Pluto, once our most distant planetary sibling in the solar system and now re-classified as one of many “dwarf” cousins, is set to get a lot less mysterious this summer. NASA’s New Horizons spacecraft will give us our first real encounter with the dwarf planet, showing scientists just how unusual they are.
Pluto, once our most distant planetary sibling in the solar system and now re-classified as one of many “dwarf” cousins, is set to get a lot less mysterious this summer. NASA’s New Horizons spacecraft will give us our first real encounter with the dwarf planet, showing scientists just how unusual they are.
But new photos from the Hubble are already giving us clues. In a study published Wednesday in Nature, researchers report on the unusual interactions of Pluto’s known moons.
Pluto’s largest and most famous moon, Charon, is weird in itself: Instead of acting like a typical planet-and-moon pair, the two are locked facing each other and spin together around a fixed point. Some have argued that Charon and Pluto are more of a binary system than a moon and a planet.
Pluto probably has at least a few moons still too small for us to see, and researchers hope New Horizons will spot them sometime before its July flyby of the area. But for now, we know of four lesser moons: Styx, Nix, Kerberos and Hydra.
According to information gleaned from these new Hubble images, the four moons that orbit Pluto and Charon are thrown into chaos by the pair’s unusual gravity. If our moon is a well-behaved child of Earth, the researchers say, these four are a bunch of ornery teens.
“We are learning that chaos may be a common trait of binary systems,” Douglas Hamilton, professor of astronomy at the University of Maryland and a co-author of the Nature study, said in a statement. “It might even have consequences for life on planets orbiting binary stars.”
Pluto and Charon’s intimate dance creates an ever-changing gravitational field. In other words, things are pretty wobbly. For smaller objects caught in their gravitational pull, it can be a little unpredictable. To make things even more unstable, the moons are all irregularly shaped, making them prone to chaotic orbits.
But the moons have a saving grace: Each other. Nix, Styx and Hydra are locked in orbital resonance. That means they orbit at related intervals that influence each other. The changing gravitation field of Pluto and Charon can throw things out of whack, but the partnership of the tiny moons keeps them somewhat in order. Or keeps them from plowing into each other, anyway.
“The resonant relationship between Nix, Styx and Hydra makes their orbits more regular and predictable, which prevents them from crashing into one another,” Hamilton said. “This is one reason why tiny Pluto is able to have so many moons.”
What about Kerberos? It’s the odd moon out, and not just because it isn’t part of the triad. Nix and Hydra seem likely to be bright, white objects, like Charon (Styx is too tiny for us to make out its surface reflectivity, but data from New Horizons may change that). Kerberos appears to be quite dark in comparison. It’s not clear how Pluto’s moons formed, but the differences between Kerberos and the other moons suggest that they might not have all formed the same way.
If New Horizons picks up signs of more moons — and what their compositions are like — scientists may be able to pin down what sort of events led to this wibbly wobbly little system.