Pluto Captured Its Largest Moon, Charon, in a 10-Hour Icy “Kiss”, New Study Suggests
13th Jan 2025
New research sheds light on a cosmic event billions of years ago when Pluto captured its largest moon, Charon, in a brief and unique “kiss-and-capture” collision. This theory explains how Pluto, a dwarf planet in the distant Kuiper Belt, formed an extraordinary binary system with Charon, which is half its size.
An Icy Collision with a Surprising Outcome
Researchers suggest that the event began as two icy bodies in the Kuiper Belt – a region of frozen objects at the edge of our solar system – collided billions of years ago. Instead of destroying each other, the two merged briefly, forming a spinning “cosmic snowman.”
“We were definitely surprised by the ‘kiss’ part of kiss-and-capture,” said Adeene Denton, the lead researcher and planetary scientist at the University of Arizona. “There hasn’t really been a kind of impact before where the two bodies only temporarily merge before re-separating!”
This fleeting union lasted just 10 to 15 hours. The material strength of both icy worlds prevented them from fully merging, unlike other celestial collisions that result in the creation of a single body.

A Unique Relationship in the Solar System
Pluto and Charon are unlike most planet-moon systems. Charon is so large compared to Pluto that the two are considered a binary system.
“Charon is HUGE relative to Pluto, to the point where they are actually a binary,” Denton explained. “It’s half Pluto’s size and 12% of its mass, which makes it much more similar to the Earth’s moon than any other moon in the solar system.”
This unusual size ratio posed a challenge to scientists trying to understand how such a system could form. Denton’s team used advanced simulations to show how Pluto’s structural strength, combined with Charon’s trajectory, allowed the two to remain separate yet gravitationally bound after the initial collision.
The Science Behind “Kiss-and-Capture”
During most collisions, the intense heat and mass of the colliding bodies cause them to behave like fluids, leading to either a “hit-and-run” or “merge” outcome. But Pluto and Charon’s icy compositions changed the dynamic.
“Because both bodies have material strength, Charon did not penetrate deep enough into Pluto to merge with it,” Denton said. Instead, Charon remained beyond the “co-rotation radius,” a point where the two bodies couldn’t stay fused. This resulted in Charon moving into a higher circular orbit and slowly migrating outward to its current position, roughly eight times Pluto’s width away.
Implications for the Kuiper Belt
The kiss-and-capture theory may also explain the formation of other large Kuiper Belt Objects (KBOs) with sizeable moons, such as Eris and Dysnomia or Orcus and Vanth. Denton’s team plans to investigate how the theory applies across the Kuiper Belt, where many icy bodies share a similar evolutionary history.
“We think this could be something that happens with some frequency in the Kuiper Belt, but we need to study how it varies depending on composition and mass,” Denton said.
The Collision Raises Some Interesting Geological Questions
Denton and her team are also keen to understand how the collision influenced Pluto’s and Charon’s geological development, including the potential presence of subsurface oceans.
“This was very new to us,” Denton said. “It also raised a lot of interesting geological questions that we’d like to test, because whether kiss-and-capture works depends on the thermal state of Pluto, which we can then tie to Pluto’s contemporary geology.”
Future research will focus on modelling Charon’s tidal evolution to confirm the theory and determine whether it aligns with the moon’s current orbit.
The findings, published in Nature Geoscience, offer a new lens through which to view the dynamic and surprising history of Pluto and Charon, while opening doors to understanding other icy worlds in the solar system.
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