Sealed With a Kiss: The Unexpected Origin Story of Pluto and Its Moon Charon

For billions of years, Pluto and its largest moon Charon have been facing each other in a mutual tidal lock. Since it’s about half the size of Pluto, the moon and its planet are sometimes referred to as a double dwarf planet system, with speculation that they may share a common origin story. Now, scientists may have figured out how this unusual duo came to be, rotating as a single object before being separated into a binary pair.

Pluto and Charon may have formed through a “kiss and capture” mechanism, with the two icy bodies colliding and becoming temporarily stuck together, spinning in a cosmic dance before separating into two objects that are forever bound in orbit, according to a new study published in Nature Geoscience. “Most planetary collision scenarios are classified as ‘hit and run’ or ‘graze and merge.’ 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,” Adeene Denton, a NASA postdoctoral fellow, and lead author of the study, said in a statement.

Charon is the largest of Pluto’s five moons, stretching at 754 miles (1,214 kilometers) across while the dwarf planet itself measures at 1,400 miles (2,253 km) wide. For years, it was believed that Charon formed the same way as Earth’s Moon, by way of a massive collision. With the Earth-Moon system, however, the colliding bodies behaved more like fluids due to their larger sizes and intense heat resulting from the collision. “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,” Denton said.

The researchers behind the study carried out computer simulations of different collision scenarios and found that Pluto and Charon remained largely intact during their collision. They became fused together as a single, odd-shaped object, resembling a snowman, according to the model. After being intertwined in a lopsided rotation, the pair became unstuck and Charon, being the smaller of the two, was doomed to a near-circular orbit around Pluto.

“The compelling thing about this study, is that the model parameters that work to capture Charon, end up putting it in the right orbit,” Erik Asphaug, a professor in the Lunar and Planetary Laboratory at the University of Arizona, and co-author of the new study, said in a statement. “You get two things right for the price of one.”

The study could also solve another mystery about Pluto, namely how the dwarf planet came to possess a theorized underground liquid ocean. As the two bodies separated from one another, according to the study, the collision process had caused a considerable amount of internal heat to be transferred into both celestial objects. That internal heat may have caused Pluto to develop a subsurface ocean.

“We’re particularly interested in understanding how this initial configuration affects Pluto’s geological evolution,” Denton said. “The heat from the impact and subsequent tidal forces could have played a crucial role in shaping the features we see on Pluto’s surface today.”

The team is planning on carrying out follow-up studies to explore how tidal forces may have influenced Pluto and Charon’s early evolution when they were fused together, and how a similar process could have led to the creation of other binary systems.