by
its gravity (the acceleration gained being balanced by the decelleration as you
pass by the star), but in a two-star system like Alpha-Centauri can you take
advantage of the orbital motions of the stars, especially when one star is more
massive than the other, so that you experience less gravitional acceleration as
you approach the system, and more gravitational deceleration as you pass
through and by the system? The idea is to enter the system with the less
massive star orbiting towards you, pass through the system (perhaps in such a
way that the less massive star alters your vector) and be flying away from it
with the more massive star orbiting towards you from the side and behind. I
cannot see that this obviates the need for some on-board deceleration system,
but it may allow some mass savings.
If this is feasible then having a double star system as our nearest neighbor is
truly good news for would-be interstellar colonists.
Gary Fisher
"Science is a collection of successful recipes." - Paul Valery
Frank Crary
In article <34d47837...@news.earthlink.net>,
radiospace <radio...@earthlink.netSPAM> wrote:
>>You might be surprised. Computer models of the Earth's climate suggest
>>that a 0.3% change in solar output would cause climate change. This
>>tends to come up when man-made versus natural climate change is debated.
>>(Solar variability seems to be under 0.3%, but it's close enough and
>>the models are uncertain enough, that the matter gets a fair amount of
>>attention.) I'd say a 0.2% change in heating due to Alpha Cent B
>>could cause some interesting weather/climate patters. But it certainly
>>isn't enough to make such a planet uninhabitable.
>If Alpha Centauri B has an interesting but not radical effect on this
>fictional planet's weather and climate, and it has an 80-year orbit
>around AC-A, does this mean that our little planet would have an
>80-year cycle of climate change?
Possibly. It wouldn't really be a 80-year cycle in the usual sense.
The orbit of Alpha Cent B is elliptical, so it would be reasonably close
to Alpha Cent A for perhaps a few years out of the 80 year period.
That could give you a few years of very unusual weather every 80 years.
Overall, this might or might not happen, and someone on that planet
might or might not notice. First, when climate modelers say a 0.3%
change in insolation could be significant, they mean that they turned
up the solar flux in their computer models by 0.3%, let them run,
and noticed a significant difference in the results. The models
could be wrong (in either direction), and it might take more than
a few years for a 0.3% difference to produce significant results.
As far as I know, no one has tried pulsing the extra heating on and
off. Second, the difference might not be noticeable over other
events. Here at the University of Colorado, we have a fairly large
climate modeling group. In the six years I've been here, they have
_always_ been talking about some unusual event causing unusual
weather: Only a few of those six years have been ``typical''. There
was a strong El Nino pattern in the first year or two of the 1990s,
and another one last year. There was a large volcanic eruption
in (I think) 1992, which supposedly was responsible for atypical
weather for a couple of years. So Alpha Cent B might cause unusual
weather on our hypothetical planet, for a few years out of every 80.
But there would be lots of other things causing unusual weather,
and it isn't clear if the effects of Alpha Cent B would be more
or less significant.
>What if the situation was reversed and the fictional planet is
>orbiting Alpha Cent. B? What would be the luminence and climate
>effect from Alpha Cent. A? (Clearly it would be greater {4
>times??]since it is brighter, but what of the effect of the planet
>orbiting about 1.7 AU closer/further away as it moved around AC-B, in
>addition to moving closer/further away in tandem with AC-B's orbit
>around AC-A?).
I'm afraid I don't have the orbital elements and luminosities handy...
But from what you have said, the effect on a planet around Alpha Cent B
would be stronger than the luminosity ratio implied. The larger
semi-major axis of the planet would mean that the closest approach
distance to Alpha Cent A would be smaller (i.e. than the closest
distance between a planet with the same average insolation around Alpha
Cent A and Alpha Cent B.) So the variability in insolation from the
second star would be greater.
