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About 850 extrasolar planets are known, and most of them are sterile gas giants, similar to Jupiter. Only a few have a solid surface and orbit their host stars in the habitable zone, the circumstellar belt at the right distance to potentially allow liquid surface water and a benign environment.

Rory Barnes of the University of Washington and the NASA Astrobiology Institute, and René Heller of Germany’s Leibniz Institute for Astrophysics Potsdam tackled the theoretical question whether such planets could host habitable moons.

In a new study to be published in the journal Astrobiology, the pair has found that exomoons are just as likely to support life as exoplanets.

The climatic conditions expected on extrasolar moons will likely differ from those on extrasolar planets because moons are typically tidally locked to their planet. Thus, similar to the Earth’s moon, one hemisphere permanently faces the planet.

Beyond that, moons have two sources of light—that from the star and from the planet they orbit—and are subject to eclipses that could significantly alter their climates, reducing stellar illumination.

Heller and Barnes also identified tidal heating as a criterion for exomoon habitability. This additional energy source is triggered by a moon’s distance to its host planet—the closer the moon, the stronger tidal heating.

Moons that orbit their planet too closely will undergo strong tidal heating and thus a catastrophic runaway greenhouse effect that would boil away surface water and leave them forever uninhabitable.

The scientists also devised a theoretical model to estimate the minimum distance a moon could be from its host planet and still allow habitability, which they call the “habitable edge.” This concept will allow future astronomers to evaluate the habitability of extrasolar moons.

The precision of NASA’s Kepler space telescope now makes the detection of a Mars- to Earth-sized extrasolar moon possible, perhaps imminent. Launched in 2009, the telescope enabled scientists to reveal thousands of new extrasolar planet candidates.

Source: University of Washington