Charon, Pluto’s largest moon, holds a unique place in the solar system due to its size and tight orbit with the dwarf planet, the New York Times reports.
New research offers an intriguing explanation for how this cosmic pairing came to be. A simulation, detailed in the journal Nature Geoscience, suggests that Pluto and Charon’s relationship began with a brief but dramatic collision billions of years ago.
Approximately 4.5 billion years ago, Pluto was joined by a companion in a rare celestial encounter. The two bodies collided at a relatively low speed of about 2,000 miles per hour — much slower than the impact believed to have formed Earth’s moon. This allowed Pluto and Charon to remain in contact for roughly 10 hours before separating, a process described by researchers as a “kiss and capture.”
Charon’s size—about 750 miles across, half that of Pluto’s 1,500-mile diameter—has long puzzled scientists. Such a large size ratio makes it unlikely that Charon simply formed from debris around Pluto or was gravitationally captured without collision. The study, led by Adeene Denton, a planetary scientist at the University of Arizona, incorporated the rocky and icy properties of Pluto and Charon, both of which reside in the Kuiper Belt beyond Neptune.
The team’s model suggests that the collision did not result in the two bodies merging, thanks to their structural toughness. Instead, Charon was ensnared in Pluto’s orbit. At the time, Pluto was spinning rapidly, completing one rotation every three hours. The angular momentum from this rotation helped push Charon away, eventually locking it in its current orbit.
The collision would have significantly reshaped Pluto, resurfacing much of its terrain and stripping Charon of its icy exterior. This event may have also played a role in the formation of Pluto’s four smaller moons—Nix, Styx, Kerberos, and Hydra—which are thought to have emerged from debris generated by the impact.
The findings add a new dimension to understanding moon formation. Erik Asphaug, a co-author of the study, notes that the research challenges previous assumptions about collisions in the solar system.
“It adds a new twist on the physics,” he said.
Asphaug suggested that the structural integrity of colliding bodies plays a larger role than previously believed.
Despite the insights from simulations, confirming this theory would require a closer look at Pluto and Charon. For instance, if Charon transferred some of its rocky material to Pluto during the collision, it might be detectable in gravity data. However, further exploration of Pluto is unlikely in the near future. NASA’s New Horizons spacecraft provided the most detailed observations of Pluto in 2015, but another mission to the distant dwarf planet has yet to be planned.
