Can a moon be as large as Earth? If so, it might support life. Our solar system's biggest moon—Jupiter's Ganymede—is only 41% of Earth's diameter, but other solar systems could sport larger satellites (artist's conception shown). Now, as astronomers report in work submitted to The Astrophysical Journal, they've used data from NASA's Kepler spacecraft to study seven planet-hosting stars, finding no moons at all. For four of the planets, the data are sufficiently good to rule out moons as small as Earth. The result suggests that Earth-sized moons are rare, but the sample is tiny—and so the search for supermoons will continue.
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