Pluto May Have Captured Its Biggest Moon After an Ancient Dance and Kiss

Charon is large in size relative to Pluto, and is locked in a tight orbit with the dwarf planet. A new simulation suggests how it ended up there.

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 percent 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.