Life has prospects on moons of giant extrasolar planets

Earthlike bodies may orbit distant Jupiters

Earth-sized moons in planetary systems trillions of miles away could be hotbeds for alien life, astronomers report in the January Astrobiology.

EXOLIFE Jupiter-like gas giants in other solar systems (illustrated in orange) may host moons that could support life (foreground). R. Heller/AIP

“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 that we’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 able to 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 of both 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 of energy.

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’s average 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 of all 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 and sterilize 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 he announced 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 that it will be a long time before astronomers can test the study by probing specific moons for water, carbon dioxide or other signatures of life. Such a feat would require a telescope with enough resolution to filter out the light from stars and planets and focus exclusively on a moon. “You can postulate that habitable moons are out there, but you’ll never be able to check it,” he says.

Heller concedes that investigating individual exomoons will be difficult, though he points to a recent study asserting that NASA’s James Webb Space Telescope, scheduled for launch in 2018, should be able to dissect moons surrounding small, dim stars close to Earth.

Despite sharing Kasting’s concerns, Williams focuses on the fact that the paper opens up yet another avenue for alien life to thrive. “Moons just improve the chances that life as we know it exists elsewhere,” he says. “The diversity of environments that you can have is just amazing.”


T. Dubé

The pattern of light and darkness on an exomoon would be very different from the relatively consistent 24-hour day-night pattern that prevails on much of Earth. Most exomoons could be expected to keep one face toward their host planet, just as Earth’s moon does. That side would cycle from complete darkness when eclipsed by the planet (right) to a twilit state (left), as starlight reflected onto it from the top of the planet’s atmosphere. The exomoon’s near side would also always receive heat radiated by the planet itself. The exomoon’s far side would cycle from direct starlight to complete darkness with every orbit. In the exomoon’s quarter phases (top and bottom), different parts of the surface would be exposed to various combinations of direct and reflected starlight.

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