Astronomers have started to consider exomoons, or those likely orbiting planets outside the solar system, as possible habitable worlds.
"The detection of moons orbiting extrasolar planets (“exomoons”) has now become feasible. Once discovered in the
circumstellar habitable zone, questions about their habitability will emerge. Exomoons are likely to be tidally locked to their
planet, and hence experience days much shorter than their orbital period around the star, and have seasons – all of which
works in favor of habitability," astronomers explain in their paper.
The research, conducted by René Heller of Germany's Leibniz Institute for Astrophysics Potsdam and Rory Barnes of the
University of Washington and the NASA Astrobiology Institute, will appear in the January issue of Astrobiology.
About 850 extrasolar planets — planets outside the solar system — 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.
Heller and Barnes tackled the theoretical question whether such planets could host habitable moons. No such exomoons
have yet been discovered but there's no reason to assume they don't exist.
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 the planet they orbit — 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.”
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.
Click on image to enlarge
Artist’s conception of two extrasolar moons orbiting a giant gaseous planet. (Credits: R. Heller, AIP)
They 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.
"There is a habitable zone for exomoons, it's just a little different than the habitable zone for exoplanets," Barnes said.