Billions of years ago, in the frigid outer reaches of our solar system, two icy worlds collided. Rather than destroying each other in a cosmic catastrophe, they spun together like a celestial snowman, finally separating while remaining forever linked in orbit. This is how Pluto and its largest moon, Charon, originated, according to a new University of Arizona study that challenges decades of scientific assumptions.
A study led by Adeene Denton, a NASA postdoctoral fellow who conducted the research at the U of A Lunar and Planetary Laboratory , has revealed this unexpected "kiss and capture" mechanism, which could help scientists better understand how planetary bodies form and evolve. By considering something planetary scientists had overlooked over decades – the structural strength of cold, icy worlds – researchers have discovered an entirely new type of cosmic collision.
The findings were published in the journal Nature Geoscience.
For decades, scientists 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, Denton said. 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. When we accounted for the actual strength of these materials, we discovered something completely unexpected," Denton said.
Using advanced impact simulations on the U of A's high-performance computing cluster, the research team found that instead of stretching like silly putty during the collision, Pluto and the proto-Charon actually became 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, much like two figure skaters spinning while holding hands.
"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," said Denton.
"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 senior study author Erik Asphaug , a professor in the Lunar and Planetary Laboratory.
The study also suggests that both Pluto and Charon remained largely intact during their collision, with much of their original composition preserved. This challenges previous models that suggested extensive deformation and mixing during the impact, Denton said. 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.
The research team is already planning follow-up studies to explore several key areas. The team wants to investigate how tidal forces influenced Pluto and Charon's early evolution when they were much closer together, analyze how this formation scenario aligns with Pluto's current geological features, and examine whether similar processes could explain the formation of other binary systems.
"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."
