A Novel ‘Kiss and Capture’ Event Gave Pluto Its Largest Moon, Charon, New Study Suggests
Researchers accounted for the previously overlooked structures of the dwarf planet and moon in computer simulations of a celestial collision
The “demoted” dwarf planet Pluto and its largest moon Charon make an unusual pair, and for decades, scientists have been discussing how the binary system—in which each mutually orbits the other—came to be. With Charon being half Pluto’s size, experts have struggled to explain how it ended up in the dwarf planet’s domain.
Now, a team of researchers has suggested that Pluto may have secured Charon through a newly described “kiss and capture” mechanism that challenges previous theories. Their work was published on Monday in the journal Nature Geoscience.
The new theory suggests that billions of years ago, Pluto and Charon collided in the far reaches of the outer solar system. Rather than obliterating each other, the two bodies joined together in a spinning snowman shape (the kiss) for 10 to 15 hours before separating—but ultimately, they remained trapped in each other’s orbits (the capture). Despite the collision, the dwarf planet and moon would have remained mostly intact.
“Most planetary collision scenarios are classified as ‘hit and run’ or ‘graze and merge.’ What we’ve discovered is something entirely different—a ‘kiss and capture’ scenario where the bodies collide, stick together briefly and then separate while remaining gravitationally bound,” says planetary scientist Adeene Denton, a NASA postdoctoral fellow at the University of Arizona’s Lunar and Planetary Laboratory and lead author of the study, in a statement.
“I had always assumed that any collision between planetary bodies that were hundreds of kilometers across would destroy the smaller one, if captured,” David Rothery, a planetary geoscientist at the Open University in England who was not involved in the study, tells New Scientist’s Alex Wilkins.
Previously, scientists suggested Charon had formed through another scenario: A celestial body catastrophically smashed into Pluto, generating a massive amount heat from the impact that would have made the bodies behave in a fluid-like manner, like silly putty or blobs in a lava lamp. Scientists widely agree that Earth’s moon formed in a similar way.
Pluto and Charon, however, are very different from our Earth and moon. While the moon clearly orbits Earth, Pluto and Charon orbit each other. “Charon is HUGE relative to Pluto, to the point where they are actually a binary,” Denton explains to Space.com’s Robert Lea. “[Charon is] half Pluto’s size and 12 percent of its mass.” Our moon, for comparison, is about 27 percent of Earth’s size. (But that’s still closer to Charon and Pluto than other moon-planet pairings in our solar system—Jupiter’s largest moon, Ganymede, is 1/28 the size of the gas giant.)
The differences don’t end there—the researchers also argue that Pluto and Charon are less likely to behave like fluid during a collision. The dwarf planet and its moon “are quite small, so the assumption that they are fluid bodies probably no longer applies,” Denton tells New Scientist. They are also mostly made of rock and ice—features that provide structural strength—which previous studies had overlooked.
The team ran advanced computer simulations of the Pluto-Charon impact and included these key structural features. In turn, their models revealed the novel “kiss and capture” cosmic collision mechanism for the first time.
“We’ve found that if we assume that Pluto and Charon are bodies with material strength, Pluto can indeed capture Charon from a giant impact,” Denton explains to Space.com. “We were definitely surprised by the ‘kiss’ part of kiss-and-capture,” she continues. “There hasn’t really been a kind of impact before where the two bodies only temporarily merge before re-separating!”
The model accurately predicted the binary system’s orbit today, lending further confidence in its reliability.
“Because Pluto is rotating rapidly prior to the collision, and because Charon lies mostly outside of their corotation zone, it is able to ‘push’ Charon off, and Charon starts to slowly migrate out,” Denton tells the Guardian’s Nicola Davis. She adds that the impact could have marked the start of a new geological era for Pluto, whose surface we observed in 2015 with the New Horizons space probe.
Among other things, the team now hopes to investigate how the new scenario might have impacted the bodies’ geology—including potential subsurface oceans—as well as whether the “kiss and capture” mechanism might be responsible for other binary systems in the universe.