DISCOVERY OF 2 DISTANT IRREGULAR MOONS OF URANUS

The systems of satellites and rings surrounding the giant planets in t he Solar System have remarkably similar architectures(1) Closest to ea ch planet are rings with associated moonlets, then larger 'regular' sa tellites on nearly circular orbits close to the planet's equatorial pl ane, and finally one or more distant, small 'irregular' satellites on highly elliptical or inclined orbits. Hitherto, the only departure fro m this broad classification scheme was the satellite system around Ura nus, in which no irregular satellites had been found(2). Here we repor t the discovery of two satellites orbiting Uranus at distances of seve ral hundred planetary radii. These satellites have inclined, retrograd e orbits of moderate eccentricity that clearly identify them as irregu lar. The satellites are extremely faint (apparent red magnitudes m(R) = 20.4 and 21.9), with estimated radii of only 60 and 30 km. Both moon s are unusually red in colour, suggesting a link between these objects -which were presumably captured by Uranus early in the Solar System's history-and other recently discovered bodies(3) orbiting in the outer Solar System.