The Tumbling Potato Moons of Pluto

Gravitationally Controlled Chaos Reigns at Pluto

Take two worlds in orbit around a common center of gravity and call them Pluto and Charon. Pluto’s the main one, Charon’s called a “moon”. Their motions around each other create a shifting gravitational field environment. Think of it like two tennis balls whirling around each other in a pond. If you are trying to navigate around them, you have to deal with the waves in the water.

In a similar way, anything ELSE in orbit in the Pluto system is going to be torqued around — literally — by the gravity fields interacting. That’s what’s happening with two of Pluto’s smaller worlds calle Nix and Hydra. They actually wobble quite chaotically in their orbits, and it’s quite likely that the other two moons — Kerberos and Styx — are also orbiting in a chaotic fashion.

These small, potato shaped moons are also tidally locked together, meaning that gravitational influences between them and their larger system buddies make it so if you could view the system from the surface of Nix, its sister moon Styx orbits twice for every three orbits made by Hydra, and so on. Nix, Styx, and Hydra are locked in a resonance, which means that there is a ration bwewteen their orbital periods.

So, the system isn’t completely chaotic. Nothing’s flying off into space, but they ARE tumbling wildly. The motions, as observed by the Hubble Space Telescope, are filling in details about this system and the motions of its members. This is a cool piece of information to get, now that we’re less than six weeks from the New Horizons mission to flyby of Pluto. The hits just keep on coming!

The big question now is: how did those moons get those wild orbits? The best hypothesis is that early in Pluto’s history, it collided with another dwarf-planet-like object. As usual with collisions, lots of debris was generated, and it went spinning into orbit. Gradually pieces coalesced, the resulting small moons fell into the orbits they occupy today. Of course, there are still some details to work out, but since collision and coalescence seem to be major themes in planetary formation, and not just in our own solar system, this seems to be the story that astronomers will follow up on.

One other cool thing about Hubble observations: it appears that the small moon Kerberos is dark — like charcoal. The others are much brighter, so why is this one so shady? Good question, and the answer will have to wait until New Horizons is there to take a direct “look” at the surface. Onboard instruments should uncover the chemical makeup of the surface.

Check out the Hubble Space Telescope web page about these wild moons to get more specifics on their orbits and this latest discovery!