In a study published recently in Nature Geoscience, researchers unveiled a new theory explaining the formation of Charon, Pluto's largest moon. The team from the University of Arizona proposed that Pluto captured Charon through an unusual mechanism termed "kiss and capture," suggesting a different formation process than the previously hypothesized giant impact, according to Space.com.
According to the new theory, Pluto and Charon collided gently billions of years ago in the distant Kuiper Belt, the region beyond Neptune populated by icy bodies. Instead of obliterating each other or merging into a single entity, the two celestial objects briefly united and then separated, maintaining their individuality. Science Alert noted that this scenario posits that they temporarily formed a contact binary, resembling the two lobes of the object Arrokoth—another contact binary in the outer solar system—which demonstrates their unusual relationship as one of the most unique pairs in our solar system.
"Pluto and Charon are different—they're smaller, colder and made primarily of rock and ice. When we accounted for the actual strength of these materials, we discovered something completely unexpected," said Adeene Denton, a researcher from the University of Arizona and lead author of the study, according to Phys.org.
The researchers utilized computer simulations that considered the material properties of both celestial objects. These models revealed that Pluto and Charon could have gently collided and remained largely intact, preserving their original compositions and avoiding large-scale deformation. According to Discover Magazine, this finding suggests that the collision process likely heated the interiors of both bodies, which may have allowed Pluto to develop a subsurface ocean without requiring formation in the more radioactive early solar system.
"The compelling thing about this study is that the model parameters that work to capture Charon end up putting it in the right orbit. You get two things right for the price of one," said Erik Asphaug, a professor at the University of Arizona and co-author of the study, according to Gizmodo.
Charon measures half of Pluto's diameter, making it approximately 754 miles across. This size ratio makes the Pluto-Charon system a rarity, often referred to as a binary planet system. Unlike Earth, where the Moon orbits the planet, Pluto and Charon orbit each other, forming a binary system that is more similar to the Earth-moon system than any other moon in the solar system.
The "kiss and capture" scenario contrasts with previous theories that suggested Charon formed from a giant impact, similar to the one hypothesized to have formed Earth's Moon. "Because both bodies have material strength, Charon did not penetrate deep enough into Pluto to merge with it; this isn't true when the bodies are fluids," Denton explained.
After the collision, the two bodies would have spun together rapidly for around 10 hours before separating. Charon then began migrating slowly outward to its current near-circular orbit around Pluto. Phys.org reported that this relationship is an example of how gravitational interactions can generate complex and unique worlds.
The team's simulations also indicate that the collision deposited heat into both bodies. "The impact dumps a bunch of heat into Pluto, followed by more heating as Charon starts to move away, which could be the start of a new geologic era that culminates in the surface we observed from New Horizons in 2015," said Denton, according to The Guardian.
The researchers hope that follow-up studies can confirm this new story and explore how gravitational forces might have played a role in Pluto and Charon's early evolution. They also aim to investigate whether these kinds of collisions might explain other known binary systems in the Kuiper Belt.
This article was written in collaboration with generative AI company Alchemiq