Frank Crary
CU Boulder
Frank Crary
In article <19980129183...@ladder02.news.aol.com>,
GCFisheris <gcfis...@aol.com> wrote:
>Now the question. I recall from earlier discussions that a major problem with
>interstellar spaceflight is the need to slow down when you arrive at the
>destination star. In a one-star system you are accelerated towards the star by
>its gravity (the acceleration gained being balanced by the decelleration as you
>pass by the star), but in a two-star system like Alpha-Centauri can you take
Yes, you can play that game. (In fact, you can also play it with a single star
that has a large gas giant.) But it won't help much. At best, you can get
a change of velocity on par with the surface escape velocity of the second
star/largest planet. And to get that, you would have to skim just above the
surface of the star/planet. The surface escape velocity of a G2 V star
(i.e. the Sun) is 600 km/s. So you could use this to stop at Alpha Cent
if you traveled from the Earth to Alpha Cent at 600 km/s. At that velocity,
the trip would take about 15,000 years. In practice, you would need to
travel at a much higher velocity, and using a star's gravity to do the
last 600 km/s of deceleration wouldn't be significant.
Frank Crary
CU Boulder
Sebastian
GCFisheris <gcfis...@aol.com> wrote:
>Now the question. I recall from earlier discussions that a major problem with
>interstellar spaceflight is the need to slow down when you arrive at the
>destination star.
Might get a little hot though...
Maybe use some kind of electromagnetic shields to keep hot plasma away
from the hull?
I'm no physicist, sorry if that quick brainstorm is way off the mark.
Later.
--
Sebastian
=====< Stop the world, I want to get off! >=====
frank...@delphi.com
GCFisheris <gcfis...@aol.com> writes:
>This discussion about the Alpha-Centauri system seems to be pointing to the
>idea that while stable orbits exist around these stars, tidal forces may have
>prevented planet formation.
>
>This strikes me as very good news. By the time human civilization has advanced
>to the point where we are able to mount a colonization effort to another star
>system we may not be a predominately planet-living people. Most of humanity
>may be living in space habitats on asteroids or built from asteroidial
>material. Colonists arriving at Alpha-Centauri would find:
arguments about not having to expend the (generally expensive0 energy to
descend into, and raise mass out of planetary gravity wells. But why should it
always be so? If you have the energy to cross interstellar space at all, that
required for landing on and leaving planets should be trivial.
When steamships were developed, the direction and speed of wind or
current became far less signifigant than for sailing ships....
You may need planets for large quantites of some materials too, such as
hydrogen (in the atmospheres of gas giants) to refuel your ships.
>through and by the system? The idea is to enter the system with the less
>massive star orbiting towards you, pass through the system (perhaps in such a
>way that the less massive star alters your vector) and be flying away from it
>with the more massive star orbiting towards you from the side and behind. I
>cannot see that this obviates the need for some on-board deceleration system,
>but it may allow some mass savings.
to today. (That's why we rely on Jupiter for slingshot maneuvers to speed outer
planet probes.) But again, if you can cross interstellar distances in reasonable
times, the vast majority of your velocity changes will have been done before
you've even entered the other solar system. It's like landing a shuttle orbiter
into the wind. Desirable, but what it does to shorten the landing roll is a tiny
fraction of its deceleration from orbital velocity.
Now, if you could find a pair of close orbiting neutron stars, the delta-v
you could get there would be far more useful for interstellar departure/
approaches....
Frank
frank...@delphi.com
Sebastian <sent...@ionsys.com> writes:
>>Now the question. I recall from earlier discussions that a major problem with
>>interstellar spaceflight is the need to slow down when you arrive at the
>>destination star.
>
>How about aerobraking in the star's or gas giant's upper atmosphere?
>Might get a little hot though...
>Maybe use some kind of electromagnetic shields to keep hot plasma away
>from the hull?
Wars' stories where it was necessary to get a small relativistic ship to
Alpha Centauri quickly for an espionage mission against the Kzin who controlled
the human-colonized worlds there. When the pilot was shown that there were no
provosions for deceleration from near-lightspeed, he was told matter of
factly; "Simple. You'll stop by ramming into the Sun."
A little later (after a scene change) it was explained that the ship would
encase itself in a stasis field prior to impact (rendering it indestructible,
and stopping time within), and using the upper *layers* of Alpha Centauri A
to slow down.
Something similar happened in one of Vinge's `Realtime' stories, where a
similar field protected a man who was falling into the Sun (at much lower
speeds), but he was trapped there, following the convectiions of gas until,
thousands of years later, they happened to bring him near the surface where
a rescue was possible.
Of course this is physics and technology that may never be possible, but
the idea's not new, and has already been taken to the logical extreme.
Frank