"Think "moon" and you probably envision a desolate, cratered landscape, maybe with an American flag and some old astronaut footprints. Earth's moon is no place for living things. But that isn't necessarily true for every moon. Whirling around Saturn, Enceladus spits out geysers of water from an underground ocean. Around Jupiter, Europa has a salty, subsurface sea and Titan has lakes of ethane and methane. A handful of the roughly 150 moons in the solar system have atmospheres, organic compounds, ice, and maybe even liquid water. They all seem like places where something could live—albeit something weird.
So now that the Kepler space telescope has found more than 1,000 planets—data that suggest the Milky Way galaxy could contain a hundred billion worlds—it makes sense to some alien-hunters to concentrate not on them but on their moons. The odds for life on these so-called exoplanets look a lot better—multiply that hundred billion by 150 and you get a lot of places to look for ET. "Because there are so many more moons than planets, if life can get started on moons, then that's going to be a lot of lively moons," says Seth Shostak, an astronomer at the SETI Institute.
Even better, more of those moons might be in the habitable zone, the region around a star where liquid water can exist. That's one reason Harvard astronomer David Kipping got interested in exomoons. He says about 1.7 percent of all stars similar to the sun have a rocky planet in their habitable zones. But if you're talking about planets made out of gas, like Saturn and Jupiter, that number goes up to 9.2 percent. Gaseous planets don't have the solid surfaces that astronomers think life needs, but their moons might…"
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