Astroblog: What will etxrasolar planetary atmospheres be like?

Wednesday, June 27, 2007

What will etxrasolar planetary atmospheres be like?

Over at Centauri Dreams, there is a very nice post on modelling exoplanetary atmospheres. It is pointed out that, as you move further away from the sun, the more water planets have. As well, the formation history of the planet plays a role. The small rocky planets Venus, Earth and Mars lost most of their original atmosphere when during the initial accretion phase, when rocky plantessimals slammed into the growing planets. Their subsequent atmospheres are a result of outgassing of volatile material from the accumulated rocky material and incoming comets, and weathering reactions between the atmosphere and the planet.

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).

More likely is that Gliese 581c formed further out beyond the “Snow line” where water ice forms in the protoplanetary disk, and migrated in to its current position (based on the observation of at last one other “Hot Neptune”). Then Gliese 581c atmosphere is probably going to be more like Neptune when it initially migrates to its current orbit. Neptune’s atmosphere is mostly hydrogen, helium and about 3% methane. With Gliese 581c d so close to its Sun, its atmosphere should evolve rapidly. The methane will be rapidly broken down and the hydrogen largely lost (although the hydrogen escape limit is higher for a larger mass world than earth, Gliese 581c is hotter). Gliese 581c will also have a lot of water in its composition, and will probably be covered in a global ocean, with little or no exposed land. As weathering reactions remove a lot of the carbon dioxide from Earths atmosphere, what the final atmospheric composition of a water world is not clear, but a high pressure atmosphere with at least a substantial concentration of CO₂ is entirely plausible. Look at Titan, an ice world with a thick nitrogen atmosphere.

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

Labels: exoplanet, extrasolar planet