The search for habitable worlds expands beyond planets, to moons

Credit: Rene Heller
Credit: Rene Heller
Artist rendering of two extrasolar moons orbiting a giant gaseous planet.

The search for life beyond our solar system tends to focus on finding familiar ground—a rocky, Earth-like world that sits in its star’s habitable sweet spot, where water is liquid and life could thrive.

But as the hunt for habitable worlds has gotten closer to successfully detecting planets that are the right size and distance from their stars to meet that criteria, some scientists have begun looking elsewhere: to those planet’s moons.

So far, efforts to detect those faraway moons, called exomoons, have not been successful. David Kipping, an astronomer at the Harvard-Smithsonian Center for Astrophysics who is leading the year-old Hunt for Exosolar Moons, recently reported that his team had been unable to detect moons circling their first seven candidate planets, using data from the NASA Kepler space telescope.

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That’s not a surprise; it took awhile for scientists to detect the first planets circling other stars. It’s also no trivial feat to use Kepler data to find a moon, since the telescope wasn’t designed for that purpose. Kipping said the telescope allows the team to search for moons that are about a fifth of the mass of the Earth, but noted that in our own solar system, the largest moon is Jupiter’s Ganymede, which is about 2.5 percent as heavy as the Earth.

Even though such moons don’t exist in our neighborhood, the hunt is worth it, Kipping said, because larger moons are more likely to be able to hold onto an atmosphere. Taking into account planets’ moons may also expand scientists’ basic notions of the conditions and spots in the universe that could support life.

A paper to be published this month in the journal Astrobiology recently described the factors that would determine whether moons in other solar systems could support life.

In that work, Rene Heller of the Leibniz Institute for Astrophysics Potsdam and Rory Barnes of the University of Washington introduced the concept of a “habitable edge” that could be used as a tool to estimate whether moons might provide a welcoming environment for life.

Those authors detail the ways in which moons could expand astronomers’ notions of habitable worlds. For example, planets that are themselves too cold and far from their suns to support life may have moons that are perfectly suited to life. Imagine a giant planet with a moon that’s quite close to it, such as Jupiter’s volcanic moon, Io. Jupiter itself is far from the sun’s warmth. Its clouds measure about minus-200 degrees. But Io experiences something called tidal heating, due to the gravitational pull that the massive planet exerts on its moon. This is what fuels its volcanic activity, with lava temperatures soaring above 2,000 degrees.

Gravity, Kipping said, is doing the equivalent of kneading the surface of the planet, like a piece of dough.

“In that case it’s too extreme, too hot,” Kipping said. “But you can imagine a bit cooler moon, way outside of the traditional ‘habitable zone’ that we think of.”

Life on such a moon would also have to grapple with very different daily patterns. It would receive light not only from a sun, but also from the light reflected from the planet. There would also be more frequent eclipses as the planet blocked the light from the sun, and thus more episodes of cooling.

In order to detect such moons, Kipping needs more data. Kepler scientists detect planets by looking for the dimming that occurs as the planets cross between the telescope and the star, blotting out a bit of the star’s light. They need about three planet crossings to confirm a planet. But to find a moon, scientists would need to be able to detect even minor variations and wobbles in the planets’ transits. For example, a moon would exert its own gravitational pull on the planet, meaning that the planet might sometimes appear to speed up in its journey across the star, or slow down. The variation could be on the order of seconds or minutes, so researchers need to observe more than just a small handful of planetary transits.

Kipping said there’s also the possibliity that, depending on what position the moon is in, it could cause an extra dimming of light, off to the side of the planet. By the end of 2013, he and his team hope to evaluate 50 planets in their search for habitable moons.