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Pluto and its moon Charon may have been briefly locked together in a cosmic “kiss”, before the dwarf planet released the smaller body and recaptured it in its orbit.

Charon is the largest of Pluto’s five moons, with a radius more than half that of Pluto itself, but the question of how it came to orbit Pluto has puzzled astronomers.

One prominent theory suggests that Charon formed after a vast object smashed into Pluto, spewing debris into space that later formed Charon, similar to how scientists think Earth’s moon formed. But Charon’s large size and close orbit, at eight times wider than Pluto itself, make this a challenging scenario to explain.

Now, at the University of Arizona and her colleagues have proposed that Charon may have a less destructive origin story, which they describe as a “kiss and capture”.

Previous simulations have treated Pluto and Charon as fluids – an assumption that works when modelling collisions between larger bodies. But recent research has shown that with objects of lighter mass than Earth’s moon, the material strength of their composition influences the outcome. “Pluto and Charon are quite small, so the assumption that they are fluid bodies probably no longer applies,” says Denton.

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The researchers ran simulations that take into account Pluto and Charon’s compositions of rock and ice, and found that a more likely scenario involved a gentle sticking together and parting ways.

Their model showed that a proto-Charon may have penetrated a proto-Pluto’s icy shell and the two bodies would have spun together rapidly for around 10 hours. Eventually, the spinning flung Charon back out and it settled into Pluto’s orbit.

“I had always assumed that any collision between planetary bodies that were hundreds of kilometres across would destroy the smaller one, if captured,” says at the Open University, UK.

While the kiss-and-capture scenario is interesting, says Rothery, it will need to also explain the complex geological features seen on both Pluto and Charon, such as heavily cratered surfaces and icy volcanism, which it doesn’t currently.