In the hunt for another Earth, maybe we should turn to retro-futuristic science fiction and consider living on the moon. Well, not our moon, but a moon orbiting one of the thousands of exoplanets we've identified in the past few years. If you ask some astrophysicists, it's time to start looking for exomoons.
Thanks to NASA's Kepler mission, we've identified more than 4,600 likely exoplanets, but most of them wouldn't be suitable for human life. They're gaseous, or too big, or else not within the habitable zone: an orbit around a sunlike star that would make a planet warm enough without getting too hot. About 800 exoplanets we've identified are Earth sized, but only a few dozen are potential Goldilocks planets: terrestrial, Earth-sized, and within the habitable zone.
But if we broaden our search to include the moons that might be orbiting these exoplanets (even the huge, gaseous ones), some astrophysicists think we might have better luck at finding a life-supporting world. At the very least, we'd increase the potential pool for a New Earth by a significant margin.
"In terms of numbers, they could actually be the most abundant habitat in the universe," René Heller, an astrophysicist who specializes in exomoons at McMaster University in Canada, told me over the phone. "Moons might actually outnumber planets as possible habitats."
Though we haven't been able to identify any exomoons yet, Heller said a look at our own solar system is a good indication that many exoplanets have moons—and probably multiple moons. He and fellow McMaster astrophysicist Ralph Pudritz have run simulations to study how young planets larger than Jupiter might move as they are formed. They predict that these planets could have multiple, water-covered, Mars-sized moons that could potentially make good candidates for supporting life. Their research has yet to be published, though two papers have been accepted for publication in the journals Astronomy & Astrophysics and The Astrophysical Journal, respectively.
But even if distant moons don't end up being good candidates for another Earth, Heller explained that studying moons can tell us a lot, from the state surrounding how a planet formed to the likelihood of collisions in a system.
Heller said astronomers are now starting the process of searching for these exomoons, using data already collected by Kepler. He's hopeful that data is detailed enough that they might be able to find exomoons, but they are also looking forward to searching with the European Space Agency's PLATO space telescope, to be launched in 2024, and the ground-based European Extremely Large Telescope, currently under construction.
"I have the impression that we as exomoon scientists are now at the stage that exoplanetary scientists were at in the early 90s, when discovery was possible and no one knew if it would happen today or in a few months or years," Heller said. "The detection of the first exomoon is now looming on the horizon."