Whether you want to call it a planet, dwarf planet, Kuiper Belt Object, or some or none 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 actually orbit each other around a central point outside the radius of either (if you feel like adding “binary” to whatever term you prefer to use, go right ahead) but it also has four other smaller moons in orbit that kinda sorta break the rules of how moons are “supposed to” behave.
In our Solar System — and very likely in other systems as well — natural satellites are typically “tidally locked” to planets, meaning that, over time, they have settled in to a rotation that keeps the same face always aimed in 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, notably Saturn’s Phoebe.) Pluto’s largest moon Charon even does this — 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 it.)
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 face-lock with Pluto. They tumble through space more like asteroids or comets than moons, causing scientists to wonder both why and how they got that way.
Want to know more? Watch the video below from NASA’s New Horizons team featuring Discovery News host and spaceflight historian Amy Shira Teitel: