Pandora
Pandora is Avatar's main place. This moon is effectively the grandiose theater of film's events. It's graviting around the planet Polyphemus. We know that its diameter is 11 447 km, its mass is 4,299.10²⁴ kg, its mean temperature is 30°C and its gravity is 0.72 times Earth's gravity.
1.Evocated places presentation
Polyphemus
Polyphemus is an exoplanet (planet outside the solar system), around which revolves Pandora. This gas giant gravitates around Alpha Centauri A, to 4.36 light years from Earth. It has physical caracteristics (mass, size, type...) similar to Jupiter.
Alpha Centauri A
Alpha Centauri A is actually an existing star since it's a part of Alpha Centauri's system. This binary system contains Alpha Centauri A, Alpha Centauri B (which are spinning one around the other) and Proxima Centauri, a red dwarf star loose-linked to the sytem. Its presence doesnt really matter for us.
Alpha Centauri A is similar to our Sun. Its mass is approximatively 1,100 times Sun's one, and its, like the Sun, a yellow dwarf star. Its habitable zone is included between 1 and 1,8 astronomical unit (one AU is equivalent to Earth-Sun distance).
2.Can a planet gravitate around a binary system ?
At first glance, it seems difficult to stably revolve a body in a system made of two stars. In fact, the gravitational force exerted by the stars (which depends of their mass) could cause problems and throw a wrong positionned planet far far away. That would happen, for exemple, if the planet in question was gravitating forming a 8 around the two stars, or if it was gravitating to close from one of the stars. It's orbite wouldn't be stable. That is definitly not what we're looking for with Polyphemus. So, we have searched a mean to give it a stable orbit.
It appears that a planet can, in a binary system, both revolve around the two stars (by forming an isosceles triangle whose base is the distance between the star 1 and the star 2) as around one of the two (but in this case at good distance not to be influenced by the gravitationnal attraction exerted by the second star). Polyphemus seems to be able to gravitate around Alpha Centauri A, whith the good orbite.
3.Conditions to life development
Comparison of the sizes of the different stars we're interested in
Many conditions have to be united to allow life development. So we will study temperature, mass et optimal force of gravitational tides.
The good temperature
Pandora must have a temperature that allow liquid water on the planet, in order to welcome life at macroscopic scale. So, it's temperature has to be between 32°F and 100°F.
The book entitled Encyclopédie d'Avatar inform us that the mean temperature of the planet is about 99°F, and that's an ideal temperature for life apparition. However, Pandora is a moon that gravitates around a planet, that gravitates around a star. We can expect big thermical amplitudes, which are not present in the film.
Our explanation is that the greenhouse effect maintains a constant temperature at Pandora's surface. In fact, the atmosphere of this planet is composed of 18% of azote, 18% of oxygen, 18% of carbon dioxide, 5,5% of xenon, 1% of methan, 1% of hydrogen sulfide and 61% of hydrogen cyanide.
So, there's lots more carbon dioxyde on Pandora that on Earth (whose atmosphere is composed with 0,03% of CO2). We can think all these gas are coming from Polyphemus (gas planet).
The strong presence of hydrogen cyanide (mortal gas) can also explain the toxicity of Pandora's atmosphere for humans. For pandorian life, the only choice we have is to think that they are insensible to this gas.
Optimal mass
Pandora's mass is 4,299.10²⁴ kg, that means 0,72 times Earth's mass. It's optimal for life development at human scale and also bigger, the constraint of the gravitational force is less important.
Tidal force
It is a force that compresses and stretches, in some directions, the objects that gravitate in a system. For exemple, let's take a look at the earth and the Moon. The Moon is rotating around the earth, because it's lighter. The intensity of gravitational force depends of the distance between the two objects, so the nearest point of the earth from the Moon is more attracted than the furthest. It's like a stretching of the Earth. On Earth, this stretching is translated by the presence of tides ; the oceans are attracted by the Moon when they are at the nearest point from it. The sun produces the same effect on our planet, but it's less visible because the Sun is further than the Moon.
This diagram sums up what we just said :
Now we're interested in Roche limit, the distance below which a satellite disintegrates under the action of tidal force caused by the body around which it's orbiting. In other words, a moon rotating around a planet would by disintegrated if it was under this limit.
It's complex to calculate this limit because it hangs into account the internal constitution of the planet. But according to cieletespace.fr (where the film images are analyzed), Pandora is 3,5 times above this limit.
