New moon for Uranus

The moon is ringed; the large object (bottom-left) is Uranus

By Dr David Whitehouse BBC News Online science editor

Astronomers have discovered a small moon circling the distant planet Uranus.

They say it is among the faintest objects ever detected in our Solar System.

It was first seen in August 2001, but quickly lost amid the glare from Uranus. It was only rediscovered and confirmed a few weeks ago, in August and September 2002.

"The extraordinary small moons we detected around Saturn convinced us that there should be similarly sized small moons around Uranus," Dr J Kavelaars of the National Research Council of Canada told BBC News Online. "Now we have found them."

Until 1997, Uranus was the only gas-giant planet in our Solar System without any known small, irregular moons. Now, including the latest one, six are known to orbit the planet.

Kavelaars' team included Matthew Holman, of the Harvard-Smithsonian Center for Astrophysics, Jean-Marc Petit, of the Observatoire de Besancon in France, Brett Gladman of the University of British Columbia and Dan Milisavljevic of McMaster University in Canada.

S 2001 U1: Lost and found

"We have looked at the Solar System in finer detail than anyone else before," said Dr Kavelaars.

Difficult to track

To find the moonlet, the astronomers used large telescopes in Chile.

The search technique, developed by Gladman and Kavelaars, has led to many new discoveries of faint objects in the outer Solar System.

These latest findings appear to support the theory that the small satellites are the remnants of a collision between a much larger body, orbiting around Uranus, and a passing comet.

The irregular satellites, which have no preferred orbital plane, are likely to be the chunks of material ejected from the surface of the parent object or objects.

The new discovery, officially called S 2001 U1, is just a few tens of kilometres across at most. It is so faint that even the best telescopes are having trouble tracking it.

Irregular satellites are those whose orbits are either very non-circular or very inclined with respect to the equatorial plane of the planet. They also tend to be very distant from the planet.