|Giant gas planet visible in the sky of a terraformed moon in the habitable zone of an unknown star|
Giant gas planets the size of Jupiter or larger have been found in orbit within a star's habitable zone. Although conditions on such gas planets would be adverse to any earth-like life, an orbiting moon comparable to earth's mass could exist and provide a more friendly environment.
With gas giants larger than Jupiter, moons Earth-size or larger are not out of the question. Some researchers think the capacity to detect them—and even analyze them for habitability—may be just over the horizon. Such large moons could form as planets and later be captured by a more massive planet's gravity to become a satellite. NASA's successor to the Hubble Space Telescope, the James Webb Space Telescope (JWST), currently scheduled to launch in 2018, should open up the field of exomoons, assuming they are as abundant as theory predicts. It may even be able to resolve atmospheric constituents of those moons. Avatar's moon Pandora could be real.
Moon habitability depends on a number of factors. Existence within the habitable zone is the first requirement, adequate size to hold a substantial atmosphere is another. A variable significant to the human experience would be the length of the day.
Moon Rotation and Tidal Forces
In our Solar System, all the giant planets rotate about their axis in less than one earth day. Jupiter's day is less than 10 hours. A giant gas exoplanet orbiting in the habitable zone of a star of the sun's type would experience a rotation similar to that of our Solar System's giant gas planets.
Moons orbiting this gas giant would be constrained by very different forces. Where the moon is orbiting a giant planet the size of Saturn or larger, the moon's rotation will likely be restricted by its primary planet. Based on our experience with moons orbiting the Solar System's giant planets, the moon will experience a tidal lock (similar to our moon with one side always facing the earth) by the planet. Other things being equal, a large moon will lock faster than a smaller moon at the same distance from the planet. Unless a moon is unusually distant from its primary giant planet, it will be subject to this tidal lock.