The area beyond Neptune, often referred to as the outer solar system or simply the "trans-Neptunian region", is still largely unexplored. This region's first formation is the Kuiper belt, a great ring of debris, similar to the asteroid belt but composed mainly of ice and far greater in extent, which lies between 30 and 50 AU from the Sun. This region is thought to be the place of origin for short-period comets, such as Halley's comet. Though it is composed mainly of small solar system bodies, many of the largest Kuiper belt objects could soon be reclassified as dwarf planets. There are estimated to be over 100,000 Kuiper belt objects with a diameter greater than 50 km; however, the total mass of the Kuiper belt is relatively low, perhaps barely equalling the mass of the Earth.[33] Many Kuiper belt objects have multiple satellites and most have orbits that take them outside the plane of the ecliptic. The Kuiper belt can be roughly divided into two regions: the "resonant" belt, consisting of objects whose orbits are in some way linked to that of Neptune (orbiting, for instance, three times for every two Neptune orbits, or twice for every one), which actually begins within the orbit of Neptune itself, and the "classical" belt, consisting of objects that don't have any resonance with Neptune, and which extends from roughly 39.4 AU to 47.7 AU.[34] Members of the classical Kuiper belt are classified as Cubewanos, after the first of their kind to be discovered, 1992 QB1. Pluto and Charon Pluto, and its three known moonsPluto (39 AU average), is the largest known object in the Kuiper belt and was previously accepted as the smallest planet in the Solar System. In 2006, it was reclassified as a dwarf planet by the Astronomers Congress organized by the International Astronomers Union (IAU).[35] Pluto has a relatively eccentric orbit inclined 17 degrees to the ecliptic plane and ranging from 29.7 AU from the Sun at perihelion (within the orbit of Neptune) to 49.5 AU at aphelion. Prior to the 2006 redefinitions, Charon was considered a moon of Pluto, but in light of the redefinition it is unclear whether Charon will continue to be classified as a moon of Pluto or as a dwarf planet itself. Charon does not exactly orbit Pluto in a traditional sense; Charon is about one-tenth the mass of Pluto and the center of gravity of the pair is not within Pluto. Both bodies orbit a barycenter of gravity above the surface of Pluto (in empty space), making Pluto-Charon a binary system. Two much smaller moons, Nix and Hydra, orbit Pluto and Charon. Pluto lies in the resonant belt, having a 3:2 resonance with Neptune (ie, it orbits three times round the Sun for every two Neptune orbits). Those Kuiper belt objects which share this orbit with Pluto are called Plutinos.