Analysis of ten years of data from the space telescope has revealed the unusual trajectories, and also suggests Pluto might in fact be a binary dwarf planet.
Pluto’s moons have been tracked closely for the first time, showing that they tumble unpredictably rather than keeping one face fixed on their host planet.
Astronomers also observed that Pluto, whose status was downgraded to a dwarf planet in 2006, might be better regarded as a binary dwarf as it is locked in orbit with its largest moon, called Charon.
The twin system creates an imbalanced and shifting gravitational field, which sends the tiny outer moons spinning chaotically, the measurements from the Hubble space telescope showed.
“Like good children, our moon and most others keep one face focused attentively on their parent planet,’ said Douglas Hamilton, professor of astronomy at the University of Maryland and a co-author of the study. “What we’ve learned is that Pluto’s moons are more like ornery teenagers who refuse to follow the rules.”
Charon, which has a mass about 11% of that of Pluto, and the planet orbit a common centre of mass every 6.4 days.
The tiny outer moons Styx, Nix and Hydra, which all have masses less than 0.001% of Pluto’s, take between 20 to 40 days to orbit the inner pair. Unlike almost any other moon to have been observed they do not keep a fixed face on their parent planet, but spin and wobble about their own axes.
“They speed up and slow down, rock their north pole towards the planet and back again and maybe even reverse direction,” said Hamilton. “It would be a pretty confusing system to be in.”
The erratic motion is likely to be enhanced by the moons being roughly rugby ball-shaped rather than spherical, he added.
Despite their chaotic rotations, the moons appear to be locked into stable trajectories, which the researchers said is likely to be due to them moving in resonant orbits, which line up exactly at regular time intervals. The time taken for a complete orbit for Styx, Nix and Hydra were in ratios of roughly 3:4:6 times that of Charon, respectively. “We think this is why the system is stable and what stops them from crashing into one another,” said Hamilton.
The findings, published in the journal Nature, were based on ten years of observations of Pluto from the Hubble space telescope, which the researchers re-analysed after the relatively recent discovery of the four small moons (Nix and Hydra were first observed in 2005, Kerberos and Styx in 2011 and 2012, respectively).
The data also suggests that Kerberos is as dark as charcoal, while the other moons are as bright as white sand. ‘This is a very provocative result,’ said lead author Mark Showalter, of the SETI Institute in California. Astronomers had predicted that dust created by meteorite impacts should coat all the moons evenly and that debris should be transferred between them, giving their surfaces a uniform look.
The scientists are hoping to investigate the appearance of the moons in more detail when Nasa’s New Horizons spacecraft flies by Pluto next month and beams back the first ever images of the planet at close quarters.