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Discovery of two distant irregular moons of Uranus


The systems of satellites and rings surrounding the giant planets in the Solar System have remarkably similar architectures1. Closest to each planet are rings with associated moonlets, then larger ‘regular’ satellites on nearly circular orbits close to the planet's equatorial plane, and finally one or more distant, small ‘irregular’ satellites on highly elliptical or inclined orbits. Hitherto, the only departure from this broad classification scheme was the satellite system around Uranus, in which no irregular satellites had been found2. Here we report the discovery of two satellites orbiting Uranus at distances of several hundred planetary radii. These satellites have inclined, retrograde orbits of moderate eccentricity that clearly identify them as irregular. The satellites are extremely faint (apparent red magnitudes mR = 20.4 and 21.9), with estimated radii of only 60 and 30 km. Both moons 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 bodies3 orbiting in the outer Solar System.

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Figure 1: Observed positions of the newly discovered moons relative to Uranus.


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Observations at the Palomar Observatory were made as part of a continuing collaborative agreement between the California Institute of Technology and Cornell University. We thank the telescope operators and mountain staff at Palomar for their assistance; D. Tholen, A. Fitzsimmons, C.Hergenrother and S. Lilly for their confirming observations; and T. Yokoyama for numerical integrations of the provisional orbits. Support came from the Natural Sciences and Engineering Research Council of Canada (B.G. and JJK.), and NASA's Planetary Geology and Geophysics program (P.D.N. and J.A.B.).

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Correspondence to Brett J. Gladman.

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Gladman, B., Nicholson, P., Burns, J. et al. Discovery of two distant irregular moons of Uranus. Nature 392, 897–899 (1998).

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