We talk a lot about Charon, Pluto's largest moon that's about half the size of its host planet. But what about Pluto’s other moons? They're strange, to say the least.
Pluto’s four smaller moons — Styx, Nix, Kerberos and Hydra — are differently fascinating than Charon. These moons are each 20 to about 50 times smaller than Charon, they aren’t round like Charon, and unlike pretty much every other moon in the solar system, they are not in synchronous rotation with their host planet — they don’t always show one face to their host.
Instead, they are spinning incredibly fast! Hydra, for example, the most distant of the four, spins 89 times for every orbit it makes around Pluto.
New Horizons mission scientists suspect these chaotic spin rates could be due to Charon exerting a strong torque on the bodies that stops them from settling into a more common, synchronous rotation.
But that’s not all. These moons also wobble far more than scientists expected small moons would. They behave more like spinning tops than anything else. This uncommon movement could be explained by observations that suggest these four moons were formed when multiple smaller moons merged. Which means at some point in Pluto’s history — likely in the aftermath of the theorized impact that formed Charon — the tiny planet had more than five moons, but the smaller moons coalesced into the ones we see today.
And it’s worth mentioning that these are Pluto’s only moons. New Horizons scientist expected to find more small moons – leftover debris from the impact – but none were found. Surprisingly, Pluto only has the five moons discovered from Earth.
