Ian Lynch wrote:
* The reason the atmosphere is composed as it is is partly because of gravity. Light atoms like hydrogen and helium at Earth temperatures go fast enough to escape, Nitrogen and Oxygen are slower so stay captured. So for life as we know it you need a planet not only the right distance from a similar star to the Sun but also one that has the right density to have the gravity to get the atmospheric composition right.
To trap hydrogen and helium you'd need a gas giant. Pretty much any terrestrial planet bigger than Mercury will trap everything from Oxygen onwards and fail to trap Hydrogen and Helium unless it is cryogenic cold. I think we have quite a lot more leeway on gravity compared to other factors.
For example, get the planet just a little too close to the Sun and you get a run-away greenhouse effect like Venus. In terms of size and composition Venus and Earth are nearly identical, but Venus has 100 times the atmospheric pressure of earth, and not a drop of water. The greenhouse effect made all the oceans evaporate. Having water in the atmosphere means that solar wind separates hydrogen and oxygen. Hydrogen escapes, and oxygen binds with carbon to make CO2. Earth has as much carbon as Venus, but ours is trapped in limestone - thank God.
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