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Some 4.5 billion years ago, the dwarf planet Pluto was suddenly joined by a companion. For a very brief period \u2014 perhaps only hours \u2014 they danced as if arm in arm before gently separating, a grand do-si-do that resulted in Pluto and its quintet of moons orbiting the sun together today.<\/p>\n
Astronomers have long wondered how Charon, the largest of those moons came to orbit Pluto. A paper published Monday in the journal Nature Geoscience described a possible sequence of events that may resolve the question.<\/p>\n
\u201cThe reason that Pluto and Charon are so interesting is because Charon is 50 percent the size of Pluto,\u201d said Adeene Denton, a planetary scientist at the University of Arizona who led the paper. \u201cThe only comparable system is Earth and its moon.\u201d<\/p>\n
Charon is about 750 miles across, while Pluto is nearly 1,500 miles in diameter. That proportion in sizes suggests that a number of conventional scenarios explaining how moons form are unlikely, including theories that Charon formed from debris around Pluto or was captured by its gravitational pull. Could Charon\u2019s existence instead be explained by the kind of collision that is believed to have formed Earth\u2019s moon?<\/p>\n<\/div>\n<\/div>\n
The sizes of Pluto and Charon meant that it was difficult to work out how they \u201cdidn\u2019t just merge like two blobs of liquid,\u201d the most likely outcome of such an explosive scenario, said Erik Asphaug, also a University of Arizona planetary scientist and a co-author on the paper.<\/p>\n
Pluto and Charon are in a region of the outer solar system beyond Neptune called the Kuiper belt, which makes them both very rocky and icy. By including these properties in their model, the research team devised a scenario where the two bodies collided and became ensnared without merging.<\/p>\n<\/div>\n<\/div>\n
If Charon hit Pluto at a relatively sedate speed of about 2,000 miles per hour \u2014 10 times as slow as the Earth\u2019s moon-forming impact \u2014 the two would have remained in contact for about 10 hours before gradually separating but remaining together. The researchers described this encounter as a \u201ckiss and capture.\u201d<\/p>\n
The toughness of the two bodies prevented them from breaking apart, Dr. Denton said.<\/p>\n
Pluto would have been rotating once every three hours at the time (the length of a day on Pluto today is some 150 hours), so the two would have swung around three times while joined together. The angular momentum of the spinning Pluto would then have pushed Charon slowly away but, crucially, left it trapped in Pluto\u2019s orbit.<\/p>\n<\/div>\n<\/div>\n
Bill McKinnon, a planetary scientist at Washington University in St. Louis, said that such a scenario \u201cmakes sense\u201d given the large number of objects believed to be drifting around the Kuiper belt in the early solar system. \u201cCollisional capture is probably a common process,\u201d he said, with many other large binary objects also thought to exist in the Kuiper belt.<\/p>\n
The impact would have meant that \u201cbasically the entire surface of Pluto gets resurfaced,\u201d Dr. Denton said, with Charon losing most of the ice on its surface to its companion. \u201cThe impact is a geologic reset to the system,\u201d she added.<\/p>\n
It might also have resulted in the formation of Pluto\u2019s four other known moons \u2014 Nix, Styx, Kerberos and Hydra \u2014 which are tiny compared with Charon and were glimpsed when NASA\u2019s New Horizons spacecraft flew past Pluto in 2015.<\/p>\n
The team\u2019s model could offer a new explanation for how some moons end up orbiting other worlds. \u201cIt adds a new twist on the physics,\u201d Dr. Asphaug said. \u201cWe\u2019ve had this idea that strength doesn\u2019t matter in collisions. We have to revisit that assumption, even for our moon\u2019s formation.\u201d<\/p>\n
Studying Pluto in more detail might tell us whether its cosmic dance really did take place, although it is likely to be a long time before another spacecraft visits the dwarf planet.<\/p>\n<\/div>\n<\/div>\n
\u201cIf Charon deposited some of its rock into Pluto, you would be able to see that in gravity data,\u201d said Dr. Denton. \u201cUnfortunately we would need to go back to Pluto to test this.\u201d<\/p>\n<\/div>\n<\/div>\n