Is a habitable moon like Pandora possible?
Many of us remember the movie Avatar and the lush habitable moon called Pandora that orbited a gas giant in the Alpha Centauri system. Is something like that possible? That's a good question that has suddenly come to the forefront of the task of finding habitable worlds for humans to colonize. Such a moon would be known as an Extrasolar moon, and it would more than likely be orbiting a massive gas giant around a star similar to our sun.
The articles I've read about this idea indicate that such a moon would have to be near Earth size, have a sizable atmosphere and liquid water on its surface. These are what one would predict for a habitable planet.
But, for it to be a moon, very special conditions would be necessary for habitability. For one, the gas giant that the moon orbits would have to be in the habitable zone of the star it orbits, and since the moon would have to be in orbit around the gas giant far enough away to prevent extreme tidal effects such as upheaval or flexing of the crust, the problem of a stable orbit that would insure a stable climate would become the main issue with such a moon. Think of it this way. Let's say that the moon orbits out from a Jupiter sized planet a million miles. A good example of this type of moon is Ganymede, the largest Jupiter moon. Ganymede has a magnetosphere, probably from a molten iron core. The moon is tidally locked to Jupiter, which means that it keeps one side facing Jupiter. Now, this wouldn't cause the moon to not have a night and day but it would be long days and nights because Ganymede takes seven days to orbit Jupiter once. Some theories say that the habitable moon wouldn't be habitable if the moon's orbit was longer than four days because the atmosphere would be subjected to too much heat from the star during the long day and then cool too much during the long night.
One theory proposes that a habitable moon in a binary star system would make the moon more habitable because two close stars' solar winds would buck each other and thus help lessen radiation effects on the moon from the stars. The good news is that most stars belong to binary systems. The other thing to consider is that a star's habitable zone would be further out from the main star in a binary system because of the heat and light from the secondary star. That's good because a star could easily rip the moon away from a close gas giant. I might add here that no exomoons have been discovered, but I think it's safe to say that there are large moons orbiting gas giants that are orbiting stars in the habitable zone. This might be the norm for most solar systems. Many of the planets found around distant stars are gas giants closely orbiting the star, suggesting that large moons might be the usual case.
The other thing that a habitable moon must have is a strong magnetosphere because gas giants blast out massive amounts of energetic particles and these could easily sputter (erode the gas) the atmospheres of habitable moons. Remember that Jupiter has a massive magnetosphere that forms radiation belts that would sputter away the atmosphere of an orbiting moon in only a few hundred million years.
Some theories say that a Mar's sized exomoon would face many problems depending on its orbital distance and whether is has a magnetosphere, but not as many problems as an Earth-sized moon. This is based on the problem of gravitational forces between the moon and gas giant. At this point, these are only theories. Perhaps in the next few years, astronomers will be able to locate large moons orbiting gas giants around distant stars. Until then, we can only guess.
Thanks for reading.