Some giant gas planets have been found in orbit within a star's habitable zone. Although conditions on such planets would be adverse to any earth-like life, an orbiting moon comparable to earth's mass could provide a friendly environment.

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 exoplanet orbiting in the habitable zone of a star of the sun's type would be too distant from the star to experience a tidal lock. Its rotation would most probably resemble that of our Solar System's giant gas planets.

Moons orbiting this giant exoplanet 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.

The day of a moon tidally locked on its primary planet will equal its time in orbit about the planet. The moons orbiting closest to the planet will have the fastest orbital speed and thus the shortest day. For most moons orbiting giant planets (except for those rare instances where there is no tidal lock), the length of the moon's day will be dependent on the orbital distance to the planet.

Tidal Lock with Jupiter
	Io	Europa	Ganymede	Callisto
Moon Radius (km)	1,815	1,569	2,631	2,400
Distance from Jupiter (km)	421,600	670,900	1,070,000	1,883,000
Orbital Time (Earth days)	1.769138	3.551181	7.154553	16.68902

Using the four largest moons of our largest planet, Jupiter, as an example, the table to the right puts this situation in perspective. The moons are listed in the order of their orbital distance from Jupiter, the closest Io being first. All four are tidally locked and thus rotate about their axis in the exact same time that they orbit Jupiter. Rounding off, their respective rotational times are 1.8, 3.6, 7.2, and 16.7 Earth days.

As can be seen, if we want to find a moon with a day similar to that of earth, it will need to orbit very close to its primary giant planet. In the case of Jupiter, Io comes closest with a rotational speed of less than two days. These tidal forces actively impact Io in ways beside its rotational speed, with large volcanoes continually remaking the surface.

Where the moon's orbit is circular, such propinquity results in a fixed tidal bulge on the side facing the planet. Where the orbit is more elliptical, the moon will approach and recede from the planet at regular intervals. The changing gravitational forces will result in a rhythmic compression or kneading of the moon's innards, generating a lot of heat. A moon close enough to the primary planet to rotate in one day would experience significant volcanic and earthquake disruption as these powerful tidal forces heated its interior.

Roche Limit - Another limitation on very close moon orbits is the Roche limit, the closest distance an object can come to another object without being pulled apart by tidal forces. The Roche limit is the orbital distance at which a satellite will begin to be tidally torn apart by the body it is orbiting.

If a planet and a satellite have identical densities, then the Roche limit is 2.446 times the radius of the planet. For Jupiter, the Roche limit for a moon the same density as Jupiter is 175,000 kilometers.

If the satellite is more than twice as dense as the primary (as can easily be the case for a rocky moon orbiting a gas giant) then the Roche limit will be inside the primary and hence not relevant. However, as we can see from Io, although the moon may survive at a distance equivalent to a day's rotational speed, its insides will certainly be shaken up by the tidal forces.

Variable daily tides will move across the moon due to the local sun, or suns, and other large moons. These fellow moons exert varying gravitational forces as their orbits bring them closer or further from our subject moon. As on Earth, oceans on the moon's surface could rise and fall in response to these latter tidal forces.

A noteworthy aspect of life on these moons is that inhabitants of one hemisphere will always see the planet in their sky. The other side's inhabitants will never see it.

Interesting Possibilities

An obvious difference from life on earth will be the frequency and length of solar eclipses. Given the size of the planet, inhabitants of a moon (those living on the side facing the planet) should experience numerous and lengthy periods when the planet blocks their view of the sun. Eclipses will be most frequent when all three bodies, the primary star, giant planet and our subject moon, are in the same orbital plane.

Given that large moons occur in groups among the gas giants in our Solar System, habitable moons could also occur in sets of two or more per planet. The prospect of two or more habitable, Earth-size moons in orbit around a giant super Jupiter certainly stimulates the imagination.

Known Candidate Giant Planets

Described below are giant planets discovered within the habitable zone of stars similar to our sun. Although life as we know it is unlikely on the planets themselves, their larger moons could contain an environment conducive to life. They are listed in the order of their distance from earth.

55 Cancri f

The planet 55 Cancri f is twice the mass of Neptune, or half the mass of Saturn, almost definitely a gassy or icy giant, almost certainly not a solid-surfaced world. It is one of five planets discovered around this star. The star, 55 Cancri, (44 light years from earth) is a main-sequence G type star, like our own sun, but a little cooler.

The planet is fourth out from the star in a possible habitable zone. This habitable zone is closer to the parent star than our sun's given the star's cooler temperature. 55 Cancri f is an average 0.738 AU from the star with a a year of 261 days. The orbit is slightly eliptical (0.2) with an average distance from the star similar to Venus (year of 224 days) in our own system.

HD 128311 b

HD 128311 is a orange to red type K star slightly cooler than the sun, 54 light years from earth. It has two known planetary companions. One of them, HD 128311 b, has an average distance (1.01 AU) from its star almost exactly equal to that of the earth from the sun.

Its mass has been calculated as equivalent to 2.63 Jupiters. However, the orbit is moderately eliptical with an eccentricity of 0.25 and a year's orbit equal to 448 days. The planet moves inward to the equivalent of Venus and outward to a distance equivalent to halfway beween Earth and Mars. Given the slightly cooler character of the star, a moon in orbit of this planet may be habitable with seasons across its entire surface.

HD 142 b

The star HD 142 is 67 light years from earth and could be an identical twin of our sun. A single planet has been found in orbit. The jovian size planet (one third larger than Jupiter) is within the habitable zone, with a year slightly shorter than earth's (338 days) and an average distance from the star of 0.98 AU.

The orbit is significantly eliptical (0.38), dipping in closer than what Venus would be and then moving outward toward an orbit close to Mars. Orbital eccentricity is a potential barrier to habitability. However, it is unclear to what extent orbital eccentricity would result in yearly seasons supportive of life rather than be deadly to its existence.

HD 92788 b

HD 92788 is a yellow dwarf star in the constellation Sextans and 107 light years from earth. It is more massive and slightly smaller than the Sun with high metallicity.

The planet HD 92788 b is a gas giant about the size of 3.86 Jupiters. The average distance to the star (0.93 AU) is slightly less than earth's to the sun. It has a year of 378 days. The planet has a moderate eccentricity of 0.27. Despite the eccentricity, the entire orbit is potentially within the habitable zone.

HD 28185 b

HD 28185 is a yellow dwarf star of the G5 type similar to our Sun located about 129 light-years away. The planet HD 28185 b is 5.7 times Jupiter in mass with a year of 383 days and an eccentricity of orbit of 0.07.

The slightly eliptical orbit (1.03 AU) moves inward about halfway between an Earth/Venus equivalent and then slightly outward of Earth equivalent. HD 28185 b is the first exoplanet discovered with an almost circular orbit within its star's habitable zone.

While it is unknown whether gas giants such as HD 28185 b can support life, simulations of tidal interactions suggest that HD 28185 b could harbour Earth-mass satellites in orbit around it for many billions of years. Such moons, if they exist, may be able to provide a habitable environment. Additionally, a small planet in one of the gas giant's Trojan points could survive in a habitable orbit for long periods.

The amplitude of the radial velocity oscillations used by scientists to determine a planet's mass yields only a minimum value on that mass. Determining the actual mass requires knowledge of the planet's orbital inclination to our line of sight. In the case of HD 28185 b, the true mass of the planet may be much greater than this lower limit of 5.7 times Jupiter's mass.