Discovery of Three Irregular Uranian Moons

Three additional candidate irregular satellites of Uranus have been discovered during July 1999 using the Canada-France-Hawaii telescope on top of Mauna Kea in Hawaii.  Adding to the first 2 irregular satellites of the planet (found by members of the same team in 1997), this brings the uranian inventory to 16 regular and 5 irregular satellites, the most for any planet in the solar system. Irregular satellites are those which do not orbit in nearly-circular prograde orbits in the equatorial plane of the planet. The three new moons are smaller (diameters of roughly 20-40 km) than those found in 1997.

The discovery team consisted of an international collaboration of astronomers from the Observatory of Nice, France (B. Gladman, J-M. Petit, H. Scholl), McMaster University, Canada (J. Kavelaars), the Harvard-Smithsonian Center for Astrophysics, USA (M. Holman), and Cornell University (P. Nicholson, J.A. Burns). Follow up observations were obtained at the Mount Palomar 5-meter and Kitt Peak 4-meter telescopes, the atter in conjunction with D. Davis and C. Neese of the Planetary Science Institute (Tucson, USA). B. Marsden and G. Williams of the IAU Minor Planet Center computed preliminary orbits for the reported objects.

The three new candidate satellites were discovered in a search using the world-class CFH12K wide-field imaging camera, which is a mosaic of CCD cameras covering a very large patch of sky (currently 35x28 arcmin). This instrument allowed the team to explore more than 90 percent of the entire region around Uranus in which satellite orbits are stable, making this search almost, but not quite, complete to the limiting size visible to the CFHT telescope (corresponding to satellites with radii of 10 km or larger).

The term `candidate' is being used here because until the moons can be tracked for an appreciable fraction of their orbital period around the planet we cannot prove definitively that they are not Centaurs (comets on planet-encountering orbits in the outer solar system) that were fortuitously passing close to the planet. However, given the motion of the 3 new objects over the 2 months since their discovery, this possibility is becoming increasingly unlikely. By the time of the 1999 Division of Planetary Sciences meeting, we hope to be able to confirm the identity of these new denizens of the uranian system.