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Life has prospects on moons of giant extrasolar planets

Earth-sized moons in planetary systems trillions of miles away could be hotbeds for alien life, astronomers report in the January Astrobiology.
"It's the most thorough look at exomoon habitability I've seen," says Darren Williams, an astronomer at Penn State Erie who was not involved in the research. “I’m encouraged by the paper thatwe’ll find exomoons in abundance and that a fraction of them could be habitable.”  
Astronomers have found about 3,600 confirmed or probable planets orbiting other stars, none of which have the ideal combination of size and temperature to support life. However, more than 150 of them are gas giants in orbits where liquid water could exist, if only it had a solid surface to puddle on. Life might be ableto survive on the rocky moons of those Neptune- and Jupiter-like planets.
This bounty of temperate giants led astronomers René Heller of Germany's Leibniz Institute for Astrophysics Potsdam and Rory Barnes of the University of Washington to examine all the factors that determine the habitability of exomoons. Moons are substantially more complicated than planets because they are at the mercy ofboth their host planet and star: The star pelts them with radiation, and so does the reflection off the top of their planet’s gaseous clouds. (Jupiter, for example, reflects about a third of solar radiation that strikes it.) Moons also get squeezed and deformed by the gravitational pull of their massive planetary companions, a phenomenon called tidal heating that supplies yet another source ofenergy.
These interactions with planet and star lead to some downright odd conditions. At times one region of the moon could have both the star and planet in its sky, providing plenty of energy, while another region would be in complete darkness. Minutes- or hours-long eclipses of the star by the giant planet would be a frequent occurrence. Nonetheless, Heller and Barnes found that an exomoon’saverage influx of energy could be sufficient to support life.
Perhaps the biggest obstacle to the habitability of extrasolar moons is their size. Heller and Barnes show that a moon needs to be roughly the mass of Earth to maintain an atmosphere and a magnetic field that could deflect deadly radiation from the giant planet next door. That could be a tough requirement to meet: Jupiter's moon Ganymede,the largest moon in the solar system, is only 2 percent as massive as Earth.
However, astronomers have proposed ways that giant planets could develop bulkier moons. In a soon-to-be-published study, Williams describes how gravitational interactions could lead to a gas giant capturing a terrestrial planet that would then become its moon. Heller agrees that such moons should exist: "When I think ofall the weird planets we've found — hot Jupiters, planets orbiting two stars — why shouldn't we be able to find a large moon around a gas planet?"
Heller and Barnes combined all these factors to come up with a new measure called the habitable edge, the minimum distance between a given planet and moon that would allow for life on the moon. Get any closer and tidal heating will take over andsterilize the moon. Heller says that when astronomers eventually discover exomoons in an environment warm enough for liquid water, they can use the bodies’ properties to see if there is a chance for life.
David Kipping, an astronomer at the Harvard-Smithsonian Center for Astrophysics, is leading a dedicated search for exomoons based on data from the planet-hunting Kepler space telescope. Last week heannounced at an American Astronomical Society conference in Long Beach, Calif., that his analyses of seven Kepler planets showed no signs of moons, but he plans on studying over 100 more planets. “It’s brilliant to see this kind of research done,” Kipping says of Heller and Barnes' study.
Penn State planetary scientist James Kasting shares Kipping’s optimism over the new work, but he laments...