Scientists Recreated the Moment How Pluto May Have Captured Its Largest Moon Charon

Far beyond Neptune, at the cold and distant edge of our Solar System, two worlds travel together in an extraordinary partnership. Pluto and its largest moon, Charon, are so closely connected that scientists often describe them as a binary system rather than simply a planet and its moon.

Now, numerical simulations of their possible formation suggest a remarkable story: Pluto and Charon may have been created through a giant collision in which the two worlds briefly touched, avoided merging, and eventually settled into orbit around each other. Researchers describe this unusual process as a “kiss-and-capture” scenario.

The idea could also explain why Charon appears to have retained much of its original interior—and raises the possibility that the moon is nearly as old as Pluto itself.

A Very Unusual Pair

Pluto and Charon are the largest known binary system among trans-Neptunian objects. They are located in the distant region beyond Neptune known as the Kuiper Belt, where numerous icy bodies orbit the Sun.

Their sizes make the relationship particularly unusual. Pluto has a radius of roughly 1,200 kilometers, while Charon has a radius of about 600 kilometers—around half Pluto’s radius.

Charon also does not orbit Pluto like many ordinary moons orbit their planets. The two bodies revolve around a point in space located between them. Their orbital relationship is therefore more like two objects dancing around a shared center of mass.

This unusual arrangement provides important clues about how the system formed.

The Mystery of Charon’s Origin

Scientists have long considered a major collision as one possible explanation for the Pluto–Charon system. A powerful impact could have removed material from Pluto and placed some of it into orbit, eventually allowing a moon to form.

But there is a problem.

Charon is large and relatively massive compared with Pluto. Creating such a large companion through a collision requires the impact to transfer a substantial amount of material into orbit. It also requires enough angular momentum—the rotational motion associated with an object's mass and movement—to produce the wide orbit observed today.

Charon now travels around Pluto at roughly 16 Pluto radii, a surprisingly large distance compared with the size of the two bodies.

So what kind of collision could create such a system without simply destroying or merging the two worlds?

A Collision Unlike a Simple Smash

Adeene Denton and colleagues investigated this question using detailed computer simulations. Their models included something that is particularly important for planetary collisions: material strength.

Many simplified collision models treat planetary bodies as fluids or collections of particles that can easily deform. But real planetary materials have strength. Ice and rock can resist deformation, fracture and maintain their structure during an impact.

Including this strength produced an intriguing result.

For certain collisions, particularly those involving a Pluto that was already rotating in the appropriate direction, friction could spread the momentum of the impact through the two bodies. Instead of Charon immediately plunging into Pluto or being completely destroyed, the two objects could become temporarily connected.

In other words, they could touch without immediately becoming one body.

This is the basis of the “kiss-and-capture” scenario.

What Does “Kiss-and-Capture” Mean?

Imagine two icy worlds approaching one another after a massive collision. Their surfaces make contact, but the impact does not have enough effect to completely merge their interiors.

Because the bodies have material strength, they can resist immediate coalescence. Friction redistributes some of the collision's energy and momentum.

For a suitable impact angle and a Pluto rotating in the same general direction as the resulting orbital motion, the two bodies can become temporarily linked.

They have effectively “kissed.”

But the encounter does not end there.

Instead of merging permanently, Charon can become gravitationally captured by Pluto. Over time, tides—the gravitational interactions between the two bodies—change their motion.

This provides a possible pathway from a violent collision to the remarkably orderly system we see today.

Tides Help Build the Final Orbit

After Charon is captured, tidal interactions become crucial.

When two large bodies orbit close to each other, each produces gravitational effects that can deform the other. These deformations create tidal forces that gradually transfer energy and angular momentum between rotation and orbital motion.

In the simulations, Charon becomes tidally decoupled from Pluto and moves into a near-circular orbit. From there, tidal evolution can drive Charon gradually outward.

This is important because Charon's present orbit is relatively distant and nearly circular.

Rather than requiring Charon to have formed directly at its current distance, the model provides a possible evolutionary path: collision, temporary contact, capture, tidal circularization and outward migration.

The collision also has to preserve enough angular momentum to produce the final binary system. The simulations indicate that the direction of Pluto's rotation is an important part of this process.

Charon May Have Survived Remarkably Intact

Perhaps the most interesting result is what happens inside Charon.

In the “kiss-and-capture” simulations, Charon does not need to be completely broken apart and rebuilt from debris. Instead, it can remain relatively intact, retaining its core and much of its mantle.

That detail has major implications for Charon's history.

If Charon formed during the same early period as Pluto and survived the collision largely intact, it may not be a moon assembled long after Pluto. Instead, the two bodies could have originated during the same ancient era and remained together ever since.

This means Charon could be almost as old as Pluto.

A New Way to Think About Planetary Collisions

The study highlights an important point about planetary formation: collisions are not always simple cases of one object smashing into another.

Depending on their sizes, speeds, composition, rotation and impact geometry, collisions can produce very different outcomes. Bodies can merge, fragment, exchange material—or, in this case, potentially touch and become gravitationally bound without immediately merging.

Material strength appears to be especially important. By resisting immediate deformation and mixing, the strength of the icy bodies can change the outcome of the collision dramatically.

That makes the Pluto–Charon system a valuable natural laboratory for understanding how binary worlds form throughout the outer Solar System.

A Cosmic Partnership With an Ancient Beginning

Pluto and Charon may look like a small pair of frozen worlds in a distant corner of the Solar System. But their unusual relationship contains clues about events that occurred billions of years ago.

The “kiss-and-capture” model offers a coherent explanation for several features at once: Charon's large size, its substantial mass, Pluto's rotation, the system's angular momentum and Charon's wide, nearly circular orbit.

Most importantly, the scenario suggests that Charon could have survived the original collision with much of its interior intact.

If this interpretation is correct, Pluto and Charon are not simply a planet and its unusually large moon. They may be ancient survivors of a single dramatic encounter—a cosmic collision that did not end in destruction, but in partnership.

Reference' Denton, C.A., Asphaug, E., Emsenhuber, A. et al. Capture of an ancient Charon around Pluto. Nat. Geosci. 18, 37–43 (2025). https://doi.org/10.1038/s41561-024-01612-0