More than 1,000 KBOs have been identified since 1992. (They are sometimes called Edgeworth Kuiper Belt objects, acknowledging another astronomer who also is credited with the idea, or they are simply called transneptunian objects (TNOs.)
No spacecraft has ever traveled to the Kuiper Belt, but NASA's New Horizons mission, planned to arrive at Pluto in 2015, might be able to penetrate farther into the Kuiper Belt to study one of these mysterious objects.
Probably the most famous of the Kuiper Belt Objects (KBOs) is Pluto, once considered a planet.
Pluto was discovered by American astronomer Clyde Tombaugh in 1930, while working at Lowell Observatory in Flagstaff, AZ. The founder of the observatory, Percival Lowell had theorized that a planet beyond Neptune was affecting its orbit as well as that of Uranus. Tombaugh spent months studying images of the sky, looking for that needle in a haystack. Although Lowell was wrong about a planet affecting Uranus and Neptune's orbit, Tombaugh discovered Pluto, anyway.
After many other suggestions and much debate, the planet was named Pluto partly due to its distance from the sun, which keeps it perpetually in the dark, and partly because "PL" are the initials of Percival Lowell.
Pluto takes 248 years to orbit the Sun. Pluto's most recent close approach to the Sun was in 1989. Between 1979 and 1999, Pluto's highly elliptical orbit brought it closer to the Sun than Neptune, providing rare opportunities to study this small, cold, distant world and its companion moon, Charon.
On August 24, 2006, the International Astronomical Union (IAU) formally downgraded Pluto from an official planet to a dwarf planet. According to the new rules a planet meets three criteria: it must orbit the Sun, it must be big enough for gravity to squash it into a round ball, and it must have cleared other things out of the way in its orbital neighborhood. The latter measure knocks out Pluto.