Unexpected Mechanism Explains Formation of Pluto-Charon System

Planetary scientists at the University of Arizona say they have discovered an entirely new type of cosmic collision.

Pluto and Charon are the largest binary system in the known population of trans-Neptunian objects in the outer Solar System. Their shared external orbital axis suggests a linked evolutionary history and collisional origin. Their radii, 1,200 km and 600 km, respectively, and Charon’s wide circular orbit of about 16 Pluto radii require a formation mechanism that places a large mass fraction into orbit, with sufficient angular momentum to drive tidal orbital expansion. Denton et al . numerically modeled the collisional capture of Charon by Pluto using simulations that include material strength. Image credit: Denton et al ., doi: 10.1038/s41561-024-01612-0.

For decades, planetary researchers have theorized that Pluto’s unusually large moon Charon formed through a process similar to Earth’s Moon — a massive collision followed by the stretching and deformation of fluid-like bodies.

This model worked well for the Earth-Moon system, where the intense heat and larger masses involved meant the colliding bodies behaved more like fluids.

However, when applied to the smaller, colder Pluto-Charon system, this approach overlooked a crucial factor: the structural integrity of rock and ice.

“Pluto and Charon are different — they’re smaller, colder and made primarily of rock and ice,” said Dr. Adeene Denton, a postdoctoral researcher with the Lunar and Planetary Laboratory at the University of Arizona.

“When we accounted for the actual strength of these materials, we discovered something completely unexpected.”

Using advanced impact simulations, the authors found that instead of stretching during the collision, Pluto and the proto-Charon temporarily stuck together, rotating as a single snowman-shaped object before separating into the binary system we observe today.

A binary system occurs when two celestial bodies orbit around a common center of mass.

“Most planetary collision scenarios are classified as ‘hit and run’ or ‘graze and merge’,” Dr. Denton said.

“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.”

“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 Professor Erik Asphaug, also from the Lunar and Planetary Laboratory at the University of Arizona.

The study also suggests that both Pluto and Charon remained intact during their collision, preserving much of their original composition.

This challenges previous models that suggested extensive deformation and mixing during the impact.

Additionally, the collision process, including tidal friction as the bodies separated, deposited considerable internal heat into both bodies, which may provide a mechanism for Pluto to develop a subsurface ocean without requiring formation in the more radioactive very early Solar System — a timing constraint that has troubled planetary scientists.

“Charon is captured relatively intact in our scenario, retaining its core and most of its mantle, which implies that Charon could be as ancient as Pluto,” the researchers said.