Snapshot of Pluto and Charon during kiss-and-capture (Credit: Robert Melikyan and Adeene Denton)
Billions of years ago, in the frozen abyss of our solar system’s distant frontier, two icy titans collided in a moment of cosmic destiny. Yet, instead of annihilating one another in a violent cataclysm, they intertwined, merging in a mesmerizing celestial dance. Bound by gravity, they spun together like a frozen embrace, only to part as distinct entities, forever tethered in orbit. This dramatic encounter, according to a groundbreaking study from the University of Arizona, reshapes our understanding of Pluto and its largest moon, Charon—challenging decades of scientific dogma and rewriting the tale of their origins.
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.
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For decades, scientists believed that Charon, Pluto’s unusually large moon, was born through a cosmic process mirroring the formation of Earth’s moon—a violent collision, followed by the fluid-like melding and reshaping of molten bodies. As researcher Denton explained, this model fit perfectly for the Earth-moon system, where immense heat and colossal masses rendered the colliding bodies more like liquid than solid. But when this theory was applied to the frigid, diminutive Pluto-Charon system, it faltered. It ignored a vital truth: in the icy depths of the outer solar system, rock and ice do not yield like molten metal—they hold firm, defying the assumptions of this long-accepted paradigm.
“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.
