About 4.5 billion years ago, the dwarf planet Pluto was suddenly joined by a companion star. For a very short time, perhaps only a few hours, they danced as if arm in arm, and then gently parted. The result of that grand do-si-do is that today Pluto and a quintet of its moons orbit the sun together.
Astronomers have long wondered how Charon, the largest of these moons, came to orbit Pluto. A paper published Monday in the journal Nature Geoscience describes a series of events that may solve this question.
“The reason Pluto and Charon are so interesting is because Charon is 50 percent the size of Pluto,” said Adine Denton, a planetary scientist at the University of Arizona who led the paper. “The only comparable star systems are Earth and its moon.”
Charon's diameter is about 1,200 miles, and Pluto's diameter is about 1,500 miles. This size ratio suggests that there are many traditional scenarios explaining how the moon forms, including theories that Charon formed from debris around Pluto or was captured by its gravity. This suggests that this is not the case. Could Charon's existence be explained by the type of impact thought to have formed Earth's moon?
Eric Asforg, also a planetary scientist at the University of Arizona, said the size of Pluto and Charon suggests that the most likely outcome of such an explosion scenario is that they “didn't simply merge like two liquid blobs. “It's difficult to figure out how,'' he said. He is also a co-author of the paper.
Pluto and Charon are located in the outer solar system beyond Neptune in a region called the Kuiper Belt, and both are extremely rocky and icy. By including these properties in their model, the researchers devised a scenario in which two objects collide and become trapped instead of merging.
If Charon were to impact Pluto at a relatively modest speed of about 2,000 miles per hour (10 times slower than Earth's moon-forming impact), the two would remain in contact for about 10 hours, after which they would gradually separate but remain together. It would have stayed there. Researchers described the encounter as a “kiss and capture.”
Dr Denton said the two bodies were strong enough to not be dismembered.
At that time, Pluto would have rotated once every three hours (the length of a day on Pluto today is about 150 hours), so the two orbited together three times. Pluto's rotating angular momentum slowly moves Charon away, but crucially, it remains trapped in Pluto's orbit.
Bill McKinnon, a planetary scientist at Washington University in St. Louis, said such a scenario “makes sense” given the large number of objects thought to have drifted around the Kuiper belt in the early solar system. . “Capture by collision is probably a common process,” he said, adding that there are likely many other large binary objects in the Kuiper Belt.
The collision would mean “basically the entire surface of Pluto would be resurfaced” and Charon would lose most of its surface ice to its companion star, Dr Denton said. “The effect is a geological reset to the system,” she added.
It may also have led to the formation of Pluto's other four known moons: Nix, Styx, Cerberus, and Hydra. These moons are smaller than Charon and were glimpsed by NASA's New Horizons spacecraft as it passed Pluto in 2015.
The researchers' model could provide a new explanation for how some satellites end up orbiting other worlds. “This puts a new spin on physics,” Dr. Asfag said. “We had the idea that strength didn't matter in impacts. We need to reconsider that assumption, even for the formation of our moon.”
A closer study of Pluto may reveal whether Pluto's cosmic dance really happened, but it will likely be a long time before another spacecraft visits the dwarf planet.
“If Charon deposited some of its rock on Pluto, we would be able to see it in the gravity data,” Denton said. “Unfortunately, we need to return to Pluto to test this.”
