Wednesday, June 27, 2007
What will etxrasolar planetary atmospheres be like?
The giant outer planets have atmospheres very similar to the composition of the primordial solar nebula, as the larger mass worlds could hold onto more of the primordial atmosphere.
For exoplanets, we expect similar relationships to hold. Large, Jovian style worlds should have atmospheres very similar to Jupiter or Saturn for example. However, in many extrasolar systems the worlds have migrated inwards to much hotter regions, so their atmospheres will probably have evolved to some degree from that of Jupiter. Hot super Jupiters like HD 149026b are very dark, as dark as charcoal, suggesting their atmosphers are no longer Jupiter standard .
To give an idea of how this may affect a world, lets take the super-earth Gliese 58 c. Just to refresh your memory, Gliese 581 C is a 1.5 Earth radii planet that was originally thought to be in the habitable zone of its host star, but outside the orbit of a "hot Jupiter" Gliese 581b. The history of Gliese 581c's formation will be critical to its atmospheric composition. If Gliese 581c was formed from the debris of a terrestrial-like world shattered when Gliese 581b migrated to its present star hugging location (see Mandell 2007), Gliese 581c would have lost most of its original atmosphere, and had to develop a new one from out gassing of volatiles in the rocks that formed it. So if Gliese 581c is a terrestrial world, then it could have a predominantly CO2 atmosphere. How dense is a good question, as we still don't have a clear idea of the original atmosphere s of Earth, Venus or Mars when they formed, anywhere between between 1 Bar and 20 Bars (where 1 Bar is the atmospheric pressure of current Earth, Venus is around roughly 90 Bar).
So while we have a fairly good idea of what an exoplanetary atmosphere may be like, the evolution of these worlds atmospheres may be far from simple, and surprises may wait us when we finally get in a position to look at the m in some detail.
Formation of Earth-like Planets During and After Giant Planet Migration Mandell AM et al. The Astrophysical Journal, volume 660, part 1 (2007), pages 823–844