What makes it Vulcanlike, though, is that it circles its star, Kepler-10, once every 20 hours, in an orbit a twentieth the size of Mercury's.

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Once the two GRAIL satellites are in orbit, their next step is to begin a five week phase in which they increase the speed at which they orbit the Moon.

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Once again, these worlds were identified using data from Nasa's Kepler telescope, which was put in orbit in 2009 with the specific goal of hunting down small planets.

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