In the frigid expanse of the Kuiper Belt, Pluto and Charon —one a dwarf planet, the other its unusually large moon— orbit each other like cosmic dance partners locked in a gravitational embrace. Scientists have spent decades trying to piece together how this system formed. Now, a new study from the University of Arizona published in Nature Geoscience proposes a surprising answer: a process called “kiss and capture.”
For years, the leading idea was that Pluto and Charon formed in a massive collision, similar to how Earth’s Moon came into existence billions of years ago. But the University of Arizona Lunar and Planetary Laboratory researchers found that there’s a problem with that explanation. Pluto and Charon are small, cold, and made mostly of rock and ice, which behave very differently from the hot, molten material involved in Earth’s collision with a Mars-sized body billions of years ago.
Instead of smashing together and behaving like fluids, Pluto and the object that would become Charon collided in a way that allowed them to briefly stick together before separating into the binary system we see today. This is what researchers are calling “kiss and capture.” The collision wasn’t so energetic that it destroyed the bodies, nor so gentle that they simply bounced apart. Instead, it was just the right amount of force to make them gravitationally bound.
What makes this scenario possible is the material strength of icy and rocky worlds. When Pluto and proto-Charon collided, their solid, rigid surfaces resisted the extreme deformation typical of molten large bodies following a major planetary collision. As the two objects made contact, they temporarily formed a snowman-shaped structure, rotating together before separating. Computer simulations run by the researchers showed that this process naturally led to the formation of a stable binary system.
“Pluto and Charon are different – they’re smaller, colder and made primarily of rock and ice,” said Adeene Denton, the study’s lead researcher. “When we accounted for the actual strength of these materials, we discovered something completely unexpected. 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.”
The simulations not only discovered the “kiss and capture” process; it also helped explain how Charon was formed and accounts for its current 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,” said senior study author Erik Asphaug, a professor in the Lunar and Planetary Laboratory. “You get two things right for the price of one.”
While Pluto and Charon remained largely intact, the energy of the collision heated both bodies internally. This heat may have melted some of Pluto’s interior, potentially creating a subsurface ocean beneath its icy crust. Tidal forces—caused by the gravitational tug-of-war between the two bodies as they spiraled apart—may have added even more heat, shaping the geological features we see today. Charon’s system of massive fractures and Pluto’s complex, varied surface could both be linked to these early interactions.
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This discovery might also explain how other binary systems in the Kuiper Belt formed. Objects like Eris and Dysnomia or Orcus and Vanth could have undergone similar processes. The “kiss and capture” mechanism shows how the unique conditions in the far reaches of the solar system—cold temperatures, low gravity, and icy compositions—can lead to outcomes that differ dramatically from those closer to the Sun.
The researchers are now expanding their simulations to explore other binary systems and study how the collision may have influenced the long-term evolution of Pluto and Charon. They also hope future missions to the Kuiper Belt will provide more data to refine these models.