(Updated post from 2016)
Whether you call it a planet, a dwarf planet, or a Kuiper Belt or Trans-Neptunian Object—or all of the above—there’s no denying that Pluto and its family of moons are true curiosities in the Solar System. Not only does little Pluto have one moon, Charon, that’s so massive in comparison that they both orbit each other around a point outside the radius of either, but it also has four other smaller moons in orbit that break the general rules of how moons are “supposed to” behave.
In our Solar System — and very likely in other systems too — most natural satellites are tidally locked to planets, meaning that over time they have settled into a rotation that keeps the same face always aimed toward their host planet as they orbit.
Our Moon behaves this way, as do the moons of Mars and most of the moons of the gas and ice giants (with the exception of some very small and far-orbiting moons, like Saturn’s Phoebe.)
Pluto’s largest moon Charon does this — and in fact Pluto and Charon are tidally locked to each other. If you were to stand on Pluto in sight of Charon it would always be in the same place in the sky as it went through its phases and the background stars passed behind; if you were on Charon, likewise with Pluto. (And if you stood on either in a place where the other wasn’t visible, you’d literally never see the other.)
But Pluto’s four wee moons — Nix, Hydra, Kerberos, and Styx, all of which were discovered between 2005 and 2012 — don’t rotate like that. Instead they each have their own rotation rates and resonances, none of which are in tidal lock with Pluto. They tumble through space more like asteroids or comets than moons, causing scientists to wonder why and how they got that way.
In fact Pluto’s outermost moon, Hydra, is very not-tidally-locked…it rotates 89 times on its axis for every one time it orbits Pluto!
It’s thought that the gravitational torque from the relatively enormous Charon is responsible for the unsettled movements of Pluto’s smaller moons. Read more here.
Want to know more? Watch a video below from NASA’s New Horizons team featuring spaceflight historian and author Amy Shira Teitel: