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Astronomers have discovered planets orbiting distant stars, but they've never found a moon outside of our solar system. NPR's Nell Greenfieldboyce reports that the hunt for the first alien moon is getting serious, in part because moons might be good places to look for life.
NELL GREENFIELDBOYCE, BYLINE: Do you take our own familiar moon for granted? David Kipping doesn't.
DAVID KIPPING: We're lucky to have the moon. I love looking up at the moon. It's lovely to look up in the sky and be able to see with your own eyes features of another world.
GREENFIELDBOYCE: Kipping is an astronomer at Columbia University. And he says our moon is more than just a pretty face. It controls our ocean's tides. It keeps the tilt of our planet stable. Take away our moon, and the climate goes haywire.
KIPPING: So we have a lot to thank the moon for.
GREENFIELDBOYCE: And a lot of reasons to hunt for more moons beyond our solar system. After all, the presence of a moon might make a planet more Earth-like. And if a rocky moon was in a sweet spot that let it have liquid water on its surface, it could be its own little habitable world. Around five years ago, Kipping started thinking about how to find alien moons.
KIPPING: I think it was seen as a little bit wild. I don't know that I would say that I was ever mocked. But, certainly, like, OK, fine, he wants to do that, but he's probably wasting his time sort of thing.
GREENFIELDBOYCE: Since then, attitudes have changed because of NASA's Kepler mission. Kepler is a space telescope that was sent up in 2009 to hunt for planets. The telescope was designed to stare at a bunch of stars, looking for little eclipses that happen when a planet passes in front of the star. Kipping says Kepler has been an incredible success.
KIPPING: Kepler has discovered planets which are as small as our own moon.
GREENFIELDBOYCE: Suddenly, hunting for moons seemed a lot less crazy but still pretty daunting. Kipping heads a small team that's doing the only systematic search. They take each planet that Kepler detected and check for subtle signs of a moon. So far, his team has surveyed about 60 planets and found no moons. But their hunt is speeding up. They now have more experience and access to one of the world's fastest supercomputers. Kipping says by the end of this year, he expects to have searched for moons around a thousand planets. He says the search ought to be good enough to catch even some pretty small moons, like the ones that orbit Jupiter.
KIPPING: If we have that sensitivity that we expect to have and we don't see anything, I would be very surprised.
GREENFIELDBOYCE: If they do find a moon, you know what the very first question is going to be.
RORY BARNES: Is there life on it?
GREENFIELDBOYCE: Rory Barnes is an astrobiologist at the University of Washington in Seattle. He says to have a shot at knowing whether a moon that's light-years away might be habitable, you'd like to see certain characteristics.
BARNES: I'd certainly like to see a moon that is at least the size of Mars. When you get to that size, then you can start talking about moons that maybe hold onto an atmosphere for a long period of time.
GREENFIELDBOYCE: A telescope could study that atmosphere and look for signs of living, breathing moon creatures. Other moons would be harder to study and will remain more mysterious. Even in our own backyard, Jupiter and Saturn are circled by icy moons that are thought to have hidden underground oceans. But at the moment, scientists can only wonder what might be swimming around down there.
BARNES: It could be that 100 years from now, we'll know that the earth is one of a handful of bodies in the solar system that is habitable, and most of them are moons.
GREENFIELDBOYCE: He says the same could hold true for other solar systems. When it comes to places that life could call home, planets may be outnumbered by moons. Nell Greenfieldboyce, NPR News.
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