Phoebe (fēˈbē) [key], in astronomy, one of the named moons, or natural satellites, of Saturn. Also known as Saturn IX (or S9), Phoebe is 137 mi (220 km) in diameter, orbits Saturn at a mean distance of 8,047,985 mi (12,952,000 km), has an orbital period of 550.5 earth days, and rotates on its axis in about nine hours. The outermost of the known satellites, Phoebe was discovered by American astronomer William H. Pickering in 1898. Phoebe's reflectivity is very low, and it appears to be composed of a mix of ice and rock. Its orbit is inclined 175° to Saturn; that is, its north pole is almost aligned with the planet's south pole. Phoebe orbits with retrograde motion, i.e., opposite to that of the planet's rotation. The sharp inclination of its orbit, its retrograde motion, and the resemblance of its apparent density and composition to Kuiper belt objects (see comet) suggest that Phoebe is a captured object similar to a comet or asteroid rather than a native satellite.