The belt exists between about 30 AU, the outer edge of Neptune's orbit, and 50 AU, where Neptune's orbital resonance causes the number of objects to drop off rapidly. Farther out from the Kuiper Belt are the so-called scattered disk and the Oort Cloud.
Over the past 26 years, the number of Kuiper Belt objects (KBOs) found has grown to more than 1,000. Among the noteworthy KBOs are Pluto and its largest moon, Charon, which led to the debate over whether Pluto should be classified as a planet or an asteroid.
Objects in the Kuiper Belt that have a 3:2 orbital resonance with Neptune (that is, whose orbital period measures 50 percent longer than Neptune’s), like Pluto, are called plutinos. The first plutino discovered (after Pluto itself) was 1993 RO. The largest known plutino (again after Pluto), 90482 Orcus, was found in 2004. Based on observations made using the Spitzer Space Telescope in 2008, this body measures about 1,000 miles (1,600 kilometers) across. For comparison, in 2015, the New Horizons spacecraft showed Pluto has a diameter of 1,475 miles (2,375 km).