This Week In Science writes "When Pluto's moon, Charon, occulted a distant star on July 11, 2005 two groups of astronomers were there to witness the fleeting moment. Based on what they saw, the scientists were able to determine that Charon has very little atmosphere, has a radius of 605 km, and is about 1.7 times denser than water. The density calculation implies that Pluto and Charon were once two separate proto-planets that because of a chance encounter are now bound by gravity."
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