Published online 31 October 2005 | Nature | doi:10.1038/news051031-1

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Two extra moons for Pluto

Hubble spies tiny satellites around icy planet.

Spotting the distant moons is the Hubble telescope's latest achievement.Spotting the distant moons is the Hubble telescope's latest achievement.© NASA

Astronomers using the Hubble Space Telescope to gaze at the edges of the Solar System have spotted two small moons orbiting Pluto, raising its satellite count to three.

Pluto is part of the Kuiper belt, a distant ring of icy rubble beyond Neptune's orbit that is left over from the formation of the Solar System. Pluto was thought to be a loner when it was discovered as the ninth planet in 1930, but in 1978 its companion moon Charon was spotted.

The two new moons, provisionally called S/2005 P1 and S/2005 P2, are about 5,000 times fainter than Pluto. P1, which is between 55 and 160 kilometres across, is about 20% larger than P2, a relative tiddler at between 45 and 130 kilometres.

Their sizes are judged by the amount of light they reflect, but are hard to pin down because astronomers don't yet know whether the moons' surfaces are coated with reflective ice, tarry hydrocarbons or something in between.

Long time looking

The tiny satellites are at least twice as far from Pluto as Charon.The tiny satellites are at least twice as far from Pluto as Charon.© NASA

"I couldn't quite believe it at first," says Hal Weaver of Johns Hopkins University in Baltimore, Maryland, part of the team that discovered the moons. "People have been searching for satellites around Pluto for a very long time."

The team first spotted the satellites on 15 May, and subsequent observations have proved that they orbit Pluto at least twice as far away as Charon. P2 stays about 49,000 kilometres from Pluto, and takes about 25.5 days to travel around the planet; P1 lies even further away at 65,000 kilometres and orbits once every 38 days.

"This is the first Kuiper belt object known to have multiple satellites," says Weaver. The find could help astronomers to work out how the Pluto-Charon system first formed, he adds. Charon is an unusually large moon relative to its parent planet, being some 15% of the mass of Pluto. Our own Moon, in contrast, weighs in at just 1.2% of Earth's mass.

Crash course

Some astronomers think that Pluto acquired its mighty moon after a Pluto-sized body collided with the young planet, leaving behind a fragment of debris that became Charon. Although the new find does not confirm this picture, the fact that the moons orbit in the same plane as Charon suggests that they all got together during the same event, says Weaver.

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"If I had to place bets I'd go with an impact origin," agrees Robin Canup, an astrophysicist at the Southwest Research Institute in Boulder, Colorado, who has produced computer simulations to explain the formation of the Pluto-Charon system1. "But I don't think that we can say that firmly yet," she adds, pointing out that P1 and P2 may have been objects from the Kuiper belt that were merely captured by Pluto.

Recent advances in telescope power have led to a rash of newly discovered Kuiper belt objects. The discovery this year of 2003 UB313, which is larger than Pluto and has a more distant orbit from the Sun, re-ignited the debate over whether Pluto truly deserves to be called a planet (see ""Astronomers reject the term 'planet":http://www.nature.com/news/2005/050919/full/437456a.html'").

So might even more plutonian moons be lurking in the distant gloom? "These Hubble images represent the most sensitive search yet for objects around Pluto," says team member Andrew Steffl of the Southwest Research Institute, "and it is unlikely that there are any more moons larger than about 10 miles (16 kilometres) across in the Pluto system."