According to preliminary orbits that may be determined from the available observations, the brighter object is moving along a highly eccentric and inclined path that is more than 8 million km from Uranus; the fainter object travels more than 6 million km from the planet. That is, respectively, they lie more than 300 and 200 planetary radii out. Each of the other giant planets in the outer solar system has been known for some time to possess similar moons, called irregular satellites, but these are the first ones in this class to be discovered about Uranus. Such moons are believed to have been captured from interplanetary orbits early in the solar system's life.
Prior to these discoveries, Uranus had been thought to have 15 moons, five of them identified by ground-based telescopes (the last in 1948) and ten found by the Voyager spacecraft during its 1986 flight through the system. These known moons travel along nearly circular, equatorial orbits that lie close to the planet, between 2 and 23 planetary radii.
The most recent discoveries of irregular satellites were those of the Jovian satellites Leda in 1974 (also at Palomar Observatory) and Ananke in 1951, and the Neptunian moon Nereid in 1949. The new Uranian satellites are the faintest ever imaged from the ground, and their discoveries were made possible by the use of a large telescope and a very efficient detector.
The diameters of the new satellites are estimated to be 160 km and 80 km, assuming that they reflect about 7% of the sunlight striking them. In this case, the largest of the Uranian objects would be comparable to the largest of the irregular satellites found about each of the other giant planets (Jupiter, Saturn and Neptune). The brighter of the pair appears quite red like some other outer solar system bodies; the color of the latter objects has been taken to indicate the presence of organic molecules.