More moons. S/1999 U 1 (left) and S/1999 U 2 are Uranus's 19th and 20th satellites.

Uranus Breaks Record With Two New Moons

Move over, Saturn. Uranus is now the planet with the largest number of moons. An international team of astronomers led by JJ Kavelaars of McMaster University in Hamilton, Ontario, say they have discovered two new satellites circling the planet, bringing the total to 20. And they think there may be no more to be found.

The two small satellites--both have diameters under 20 kilometers--were found on images taken with the 3.5-meter Canada-France-Hawaii Telescope at Mauna Kea, Hawaii, on 18, 19, and 21 July, and reported today in a circular of the International Astronomical Union. They are provisionally called S/1999 U 1 and S/1999 U 2. "We're still thinking about appropriate names," says Kavelaars.

Two years ago, Kavelaars and three other scientists, using the 5-meter Hale telescope at Palomar Mountain, California, found the 16th and 17th satellites of Uranus, now named Caliban and Sycorax (ScienceNOW, 31 October 1997). And just 3 months ago, Erich Karkoschka of the University of Arizona, Tucson, found an 18th satellite on old Voyager 2 photographs (ScienceNOW, 19 May 1999), putting Uranus's retinue on a par with Saturn's. The new moons could never have been photographed by Voyager, because they're in very wide orbits, says Kavelaars; currently, they're 25 million and 10 million kilometers away from Uranus, whereas most other satellites circle the planet at distances of less than 100,000 kilometers.

The new satellites' orbits are elongated and inclined with respect to the planet's equator, but they couldn't be determined unequivocally. That means there's still a remote possibility that they are not moons at all but rather asteroids orbiting the sun, says Brian Marsden of the Harvard-Smithsonian Center for Astrophysics. But Kavelaars disagrees: "We have searched the complete area out to 100 million kilometers from Uranus, and these were the only two [new] objects we found," he says. "It would be an odd coincidence" if they were anything else than satellites. The team will make additional observations with the Hale telescope within a few weeks. "I'm pretty sure the final confirmation will come from our team," Kavelaars says.

But will he or other astronomers bag Uranus's 21st moon before long? Kavelaars thinks not. At even larger distances, a satellite's orbit would be unstable because of the sun's and other planets' gravity. "This could be the end of it," says Kavelaars.