In 1992, astronomers observed the first evidence of the Kuiper Belt, a population of icy objects — more than a thousand, at latest count — orbiting the sun at a distance of between thirty and fifty astronomical units.
After many more Pluto-sized and smaller icy objects were found orbiting beyond Neptune beginning in the 1990s, astronomers recognized that Pluto, far from being unique in its part of the solar system, is almost undoubtedly one of the larger and nearer pieces of this debris, known collectively as the Kuiper belt, that is left over from the formation of the planets.
Now, astronomers think Pluto belongs to the "trans-Neptunian objects" (TNOs), small icy objects beyond Neptune's orbit, more than 70 of which have been discovered recently.
Today astronomers have found about 1,000 other small icy objects (and there may be many more) beyond Neptune rotating around the sun, in an area called the Kuiper Belt.
After encountering the planet, the spacecraft would probably go on to one or more icy asteroidlike bodies in the Kuiper Disk, a cloud of small objects believed to be left over from the formation of the solar system.
More distant still, beyond the orbit of Neptune, is a band of icy objects called the Kuiper belt.
Its extremely elliptical orbit resembled the orbits of objects thought to exist in the Oort cloud, a distant cloud of icy objects that had been postulated by Dutch astronomer Jan Oort more than a half century before to account for the origin of comets.
Out in the Kuiper Belt, the dwarf planet is the largest object among a plethora of icy objects.
Since 1992, nearly 100 other small, icy objects have been detected in orbits beyond or crossing that of Neptune.
