An article just published in the journal “Nature” describes a research on Pluto and its moons showing how two of them, Nix and Hydra, spin in an uncontrolled and unpredictable way. This study is based on an analysis of the observations made using the Hubble Space Telescope but because of the considerable distance they’re far from complete. For this reason, it’s possible that the two other small moons of Pluto, Styx and Kerberos, are in the same situation.
The situation of Pluto and its moons is peculiar. Its largest moon is Charon, which is really huge compared to it. To have a comparison, the Earth’s moon is considered massive compared to Earth but its mass is an eightieth of the Earth’s while Charon’s mass is about one-eighth of Pluto’s. For this reason, some scientists talk about a binary dwarf planet.
Because of its huge distance and its small size, Charon was discovered only in 1978. The surprises weren’t over because four small moons were discovered only in recent years: Nix and Hydra in 2005, Cerberus in 2011 and Styx in 2012. They’re asteroids more than normal moons, with lengths of the order of tens of kilometers and their shapes are probably oval.
It’s possible that this small system was created as a result of an ancient impact between two dwarf planets that ejected material that then formed Pluto’s moons. Pluto and Charon’s gravity are far dominant compared to those of the small moons. The consequence is that this gravitational influence generates chaotic movements in them.
This situation was discovered by examining the changes in the light reflected by Nix and Hydra and noting that their brightness changed unpredictably. The analysis of the observations made with the Hubble Space Telescope between 2005 and 2012 showed an irregular spinning movement of the two moons. This effect is amplified by their oval shape.
Despite that, the Pluto’s small moons’ orbits seem regular instead. In particular, Nix, Styx and Hydra are connected by what is called resonance of Laplace. It’s a mutual gravitational influence that keeps their orbits stable and was also observed in Jupiter’s system with its moons Io, Europa and Ganymede.
This chaotic situation could be an example in a small version of the behavior of planets orbiting a binary star. Such systems have been discovered but for the moment it’s impossible to examine the behavior of exoplanets. Even the observations of Pluto and its moons are incomplete. Astronomers expect new information in the coming weeks, when NASA’s space probe New Horizons will reach them.
Here’s a NASA video that simulates the chaotic movements of Nix.