Pluto may have captured its biggest Moon after an ancient dance and kiss

Some 4.5 billion years ago, the dwarf planet Pluto was suddenly joined by a companion. For a very brief period -- perhaps only hours -- 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.

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.

"The reason that Pluto and Charon are so interesting is because Charon is 50 per cent the size of Pluto," said Adeene Denton, a planetary scientist at the University of Arizona who led the paper. "The only comparable system is Earth and its moon."

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's existence instead be explained by the kind of collision that is believed to have formed Earth's moon?

The sizes of Pluto and Charon meant that it was difficult to work out how they "didn't just merge like two blobs of liquid," 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.

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.

If Charon hit Pluto at a relatively sedate speed of about 2,000 mph -- 10 times as slow as the Earth's moon-forming impact -- the two would have remained in contact for about 10 hours before gradually separating but remaining together. The researchers described this encounter as a "kiss and capture."

The toughness of the two bodies prevented them from breaking apart, Denton said.

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's orbit.

Bill McKinnon, a planetary scientist at Washington University in St. Louis, said that such a scenario "makes sense" given the large number of objects believed to be drifting around the Kuiper belt in the early solar system. "Collisional capture is probably a common process," he said, with many other large binary objects also thought to exist in the Kuiper belt.

The impact would have meant that "basically the entire surface of Pluto gets resurfaced," Denton said, with Charon losing most of the ice on its surface to its companion. "The impact is a geologic reset to the system," she added.

It might also have resulted in the formation of Pluto's four other known moons -- Nix, Styx, Kerberos and Hydra -- which are tiny compared with Charon and were glimpsed when NASA's New Horizons spacecraft flew past Pluto in 2015.

The team's model could offer a new explanation for how some moons end up orbiting other worlds. "It adds a new twist on the physics," Asphaug said. "We've had this idea that strength doesn't matter in collisions. We have to revisit that assumption, even for our moon's formation."

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.

"If Charon deposited some of its rock into Pluto, you would be able to see that in gravity data," said Denton. "Unfortunately we would need to go back to Pluto to test this."