<i>I'm surprised at how hard it is to find info on Proxima and Alpha centauri...</i><br /><br />You and me both. Though I'm sure there must be one, I've never really heard a good and convincing argument explaining why so little attention has been paid to our nearest neighbor.<br /><br />Did you check out the
Alpha Centauri 3 site, by the way? Just now I took a look and found: '<i>Accounting for infrared radiation, the distance from Proxima where an Earth-type planet would be "comfortable" with liquid water is around 0.02 to 0.06 AU (Endl et al, 2003, in
pdf) -- much closer than Mercury's orbital distance of about 0.4 AU from Sol -- with an orbital period of two to 16 days.</i>'<br /><br />But, since much of this is speculation - or science fiction - then, just as Severian suggests, there's plenty of wiggle room. For instance, why can't a mild greenhouse effect be in play? Your atmosphered moon could have a thick atmosphere. Also, your moon is clearly planet-sized, a situation along the lines of Niven's Jinx. The world with the biosphere could be more massive than Earth. The entire Proxima I (or II?) system masses around 254 times the mass of Earth. The life-bearing world - planet, moon, call it what you will - masses at around 1.3 or so what Earth does, but is only a little bigger. Make it a little more dense than Earth to give it a higher gravity, that thick atmosphere, and plenty of metals.<br /><br />A planet similar to Jupiter may command an impressive magnetic field. Jupiter's reaches 2 or 3 million miles toward our sun, easily big enough to encompass its moons. So why not a 36-day orbit? That's basically a little more than double Titan's 16-day orbit about Saturn (placing it somewhere between Hyperion and Iapetus, if your world were part of Saturn's system - somewhere out between Callisto