Extrasolar moons will support life just as likely as exoplanets

With thousands of exoplanets discovered in just the past few years, showing that planetary systems are actually the norm in our galaxy and universe, it is very likely that there are also countless moons. Scientists now published research on the question if and how those moons could support life in the January issue of Astrobiology. They found that moons orbiting planets outside the solar system are just as likely to support life as exoplanets.

The two scientists, Rene Heller of Germany's Leibniz Institute for Astrophysics Potsdam and Rory Barnes of the University of Washington and the NASA Astrobiology Institute, tackled the question purely theoretical, since exomoons have yet to be discovered, asking the question whether exoplanets could at all host habitable moons, and how they could look like, with what implications from being moons around giant planets.

The topic is becoming relevant, since the first moons are expected to be found soon thanks to the exquisite photometric precision of NASA's Kepler space telescope. The detection of a Mars- to Earth-size extrasolar moon could be imminent. Since 2012 the first dedicated "Hunt for Exomoons with Kepler" is in progress.

Nearly one thousand extrasolar planets are already known, with several times more candidates, and many of them are gas giants, similar to Jupiter, defined as hostile to life. Only a few have a solid surface and orbit their host stars in the 'Goldilocks' zone, an orbit at the right distance to potentially allow liquid surface water and a benign environment.

Similarly, Heller and Barnes started to define a concept will allow to evaluate the habitability of extrasolar moons. "There is a habitable zone for exomoons, it's just a little different than the habitable zone for exoplanets," Barnes said. Their findings suggest that there is a "habitable edge," a minimum distance a moon could be from its host planet to still allow habitability, in a given context.

The climatic conditions assumed on extrasolar moons would differ from those on extrasolar planets, since 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 the planet they orbit. But they are also subject to eclipses that could significantly alter their climates, reducing stellar illumination. "An observer standing on the surface of such an exomoon would experience day and night in a totally different way than we do on Earth." explained Heller. "For instance stellar eclipses could lead to sudden total darkness at noon."

Heller and Barnes 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 exact title of the study: "Exomoon Habitability Constrained by Illumination and Tidal Heating," in the journal Astrobiology.