Pluto has wobbly moons, Hubble Space Telescope finds

At least two, and possibly all four, of Plato's moons are wobbling unpredictably, according to scientists at the Hubble Space Telescope.

New data shows that the moons, Nix and Hydra, are involved in an intricate dance with their parent planet Pluto. And to make it even more complicated, Pluto is considered to be a double planet itself, as it constantly orbits another object - Charon - which is almost half its size.

"Hubble has provided a new view of Pluto and its moons revealing a cosmic dance with a chaotic rhythm," said John Grunsfeld, associate administrator at NASA. "When the New Horizons spacecraft flies through the Pluto system in July we'll get a chance to see what these moons look like up close and personal."

Pluto's other two moons, Kerberos and Styx, are also likely to be locked into a similar pattern, the scientists say, embedded inside a dynamically shifting gravitational field caused by the system's two central bodies whirling about each other. The effect is intensified by the fact that most of the moons are ovid.

What's more, Hubble also found that the moon Kerberos is as dark as a charcoal briquette, while the other satellites are as bright as white sand - again to the surprise of astronomers who assumed meteorite impacts would coat each of the moons in dust of the same colours.

"Prior to the Hubble observations nobody appreciated the intricate dynamics of the Pluto system," said Mark Showalter of the SETI Institute.

The moons are so small and distant, no earth-based telescope has picked them up. Nasa's New Horizons probe is due to fly past in July - hopefully to unlock more secrets.