The name “trans-Neptunian object” (TNO) has been given to the small solar-system bodies that constitute the Kuiper belt, beyond the orbit of Neptune. The existence of this population was first proposed by Kenneth Edgeworth and Gerard Kuiper on the basis of dynamical considerations. Over 1,300 trans-Neptunian objects are presently known. The first discovery of a TNO was made in 1992 by David Jewitt and Jane Luu. Trans-Neptunian objects are classified in different categories: the “classical” objects, at 42–47 AU from the Sun, have a low inclination and a low eccentricity; the “plutinos” are in resonance with Pluto and have the same semimajor axis as Pluto; and the “scattered” objects are more distant and have larger eccentricities.