"We think the Earth-Moon system initiated when a Mars-sized object hit the Earth and led to the formation of our large Moon sometime later," said Dr. Adeene Denton, the lead researcher of the study published in Nature Geoscience. "In comparison, Mars has two tiny moons that look like potatoes, while the moons of the giant planets make up a small fraction of their total systems."

Initial simulations by SwRI Vice President Dr. Robin Canup in 2005 suggested that the Pluto-Charon system could have originated from a giant collision. However, these early models treated the material involved as a strengthless fluid. Recent advancements in impact models over the past five years incorporate material strength properties, allowing for more nuanced simulations. These new results show Pluto behaving like a rocky core encased in ice, which significantly alters the predicted outcomes.

"In previous models, when proto-Charon hit proto-Pluto, you have a massive shearing effect of fluids that looks like two blobs in a lava lamp that bend and swirl around each other," Denton explained. "Adding in structural properties allows friction to distribute the impact momentum, leading to a 'kiss-and-capture' regime."

Under this scenario, the two bodies collide and briefly form a snowman-like structure before separating. As the pair rotate together, Pluto eventually pushes Charon into a stable orbit.

"Most cosmic collisions are what we call a hit-and-run, when an impactor hits a planet and keeps going," Denton said. "Or an impactor hits a planet, and they merge, which is called a graze and merge. For the Pluto-Charon system, we have a new paradigm where the two bodies hit and then stick together but do not merge because they are behaving like rock and ice."

This collision likely involved an exchange of material between Pluto and Charon without significant loss to the solar system. Pluto retained a higher proportion of rock compared to ice, while Charon's composition is about 50% rock and 50% ice. Both bodies preserved their structural integrity, potentially maintaining ancient interior features formed in the Kuiper Belt.

"And this collision scenario supports the formation of other moons, such as Pluto's four other tiny, lumpy satellites," Denton added.

The research sheds light on the dynamics of the impact but not its timing, which remains an open question. Understanding when this event occurred is crucial for explaining Pluto's ongoing geological activity and the potential presence of a liquid ocean beneath its icy surface.

"Even if Pluto starts out really cold, which makes more sense from a solar system evolution perspective, the giant impact and the subsequent tidal forces following the separation could result in an ocean down the line," said Denton. "And that has pretty big implications for the Kuiper Belt as a whole, because eight of the 10 largest Kuiper Belt objects are similar to Pluto and Charon."

Research Report:Capture of an Ancient Charon Around Pluto

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The million outer planets of a star called Sol