Alien moon. An artist's conception of a hypothetical Earth-like exomoon in orbit around an extrasolar gas giant planet in the habitable zone of an alien solar system.

Dan Durda

Does Life Exist on Distant Moons?

Imagine life on an Earth-like moon, one so close to its gas giant host that its landscape is bathed in a dusklike planetary glow. Such places are not only possible but also probable, according to a new study, which finds that as many as 5% of gas giant planets orbiting their stars at Earth-like distances may harbor habitable "exomoons."

For the past decade or so, planet hunters have been scanning the skies, hoping to detect an Earth-like planet in a habitable orbit around its sunlike star. NASA's Kepler observatory is expected to detect dozens of such planets as they make their way across the face of their parent stars. But so far, most of the discoveries have been giant planets in Earth-like orbits, which are much easier to spot but too hot and gaseous to harbor life. The new study provides them with the possibility of life-supporting satellites.

Simon Porter, a predoctoral fellow at Lowell Observatory in Flagstaff, Arizona, and colleagues created computer models of 12,000 hypothetical planets about the size of Earth that had been gravitationally captured by gas giants like Jupiter. The simulated gas giants orbited their stars at just the right distance for life to be possible on these planets-turned-moons.

All of the simulated satellites started out on very elliptical orbits around their gas giants, the team will report in an upcoming issue of The Astrophysical Journal Letters. Over hundreds of thousands of years, half of them crashed into their planets or were spat out of orbit. But the other half evolved steady orbits, allowing them to develop stable climates over billions of years. Such climates are thought to be necessary for the evolution of complex life.

"It was eye-opening to see how many of the loose initial [orbital] configurations evolved into tight, stable orbits and how quickly they did it," says co-author Will Grundy, an astronomer at Lowell Observatory.

Astronomers have yet to spot any exomoons, however, and it's unclear how many Earth-like objects would be captured in the first place. That's "the main problem with this paper," says Alan Boss, a planetary scientist at the Carnegie Institution for Science in Washington, D.C. So far, he says, models suggest that the probability of such captures is low.

Porter counters that even a low capture rate could lead to plenty of habitable exomoons. If 10% of giant planets in habitable zones capture such Earth-like exomoons—which would likely come from the inner portion of a young solar system—and 50% of those moved into a stable orbit, then one in 20 giant planets in a sunlike star's habitable zone could harbor such moons. Kepler is expected to detect some 300 giant planets in or near habitable zones by the end of its 3½-year mission, notes Porter, adding roughly a dozen possible exomoons to the list.

Whether life could survive on such moons is another question. Jupiter and other exo-gas giant planets are thought to have strong radiation belts that could stymie life in the immediate vicinity. However, as Grundy notes, water and solid dirt block most radiation. That means life underground or undersea might thrive. Even so, Jack Lissauer, a planetary scientist at NASA Ames Research Center in Moffett Field, California, says that heat generated soon after the moon was captured would boil water off the natural satellite. "The body would need to have had an ample inventory of water prior to capture in order to retain an inventory sufficient to support life."

The real challenge, says Lisa Kaltenegger, an astronomer at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts, will be to identify such potentially habitable moons after finding them. That would require observing their atmospheres, which is not an easy task.

*This article originally stated that the Kepler observatory orbited Earth. We have corrected this phrase.