Pluto's Moons Have a Strange and Wonderful Pattern of Orbit

You think you know Pluto? Think again. NASA’s New Horizons space probe sauntered by eight months ago, but it just keeps knocking down mysteries. In yet another surprise from that distant dwarf planet, scientists at the SETI Institute have discovered that Pluto’s moons orbit in a strange and unpredictable way.

Pluto’s largest moon, Charon, has a synchronous orbit with the dwarf planet, meaning that it rotates on itself exactly once for each orbit around Pluto. Like Earth’s moon and most of the other planetary moons, it has a near side and a far side (not, contrary to popular conception, a “dark” side).

But the four smaller rocks that circle Pluto — Styx, Nix, Kerberos, and Hydra — behave differently. They rotate more than once per orbit, and they wobble like tops. “The about tides from Pluto should have slowed down and re-oriented the spins of these moons by now, but they haven’t,” said SETI scientist Mark Showalter in the release.

Over time, astronomers expect orbiting bodies to sync up, as gravity locks them into position. Imagine an Olympic track athlete winding up for the hammer throw, and this is essentially what that looks like.

But Pluto’s little moons don’t behave this way at all. Their spins are weirdly chaotic. The scientists speculate that these moons collided with other space rocks, shoving the satellites into rapid spins.

This explanation makes a good deal of sense. Although the line between planets and dwarf planets continues to be blurry, the main distinction is this: Dwarf planets, unlike planets, share their orbit around the sun with smaller objects, like comets and asteroids. The theory is that the larger rocks have been around the block enough times to annihilate anyone else in their path, whereas the dwarfs still have some competition. Pluto’s moons, by definition, would be more prone to major collision with another orbiting body.

Pluto may be smaller than Earth’s moon but it looms large in our imaginations. It’s easy to see why Pluto’s demotion from planet to dwarf planet has so many people confused and upset — it has five moons, ferchrissake! It’s best to think of Pluto’s dwarf status not as a demotion but a recognition of how it is different — and just as fascinating — as its larger cousins.