While the orbit of our own moon is reliable enough to travel or set a clock by, the same does not hold for Pluto's moons Nix and Hydra. The pair of football-shaped moons spin in an orbit as chaotic and random as a TIE fighter piloted by a drunken Vac-head.
A new analysis of data from the Hubble telescope that will appear in the journal Nature [PDF] on Thursday finds that the moons tumble, spin and wobble while changing direction and rotation seemingly at random. The analysis focused on Pluto's second- and third-largest moons.
"Like good children, our moon and most others keep one face focused attentively on their parent planet," co-author Douglas Hamilton, a professor of astronomy at the University of Maryland, said in a release. "What we've learned is that Pluto's moons are more like ornery teenagers who refuse to follow the rules."
The erratic behavior of Pluto's moons is the fault of the dynamically shifting gravitational field created by Pluto and Charon. For many years it was thought that Charon was Pluto's lone moon, but in the past decade astronomers have discovered four minor moons orbiting what is really a binary planet system, made up of Pluto and Charon mutually orbiting a center of gravity between the them.
In other words, Pluto and Charon are like twin bodies engaged in an eternal and rambunctious dance, while four smaller moons orbit the pair. But the gravitational field created by this dance is much more complex than the one generated by the simple rotation of Earth. This fact, combined with the oblong shape of the smaller moons, is what the researchers believe causes their odd movements.
While the smaller movements and orientations of the moons are hard to predict, their orbital paths around Pluto and Charon are surprisingly predictable, the scientists found. It's a bit like a person who is able to walk along a path and consistently arrive at their destination without a problem, despite the fact that they might take a drastically different approach to each step. They might progress down their path partly walking forwards, partly walking backwards, and partly walking on their hands, for example -- this is one way to think of how Nix and Hydra (and presumably the other two minor moons, Kerberos and Styx) approach their orbital paths.
Aside from providing new insight into the world of one of our solar system's most maligned bodies, the new discoveries will also help inform thespacecraft's fly-by of the region next month. It could also be helpful in understanding how distant planets orbit binary star systems.
Thanks to Kepler and other space telescopes, there's evidence that worlds straight out ofmay actually orbit distant twin stars.
"We are learning that chaos may be a common trait of binary systems," Hamilton said. "It might even have consequences for life on planets orbiting binary stars."