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Two New Moons Around Uranus

There's no moon in the sky tonight for Halloween, but creatures of the night with powerful telescopes have two new targets: A team of astronomers announced today that it has spotted two new moons around Uranus, bringing the count to 17. One of the discoverers, Philip Nicholson of Cornell University, says the satellites probably escaped detection for so long because they are so small--approximately 80 and 160 kilometers in diameter--and their eccentric orbits are far from the planet.

Nicholson and his colleagues, including Joseph Burns of Cornell, Brett Gladman of the University of Toronto, and J. J. Kavelaars of McMaster University in Hamilton, Ontario, first spotted the objects during a search for satellites of the gas giant. No one had made a systematic search for uranian moons with modern equipment, says Nicholson, and although the Voyager spacecraft had discovered 10 moons on its 1986 flyby, its view was much narrower than that of ground-based telescopes. As a side project to other observations, the team used the Hale telescope on Mount Palomar, California, to look for slow-moving objects near Uranus that might be moons.

In September, the astronomers took three exposures of the sky around Uranus, 30 minutes apart, for two nights in a row. When they analyzed their data a few weeks later, they found two objects moving slowly enough to be moons. This week, they were able to follow up their observations to confirm that the objects were not simply ill-placed comets or asteroids orbiting the sun. Observers in New Mexico and Hawaii also confirmed the sightings, and the International Astronomical Union announced the find in its Internet circular today.

The moons' eccentric orbits suggest that they may have once orbited the sun, but were captured by Uranus's gravity, says astronomer David Jewitt of the University of Hawaii. The other gas giants, Jupiter, Saturn, and Neptune, all have eccentric moons, and the new uranian satellites suggest that Uranus--even though its equator is tipped sideways--may have evolved like other gas giants. The leading theory to explain such moons holds that the planets once had much larger atmospheres, which were able to slow down and capture smaller objects as they traveled around the sun. "There is nothing funny about Uranus in this regard," Jewitt says. "It appears they are all good at capturing things."