PLUTO鈥橲 moon Charon could be a chunk of the planet that was blasted off when Pluto collided with another large body early in the life of the solar system. A similar process is thought to have formed Earth鈥檚 own moon.
鈥淎n impact is the simplest way to form the Pluto-Charon pair, which makes it appealing,鈥� says Robin Canup, at Southwest Research Institute in Boulder, Colorado. Using Charon鈥檚 present orbit and Pluto鈥檚 spin, Canup estimated that the planet once sped through a full rotation every 2 to 4 hours. Today, Pluto takes 6.4 days to rotate, and the most likely cause of the slowdown was a large object smashing into the planet.
Canup鈥檚 team calculated that the body was probably 1600 to 2000 kilometres in diameter, only slightly smaller than the planet itself. It may have come from the Kuiper Belt 鈥� the ring of icy rocks on the fringes of the solar system, where Pluto also resides.
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