A region of the outer Solar System containing an estimated 107−109 icy planetesimals, or comet nuclei. The Kuiper Belt is an inner, flattened extension of the Oort Cloud. It lies more or less in the same plane as the planets and extends outwards from around 30 AU (the orbit of Neptune) to perhaps 1000 AU. Members of the Kuiper Belt are also known as trans-Neptunian objects. Such a vast reservoir of comets beyond Neptune was proposed in 1951 by G. P. Kuiper. The Irish engineer and astronomer Kenneth Essex Edgeworth (1880–1972) wrote papers about objects beyond Pluto in 1943 and 1949, so it is also known as the Edgeworth–Kuiper belt. In 1992, the British-born American astronomer David Clifford Jewitt (1958– ) and the Vietnamese-born American astronomer Jane Luu (1963– ) discovered the first Kuiper Belt object, 1992 QB1, now numbered (15760). This has a diameter of about 200 km, semimajor axis 44.2 AU, orbital period about 294 years, perihelion 40.9 AU, aphelion 47.5 AU, and inclination 2°.2. Since then, over a thousand have been found. The Kuiper Belt is thought to be the source of most periodic comets. Members of the Kuiper Belt that pass close to Neptune can be diverted inwards by gravitational perturbations to become Centaurs, or outwards to become scattered-disk objects. See also cubewano; plutino.