Billions of years ago, two icy worlds—Pluto and Chron—collided in the farthest reaches of our solar system. But rather than smashing into pieces, Pluto and its largest moon, Charon, briefly spun together like a celestial snowman before separating, forever linked in orbit.
The surprising scenario is drawn from a study conducted by a researcher at the University of Arizona who sets decades of scientific hypotheses regarding Pluto and Charon’s origins into question.
The study shows us an entirely new type of cosmic collision capable of changing scientists’ long-held ideas about how planets and moons are formed.
The new research argues that contrary to previous beliefs that Pluto and Charon originated from a giant impact, they were “kissed” together in one collision before separating into a binary system.
“We always thought Pluto’s moon formed from a big crash that stretched the bodies like silly putty,” Adeene Denton, a NASA postdoctoral fellow who conducted the research at the U of A Lunar and Planetary Laboratory, said. “But we overlooked something important: Pluto and Charon are cold, icy worlds, not hot, fluid-like planets like Earth. When we considered their materials, we realized something completely unexpected.”
Pluto’s atmosphere could disappear soon
For decades, scientists believed that Charon, Pluto’s unusually large moon, was created through a process similar to Earth’s moon. This theory suggested that a massive collision caused Pluto to stretch, and the debris eventually formed Charon. However, this model didn’t fit the smaller, colder Pluto-Charon system, which is mainly made of rock and ice.
By running advanced simulations on powerful computers at the University of Arizona, Denton and her team found that, instead of stretching like a rubber band, Pluto and Charon became temporarily stuck together. The two bodies then rotated like a snowman before eventually separating while remaining gravitationally linked, forming the binary system we observe today.
A binary system occurs when two celestial bodies orbit a common center of mass like two figure skaters spinning while holding hands.
“This ‘kiss and capture’ scenario is completely different from what we’ve seen in other planetary collisions such as ‘hit and run’ or ‘graze and merge’,” Denton said. “Instead of just hitting or grazing, the bodies collided, stuck together for a moment, and then separated while staying bound by gravity.”
The study also found that Pluto and Charon likely stayed mostly intact during the collision. Unlike previous theories that suggested a violent impact would have caused the two bodies to deform and mix, this new model shows that their original structure was mostly preserved.
This discovery also raises new questions about how Pluto could have developed a subsurface ocean. The collision and tidal forces generated internal heat, which might have helped create the conditions necessary for a hidden ocean, even though the solar system was less radioactive at the time.
“What’s exciting about this study is that it solves a long-standing puzzle,” said Erik Asphaug, senior study author and professor at the Lunar and Planetary Laboratory. “The model explains how Charon could have formed and places it in the right orbit.”
The team’s findings were quite a revelation that paved the way for future research. However, they are still planning to investigate how tidal forces might have affected the moons’ geology after Pluto and Charon got relatively close to each other post-collision. They also want to explore how this new formation scenario relates to the unique features we see on Pluto’s surface today.
“We’re particularly interested in how the initial collision might have shaped Pluto’s geology,” Denton said. “The impact heat and tidal forces could have played a key role in creating the surface features we observe.”
This study not only redefines the origins of Pluto and Charon but also opens the door to new ideas about how other binary systems in the universe may have formed. It challenges old assumptions and shows that icy bodies like Pluto and Charon can collide and evolve in ways previously unimagined.
Journal Reference:
- Denton, C.A., Asphaug, E., Emsenhuber, A. et al. Capture of an ancient Charon around Pluto. Nat. Geosci. (2025). DOI: 10.1038/s41561-024-01612-0
