Living On The Habitable Edge
January 11, 2013

Could Life Actually Be Possible On Exomoons?

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April Flowers for - Your Universe Online

Astronomers searching for extraterrestrial life have begun to consider exomoons, or those likely orbiting planets outside our solar system, and new research has found that they are just as likely to support life as exoplanets.

René Heller of Germany's Leibniz Institute for Astrophysics Potsdam and Rory Barnes of the University of Washington and the NASA Astrobiology Institute conducted the new study which is to be published in the journal Astrobiology.

Scientists know of about 850 extrasolar planets outside of our solar system. Most of these are sterile gas giants similar to Jupiter, while only a few have a solid surface and orbit their host stars in the habitable zone. The habitable zone is the circumstellar belt at the right distance to possibly allow liquid surface water and a benign environment.

The team approached the theoretical question whether such planets could host habitable moons. The universe is so large and varied, that there is no reason to assume that just because no such exomoons have been located yet that none will be.

Because moons are typically tidally locked to their planet, the climatic conditions expected on extrasolar moons will likely differ from those on extrasolar planets. One hemisphere of each moon would also face the host planet, similar to our own Moon. Moons have two sources of light — the star and the planet that it orbits — and are 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.”

Tidal heating is another criterion for exomoon habitability. The moon's distance to its host planet triggers the additional energy source; the closer the moon to the planet, the stronger tidal heating. Strong tidal heating and thus a catastrophic runaway greenhouse effect that would boil away surface water and leave them forever uninhabitable will be suffered by moons that are too close to their host planet. The team devised a theoretical model to estimate the minimum distance a moon could be from its host planet and still allow habitability. They call this the "habitable edge," a concept that will allow future astronomers 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.