It sounds like one of Kipling’s Just So Stories but it is rooted in science: experts say they have a new theory for how Pluto got its largest moon.
Pluto – once considered the ninth planet of our solar system, but now classified as a “dwarf planet” – has five known moons, of which Charon is the largest with a diameter of about 754 miles, just over half that of Pluto itself.
Unlike Earth, where the moon orbits the planet, Pluto and Charon orbit each other, with the icy bodies found in the Kuiper Belt beyond Neptune.
Now experts say they have unpicked how the system came to be, suggesting Charon and Pluto ended up orbiting each other after a “kiss-and-capture” event.
In this scenario Charon crashed into Pluto and the pair briefly rotated together – resembling, the researchers say, a giant snowperson – before breaking apart. This left each body largely distinct and intact, albeit with some material exchanged.
“Because Pluto is rotating rapidly prior to the collision, and because Charon lies mostly outside of their corotation zone, it is able to ‘push’ Charon off, and Charon starts to slowly migrate out,” said Dr Adeene Denton, first author of the research from the University of Arizona.
The findings pushed back against previous theories that Charon was formed when a large object collided with Pluto, causing both bodies to deform and mix like blobs in a lava lamp. It was proposed that as part of this process Charon was produced, and then captured in orbit. A similar process is thought to have led to the formation of our moon after a huge Mars-sized object smashed into Earth.
Writing in the journal Nature Geoscience, Denton and colleagues report how they made their new findings after creating computer models that included crucial information on Pluto and Charon: their strength.
This, Denton said, was previously overlooked as the models were originally created to explore collisions of galaxies, giant planets and other large bodies where strength was less important and colliding bodies could be treated as fluids.
However, the team said small bodies such as Pluto and Charon would have collided relatively slowly.
“What that means is that, because they’re made of rock and ice, they respond the way those materials would under stress, and not like fluids,” said Denton.
While Denton said further work was needed to explore whether Pluto’s smaller moons arose from debris released in the collision, she added that the event could have affected the subsequent geologic evolution of Pluto and Charon – including whether they formed and sustained subsurface oceans.
“The impact dumps a bunch of heat into Pluto, followed by more heating as Charon starts to move away, which could be the start of a new geologic era that culminates in the surface we observed from New Horizons [space probe] in 2015,” she said.
“Moreover, because eight out of 10 of the largest Kuiper Belt objects have a large satellite like Charon , kiss and capture might have been a prolific event across the Kuiper Belt as the solar system formed.”
