These figures show the discovery images of the Jupiter satellite S/2001 J3. The motion of the satellite clearly stands out compared to the steady state background of stars and galaxies. Jupiter is not in the field, but is a few degrees to the West of these images. The satellite has a green circle around it to show its position in the three images. Each image is 50 arcseconds long and 90 arcseconds in height with North on top and East to the right. The images were taken on December 9, 2001 at 13:38:33, 14:28:19, and 15:04:10 UT. Click on the images in order to see them without the circles or here to see color images. Credit: University of Hawaii

Astronomers announced Thursday that they have discovered 11 additional moons orbiting the planet Jupiter, giving the giant planet more moons than any other planet in the solar system.

The Minor Planets Center of the International Astronomical Union announced the discovery of 11 small, irregular moons orbiting Jupiter in a circular issued Thursday. The discovery was made by astronomers David Jewitt and Scott Sheppard of the University of Hawaii's Institute for Astronomy and Jan Kleyna of Cambridge University.

All 11 moons are small bodies, with diameters believed to be between two and four kilometers. The moons are in inclined, highly elliptical orbits with an average distance of about 300 Jupiter radii, or 21 million kilometers, from the planet. The orbits are also all retrograde, meaning that the moons orbit Jupiter in the opposite direction of the planet's rotation.

Thursday's announcement increases the number of moons known to orbit Jupiter to 39. This puts the planet, the largest in the solar system, firmly in first place when ranking them by the number of moons. Saturn, formerly in first place, is now a distant second with 31 known moons.

This discovery was made using images taken in December 2001 at the 3.6-meter Canada-France-Hawaii Telescope atop Mauna Kea, Hawaii. The images were obtained by one of the largest digital cameras used in astronomy today, "12K", so named because the detector is 12,000 pixels across. The images were analyzed by computers to look for objects that could be moons; those candidates were later observed by a 2.2-meter telescope on Mauna Kea to confirm their identity and track their orbits.

The discovery is the latest in a string of discoveries over the last several years that have turned up dozens of small moons orbiting all four gas giant planets in the solar system: Jupiter, Saturn, Uranus, and Neptune. These discoveries have been made possible by a new generation of electronic cameras that can both scan wide areas of the sky and detect dim objects. "These combinations make them very efficient in searching large areas of the sky such as that around Jupiter for these small dim moons," said co-discoverer Sheppard.

This discovery may also help planetary scientists understand how Jupiter acquired both these moons as well as 20 other Jovian moons that also lie in irregular orbits. The orbits suggest to astronomers that the bodies were captured from the asteroid belt. However, since it's very difficult in the modern solar system for a planet to capture a passing asteroid, astronomers believe that the moons may have been orbiting the planet since the early history of the solar system.

This schematic shows the orbits of the irregular satellites of Jupiter. The outer most regular satellite of Jupiter, the Galilean Callisto, is shown for reference. The inset in the upper right shows the discovery images for one of the newly discovered satellites. Black circles have been drawn to show its location and movement against the background stars and galaxies. Credit: University of Hawaii

Scientists have proposed two models to explain how a young Jupiter could have captured these moons. Under the gas drag model, small bodies would have been slowed down by passage through the proto-Jupiter's bloated atmosphere: those bodies that did not burn up would have been captured into elliptical orbits. An alternative model, mass capture, postulates that the rapid growth of Jupiter would have allowed the young planet to capture nearly planetesimals moving in similar orbits.

Sheppard believes that studying the size distribution of these irregular moons may allow scientists to differentiate between the two models. "The gas drag mechanism predicts there should be a cut off at some small size in the satellites since these objects would feel the friction from the gas drag much more than larger objects," he explained. "The mass growth mechanism doesn't have any size dependence. Thus close examination and a complete sample of the satellites of Jupiter may allow us to prefer one theory over the other."

Sheppard said that his group currently prefers the gas drag model since the moons they have discovered are about the right size to be slowed into orbit by a bloated proto-Jupiter. However, he cautioned that the size cutoff predicted at the model may be too small than what can be currently observed.

Searches for more moons of Jupiter are planned for the near future, with every indication that there are many more moons waiting to be discovered. "Based on our surveys," said Sheppard, "we believe Jupiter has about 100 satellites larger than one kilometer in diameter."