Hubble spots Pluto's moons are a chaotic mess of tumbling rock

Astronomy fans are still split over whether it was right to declassify Pluto as a planet in 2006 and call it a dwarf. Now the latest data from the Hubble Space telescope shows that the distant rock-ball is far weirder than we first thought.

Pluto has one large moon, dubbed Charon after its discovery in 1978, and the two orbit each other as binary planets. Four other moons, only discovered this century, orbit the pair. Now a new study, published in the journal Nature, shows that at least two of them are in chaotic orbits around the dwarf planet, in part due to the influence of Pluto and Charon.

"These two bodies whirl around each other rapidly, causing the gravitational forces that they exert on the small nearby moons to change constantly," said Doug Hamilton, professor of astronomy at the University of Maryland and co-author of the study.

"Being subject to such varying gravitational forces makes the rotation of Pluto's moons very unpredictable. The chaos in their rotation is further accentuated by the fact that these moons are not neat and round, but are actually shaped like rugby balls."

Only Jupiter has similarly chaotic moons and the astronomers think that the odd movement of Pluto's moons could be a result of how they were formed. It's postulated that Pluto was once much larger and suffered an impact that formed Charon and most of the chaotic moons around the dwarf planet.

"We are learning that chaos may be a common trait of binary systems," Hamilton continues. "It might even have consequences for life on planets orbiting binary stars."

We say most, because another surprising result from the new study is that another of Pluto's moons, Kerberos, is very much the odd man out. All of Pluto's outer moons are bright and shiny, probably because of a layer of dust formed by micrometeorite impacts, but Kerberos is as black as a charcoal briquette, suggesting it may be a wanderer grabbed by Pluto's gravity.

Scientists are going to get a clearer idea once the New Horizons probe reaches the dwarf planet on July 14. The probe will gather data as it whizzes past Pluto at four kilometers a second (8,950 miles per hour) but, as is so often the way in science, the results may raise as many questions as it answers. ®