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10 additional moons discovered around Jupiter

Posted: January 8, 2000

Jupiter as seen by the Cassini probe during its close encounter last month. Photo: NASA/JPL/University of Arizona. SEE FULL PAGE IMAGE
An unprecedented surge in planetary moon discoveries continues as astronomers announced Friday the discovery of 10 more moons orbiting Jupiter.

The ten natural satellites, provisionally designated S/2000 J2 through S/2000 J11, were first spotted in late November and early December by a group of astronomers at the University of Hawaii. The astronomers, led by graduate student Sam S. Sheppard and professor David Jewitt, used a wide-field camera mounted on the university's 2.2-meter telescope on Mauna Kea to find the moons.

All ten moons are believed to be very small: based on their magnitudes and estimates for their albedo, or reflectivity, they are thought to be no more than five kilometers in diameter. All ten are in moderately elliptical orbits inclined by 15 to 30 degrees. Nine of the ten are in retrograde orbits -- orbiting in the opposite direction than the planet's other moons -- at an average distance from Jupiter of 21 to 24 million kilometers. The tenth moon is in a prograde orbit about 13 million kilometers from the planet.

The discoveries bring to 28 the total number of moons orbiting the giant planet, with 12 of the moons discovered in just over a year. In addition to the ten spotted in November and December, the same University of Hawaii group spotted another moon in late November, designated S/2000 J1. Later calculations showed this was the same moon first seen in 1975 but lost shortly thereafter. A twelfth moon, S/1999 J1, was discovered in late 1999 with the Spacewatch telescope in Arizona but originally thought to be a slow-moving asteroid; only later analysis, confirmed by observations in the middle of last year, determined the small body was a moon.

Although Jupiter is the largest planet in the solar system, even these latest discoveries do not make it the planet with the most moons. That honor goes to Saturn and its 30 moons, 12 of which were discovered since late October by another group of astronomers from France, Canada, and the United States. Like the Jovian discoveries, Saturn's newly-discovered moons are also small and in distant, elliptical orbits, suggesting in both cases that the moons are captured bodies from either the asteroid belt or the Centaurs, a family of small icy bodies orbiting between Saturn and Uranus.

The latest finds are the latest in a surge of planetary moon discoveries unparalleled in the history of astronomy. In addition to the moons found orbiting Jupiter and Saturn, astronomers have also found six more moons orbiting Uranus since 1997: five from telescopic searches similar to the ones that netted Jupiter and Saturn's moons, and one from a reanalysis of Voyager 2 images taken when the spacecraft flew by the planet 15 years ago. As recently as October Uranus held the crown for the planet with the most moons with 21; it is now a distant third.

Astronomers credit this new surge in discoveries to dedicated searches using large telescopes and improved instruments that allow them to scan large areas around planets to look for the faint signatures of these moons. They also caution, however, that more work needs to be done to follow up these discoveries, making additional observations of the moons in order to refine their orbits.