Habitable Exomoons

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.

Gas giants larger than Jupiter, with moons Earth-size or larger, are possible. Some researchers think the capacity to detect them—and even analyze them for habitability—may be just over the horizon. Earth-size moons could even form as planets and later be captured by a more massive gas 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 2020, 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.

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 factor significant for the human experience would be the length of the day. The movie Avatar's moon Pandora could exist.

Moon Rotation and Tidal Forces

 

Giant gas planet and earth-size moon in the habitable zone, H Graem © 2008

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 probably experience a rotation similar to that of our Solar System's giant gas planets. Moons orbiting this gas giant would be constrained by a number of 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 host 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 host giant planet, it will be subject to this tidal lock.

The day of a moon tidally locked on its host 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.

Using the four largest moons of our largest planet, Jupiter, as an example, the table to the left puts this situation in perspective. The moons are listed in the order of their orbital distance from Jupiter, the closest being Io. 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.

One concern with moons having orbital periods greater than 4 earth days is that the moon could be uninhabitable due to large swings in surface temperature between the day and night side. However, it is possible that even a modest amount of carbon dioxide in the moon's atmosphere would retain a tolerable temperature range despite having a day as long as several weeks. Clouds and large bodies of water should further moderate temperature extremes. 

If we want to find a moon with a day similar to that of earth, it will need to orbit very close to its host giant planet. In the case of Jupiter, Io comes closest with a rotational speed of less than two days. Tidal forces actively impact Io in ways beside its rotational speed, tidally induced heating of the moon's interior has created many large volcanoes which continually remake the moon's 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 host 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 host planet (as can easily be the case for a rocky moon orbiting a gas giant) then the Roche limit will be inside the host and hence not relevant. However, as we can see with 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.

One scientist has proposed an interesting aspect of this heating associated with tidal forces. Tidal heating may substantially expand the outer habitable zone of a star when a large moon is involved.

Prospective Moon Life

Future hypothetical moon orbiting a gas giant planet located in the
primary star's habitable zone

A noteworthy aspect of life on these moons is that inhabitants of the moon's near side will always see the planet in their sky. The far side's inhabitants will never see it if they never travel away from that side. 

Variable daily tides also 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 would rise and fall in response to these latter tidal forces. 

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 near 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, host giant planet and the 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.

Other Factors Effecting Possible Habitable Moons

Habitability of extrasolar moons will depend on stellar and reflected planetary illumination of the moons as well as the effect of eclipses on their orbit-averaged surface illumination. Beyond that, with short day moons, tidal heating could increase surface temperature, effecting a moon's habitability.

The magnetic environment of exomoons, which is critically triggered by the intrinsic magnetic field of the host planet, has been identified as another factor of exomoon habitability. Most notably, it was found that moons at distances between about 5 and 20 planetary radii from a giant planet could be habitable from an illumination and tidal heating point of view, but still the planetary magnetosphere would critically influence their habitability.