How Pluto got its biggest moon with an ancient 'kiss and capture'

UPDATED: Jan 10, 2025 00:00 IST

Scientists have uncovered a new theory to explain how Pluto and its largest moon, Charon, formed their unique relationship.

Citing a recent paper published in Nature Geoscience, the New York Times reported that the two celestial bodies may have danced in an ancient “kiss and capture” event around 4.5 billion years ago. This brief encounter — lasting possibly only a few hours — could have set the stage for their current orbital configuration.

Charon, Pluto’s largest moon, is almost half the size of the dwarf planet itself, a rare proportion in the solar system. Adeene Denton, a planetary scientist at the University of Arizona and lead author of the paper, said: “The reason that Pluto and Charon are so interesting is because Charon is 50 percent the size of Pluto. The only comparable system is Earth and its moon.” This size ratio complicates previous theories about Charon’s formation, such as the idea that it was formed from debris around Pluto or captured by Pluto’s gravitational pull.

To resolve this, Denton and her team modelled a scenario in which Pluto and Charon collided in a manner akin to the event believed to have created Earth’s moon. However, instead of merging, the two bodies “kissed” each other gently at a speed of about 2,000 miles per hour — much slower than the violent impact believed to have created the Earth’s moon. This impact would have allowed them to remain together, spinning around each other for several hours before gradually separating, with Charon staying in orbit around Pluto.

Erik Asphaug, also from the University of Arizona, co-author of the study, explained, “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 collision would not have destroyed the bodies, thanks to their tough, icy nature.

This slow collision would have caused a “geologic reset” for Pluto, as Charon stripped some of its ice, and potentially led to the formation of Pluto’s smaller moons — Nix, Styx, Kerberos, and Hydra.

Bill McKinnon, a planetary scientist at Washington University in St. Louis, praised the theory, saying that “collisional capture is probably a common process” in the Kuiper Belt, where Pluto resides.

Further studies of Pluto may eventually confirm this theory, but any future space missions to Pluto remain distant, as the dwarf planet lies far from Earth.