> Daniel Carrera wrote: >> 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. > > To expand on gravity: Anything planet from about 0.1 Earth masses up to > maybe 10 earth masses >ity altogether, it is not something we need to fine-tune very > carefully.
We don't really get the opportunity. The anthropic principle applies. We are here because Earth supports life. That's about all we can say. > Some readers may know that Mars has 0.1 Earth masses and it has minimal > atmosphere. First I note that Mars does have some atmosphere, it's just > very thin. So that proves that it can hold on to one. A thin one in which we can't survive and there seems little evidence that anything else has. Low gravity is not > the only probable reason why Mars lost its atmosphere. It is likely that > if Mars had had a stronger magnetic field, like Earth does, it would > have retained its atmosphere. Keep in mind that the main thing that > removes the atmosphere from the inner planets is not heat but the solar > wind. You can compensate for lower gravity by having a stronger magnetic > field. So the planet has to have a magnetic field, be the right distance from the sun and have reasonable temperature with not too big a variation, water, be the right gravity range and a few other things too. And in order to communicate with any other civilisation in a planet's 4 billion year history we have a window of maybe a few hundred years. Star Trek doesn't seem too likely ;-) Ian --------------------------------------------------------------------- To unsubscribe, e-mail: [EMAIL PROTECTED] For additional commands, e-mail: [EMAIL PROTECTED]
