my review of that movie, I was alerted (via a comment) regarding a book that gave some relevant details about the physical characteristics of the world of Pandora.
As it turns out, Pandora is a moon orbiting a gas giant called Polyphemus. What surprised me is that Polyphemus belongs to the Alpha Centauri trinary system. The largest star, Alpha Centauri A (or ACA, as they call it) serves as the main sun of Pandora. The third companion Alpha Centauri C, as it is written in that book, is a red dwarf. Obviously, it is not accurate with regards to the real Alpha Centauri system which is a binary system. A third star called Proxima Centauri, the nearest star to our sun is believed to be gravitationally linked to the Alpha Centauri system.
The floating mountains which i found quite odd, had some explanations on why it floats. The Hallelujah Mountains, as it is called, are laced with "Unobtainium" a high-temperature superconducting element that is levitated by powerful magnetic fields generated by Pandora's swirling molten core.
I do find the name of that fictitious substance funny, and for a perfect reason: Unobtainium may never be obtained, because it will never occur in nature, well at least in the earth-like temperature of Pandora. Atoms would never align uniformly to make a perfect superconductor specially with Pandora's climate. And not even the frigid icy moon like Titan or Europa would produce anything close to superconductors.
More details can be found in that book regarding the Geology, Atmosphere and Chemistry of Pandora, and still more from around the web.
Of course, nothing would sway me from simply enjoying the movie's impressive special effects. I would simply stop my mind from analysing and just admire the view. I would watch it again in 3D and immerse myself in a world of imagination, the way it's supposed to be.