By Dr David Whitehouse BBC News Online science editor

Astronomers have discovered 11 new, small moons circling Jupiter.

They were discovered by a team led by Scott Sheppard and David Jewitt from the University of Hawaii's Institute for Astronomy.

When added to the 11 found last year by the same team, they bring the total of Jupiter satellites to 39, more than any other planet.

"We found them in a deliberate survey with the world's largest digital camera last December," Dr Jewitt told BBC News Online.

'Irrefutable satellites'

This instrument was attached to the Canada-France-Hawaii 3.6-metre (11.8 feet) telescope to obtain sensitive images of a wide area around Jupiter.

We are more familiar with the major moons like Io and Europa

"By following suspect objects for a few months, we were able to eliminate asteroid interlopers to settle on our final set of 11 irrefutable satellites," says Dr Jewitt.

He adds: "They're faint and probably small. The best guess is that they are just a few kilometres or miles in diameter."

The researchers say that the 11 new objects all belong to the "irregular satellite" class, meaning that they have large elliptical orbits around the giant planet.

All are in retrograde orbits (they orbit in the direction opposite to the rotation of the planet). A smaller number of objects are in prograde obits.

"We now know of 25 retrograde irregulars versus six progrades."

Random capture?

One possibility is that these new objects are captured asteroids.

David Jewitt: "How many more are there?"

"In random capture, we'd expect a 50:50 split - so we'll be thinking about why the retros are favoured in some way," says Dr Jewitt.

"One factor is that the retrograde orbits at large radii appear to be more stable over long timescales."

The irregular satellites are grouped into distinct families or clusters. This suggests that the individual satellites are pieces of a few precursor bodies that have been shattered.

Scientists speculate that the fragmentation took place either during the process of capture or possibly after capture due to collisions with Jupiter-crossing comets.

The new bodies bring the known total of Jupiter satellites to 39, of which 31 are irregulars. The eight regular satellites include four large objects discovered by Galileo and four small objects in circular orbits interior to that of the major volcanic moon Io.

Jupiter's nearest rival for having the largest number of known satellites is Saturn, with 30 (of which 13 are irregular).

"How many more are there?" asks Dr Jewitt. "We need a new, deep survey to establish the true population.

"We would like to make physical measurements. Are they all the same colour? Are there differences between them? We will try to answer those questions with the world's largest telescope, the Keck, later this year," he told BBC News Online.