Pluto’s four smallest moons might have formed after its biggest moon, Charon, suffered an impact, rather than being a chip off the dwarf planet.
The Pluto system is an oddity in the solar system. The largest of its five moons, Charon, is 1200 kilometres across, almost half the size of the dwarf planet. By comparison, Pluto’s four smaller moons – Styx, Nix, Kerberos, and Hydra – are tiny, measuring no more than 50 kilometres across. They also orbit very close to both Pluto and Charon, just tens of thousands of kilometres away.
Explaining how these smaller moons came to be has been difficult, but Benjamin Bromley at the University of Utah and Scott Kenyon from the Smithsonian Astrophysical Observatory in Cambridge, Massachusetts, think they have an answer: the moons weren’t formed from Pluto, but Charon.
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The leading theory for Charon’s formation is that it resulted from a grazing collision on Pluto, with the four smaller moons also resulting from this event. But, says Bromley, Pluto and Charon would have begun life closer than the 20,000 kilometres between them now. As they gradually separated, it would have gravitationally disturbed the system, and kicked out any smaller moons arising from the collision.
This isn’t the case if the smaller satellites formed in a later impact on Charon, says Bromley – about a billion years after the large moon formed. The pair’s model estimates the impactor would have been up to 100 kilometres in size, throwing out enough debris to surround both Pluto and Charon and forming the four smaller moons.
If so, it could give us a valuable insight into how planets form around binary stars, with Pluto and Charon being a scale model of such systems. “Our paper shows a way for that kind of process to work,” says Bromley.
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