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I have this little project of my own to determine to how extent is Avatar's story-line scientifically possible. Pandora, where the story-line of Avatar takes place is depicted as a moon orbiting around the gas giant Polyphemus that orbits around the star Alpha Centauri A.

In this project, I have to find a large amount of facts unspecified/not explicitly specified in the film. Among these many facts, one would be the size of the orbit of Polyphemus, the gas giant which Pandora orbits around.

We currently know that Alpha Centauri A is a star slightly larger than our Sun, and it's one of the stars in a binary system. The size of the orbit of Polyphemus will tell us how far Pandora is from its host star, and thus how much radiation it gets. Then we can determine whether or not Pandora can support life.

So, how far is Pandora (and Polyphemus) from its host star, Alpha Centauri A?

This will be the start of a series of questions (a multiple question, now deleted) I have.

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  • You're doing this the wrong way round. You can't use the distance of Pandora from Alpha Centauri A to determine whether or not it can support life, because we already know it can support life. What you can do is use the fact that it can support life to determine its distance from Alpha Centauri A. Since Alpha Centauri A's luminosity is 152% that of our sun, and the Earth is near the inner edge of the habitable zone, it's probably somewhere between 1.5 and 2.5 AU. – Mike Scott Sep 15 '14 at 14:38
  • @MikeScott you have the inverse square law the wrong way 'round. – Nick T Sep 15 '14 at 14:42
  • @MikeScott be careful, as gas giants may emit more energy that they absorb from their main star, therefore invalidating those calculus. V.g. Jupiter. A more massive planet would irradiate more heat (with the upper limit being the planet becoming a brown dwarf). – SJuan76 Sep 15 '14 at 18:23
  • @SJuan76 Yes, that's a fair point, in which case it could be significantly further out. Except looking at the comments on the answer, with 18% CO2 in the atmosphere and a 2AU upper bound for stability, it can't be picking up much heat from Polyphemus or it would be too hot. – Mike Scott Sep 16 '14 at 5:56
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Nothing is specifically stated on the "JC's Avatar" Wikia page, but it does say Polyphemus orbits in the habitable zone of Alpha Centauri A (ACA). ACA is only slightly larger (110%) and brighter (152%) than the sun, so...

[A]ny suspected Earth-like planet around Alpha Centauri A would have to be placed about 1.25 AU away – about halfway between the distances of Earth's orbit and Mars's orbit in the Solar System – so as to have similar planetary temperatures and conditions for liquid water to exist.

—Alpha Centauri / Theoretical planets

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