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Message: <blockquote data-quote="Agemegos" data-source="post: 2809598" data-attributes="member: 18377">The trick actually requires a certain escape velocity, and the escape velocity of a sphere is equal to its surface gravity times its radius. So moons (which are presumably small) need higher surface gravity than planets (which are presumably larger).For a world to be habitable (and to stay terraformed after it has been terraformed) the escape velocity has to be low enough that excess hydrogen and helium (with molecular weights of 2 amu and 4 amu respectively) will boil off by the process called (if memory serves) Jeans Escape, but that water vapour (with a molecule weight of 18) will not. This depends on the mean velocity of the molecules in a gas at the temperature of the atmosphere being well less than escape velocity, and molecular velocity is inversely proportional to molecular weight (the temperature dependency is fixed by the narrow range of comfortable temperatures. Anyway, the margin between the molecular weights of helium and water vapour is more than a factor of fourt, which gives a comfortably wide margin. Judging by the fact that helium is rare in Earth's atmosphere (but abundant in the primordial nebula) we can say with fair confidence that if a planet were the same size as Earth by 1/4 the gravity (requiring 1/4 the density) all its water would undergo Jeans escape, and that if it were the same surface gravity as Earth but 1/4 the size (requiring four times the density) the same would happen. Given that any habitable planet is likely to be about as dense as Earth (maybe a little less dense, like almost all the other objects in the Solar System), the lower bound is somewhere above a surface gravity of 1/2 g.An object with an equable surface temperature that has less than  1/2 the surface gravity is Earth is going to lose its all its water to Jeans escape in a time that is short compared to the age of the Earth. So it is overwhelmingly likely that any such object discovered is going to be desiccated and deficient in hydrogen. But Jeans Escape is a slow process by human standards, and it is conceivable that such an object much be terraformed by hauling in huge quantities of ice from out where temperatures are lower (and then introducing algae to make oxygen out of the ice and waiting a century or so). If this were done, it might retain its artificial atmosphere for a time long compared with human lives.So we might see a terrafomed world with less than 1/2 of Earth's surface gravity.</blockquote>

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