Astronomers have for decades tried to figure out how Pluto captured its largest moon. Now, there’s a new theory

By Ashley Strickland, CNN

(CNN) — For decades, astronomers have tried to determine how Pluto acquired its unusually large moon Charon, which is about half the size of the dwarf planet. Now, new research suggests that Pluto and Charon briefly came together billions of years ago in a newly discovered “kiss and capture” collision.

Scientists have long theorized that Charon formed much like Earth’s moon, which was created after a Mars-size object slammed into our planet. The impact sent molten debris from Earth flying into space, which eventually came together in orbit around our world and cooled to form the moon.

But these theories didn’t account for the fact that Pluto and Charon may have more structural integrity as icy, rocky bodies on the frigid edge of the solar system. “Pluto and Charon are different — they’re smaller, colder and made primarily of rock and ice,” said Adeene Denton, lead study author of the research published Monday in the journal Nature Geoscience.

Instead, Pluto and Charon likely remained much the same after they collided, spinning together to form an object shaped like a cosmic snowman before separating into the binary system they have today, the study authors said. Binary systems form when two celestial bodies orbit around a common center of mass, like figure skaters who spin while holding hands, according to the authors.

“Most planetary collision scenarios are classified as ‘hit and run’ or ‘graze and merge,’” said Denton, a NASA postdoctoral program fellow at the Southwest Research Institute in Boulder, Colorado. “What we’ve discovered is something entirely different — a ‘kiss and capture’ scenario where the bodies collide, stick together briefly and then separate while remaining gravitationally bound.” (Denton conducted the research as a postdoctoral researcher at the University of Arizona Lunar and Planetary Laboratory.)

This new type of celestial collision uncovered by Denton and her colleagues could shed light on how planets form and evolve as well as how other bodies in the outer reaches of our solar system gained their own moons. And the new research may offer evidence for a subsurface ocean beneath Pluto’s icy crust.

How planets gain moons

Charon and Earth’s moon are both a large fraction of the size of the main body they orbit, which is unlike other smaller moons orbiting planets throughout our solar system. (Pluto has four smaller moons besides Charon.)

For example, Mars is orbited by two tiny potato-shaped moons called Phobos and Deimos, which are likely two asteroids that were captured by the red planet’s gravity.

But that type of formation is unlikely for objects as large as our moon or Charon, Denton said. Other examples of collisions are the hit and run, when an impactor hits a planet and keeps going, or the graze and merge, when an object strikes a planet and they come together, Denton said.

Simpler models from the early 2000s simulating how Pluto and Charon came to be in their current orientation supported the idea that some object collided with Pluto to create Charon. In that scenario, the colliding material was regarded as a strengthless fluid, meaning Pluto and Charon would have resembled two swirling, bending blobs, like in a lava lamp, Denton said.

Within the last five years, there have been advancements in simulations for impact formation models, which allowed researchers to include material strength properties of celestial bodies — such as Pluto as a rocky core covered in ice. Pluto contains more rock than ice, while Charon is 50% rock and 50% ice, Denton said.

When Denton and her colleagues simulated a collision including the structural properties of Pluto and Charon, they were better able to determine the impact momentum exchanged between both bodies as well as the end result.

While Pluto and Charon likely exchanged some material during the impact, both remained largely intact because of their composition, sticking together to form a snowman-like figure and rotating as one body until Pluto essentially pushed Charon into a more distant orbit over time.

“We were definitely surprised by the ‘kiss’ part of kiss-and-capture,” Denton said in an email. “There hasn’t really been a kind of impact before where the two bodies only temporarily merge before re-separating.”

The findings suggest that Charon could be as ancient as Pluto. Researchers aren’t entirely sure when the impact occurred, but it likely happened about 4 billion years ago, early on in the history of the solar system, which is estimated to be 4.6 billion years old.

“We know that it happened fairly early on, because that’s when giant collisions happened — and by early, I mean in the tens of millions of years after solar system formation, around the same time as the (formation of Earth’s moon),” Denton said.

The study is a “great example of advances in numerical geophysics,” said Dr. Katarina Miljkovic, professor and deputy head of the School of Earth and Planetary Sciences at Curtin University in Australia. She was not involved in the new research.

“They provide an elegant solution to the origin of the Pluto-Charon system, where by treating the physical properties of the planetary bodies with higher fidelity resulted in a more feasible impact and capture scenario for the origin of this system,” Miljkovic wrote in an email.

A subsurface ocean

Pluto belongs to a group of objects that distantly orbit the sun called the Kuiper Belt, where thousands of icy remnants left over from the formation of the solar system linger. Eight of the 10 largest Kuiper Belt Objects have large moons like Charon, Denton said, which means that “kiss and capture” collisions may have occurred across the Kuiper Belt as the solar system formed.

“Adding more physical realism to the impact models — which we sometimes have to wait on while technology catches up with the complexity of the natural world — changed the answer and opened up a new suite of possible histories for Pluto, Charon, and a host of other Kuiper Belt Objects,” said Alyssa Rhoden, a staff scientist at the Southwest Research Institute, in an email.

Rhoden and Denton both began working through the NASA Astrobiology Postdoctoral Program in September, but Rhoden was not part of the Pluto study.

Some scientists suspect an ocean could lie beneath Pluto’s thick crust of ice, but questions have been raised about how such an ocean could form in this frigid world.

The new collision scenario supports the formation of an ocean because such impacts heat up celestial bodies, Denton said. For example, Charon’s collision with Pluto would have raised the temperature of the the dwarf planet’s ice shell, which may have caused it to melt and form a subsurface ocean.

Rhoden said she is curious whether such changes also occurred on other Kuiper Belt Objects with large moons.

“Is there something special about the Charon-forming impact — or the characteristics of Pluto and the impactor — that led Pluto to become a rare (Kuiper Belt Object) ocean world?” Rhoden said.

It’s also possible that after Charon separated from Pluto, moving from a close orbit to a more distant one, both bodies would have experienced tidal heating. This process occurs when the interiors of planets or moons are heated due to the gravitational forces between two bodies in orbit.

“This is important because we’re not sure what thermal conditions Pluto may have possessed prior to impact,” Denton said. “If Pluto formed as a colder body without an ocean, then the giant impact may provide the key tipping point to then push Pluto towards forming and sustaining a subsurface ocean.”

The-CNN-Wire
™ & © 2025 Cable News Network, Inc., a Warner Bros. Discovery Company. All rights reserved.