What lies beyond Neptune: the Kuiper belt and the Oort cloud?

Beyond Neptune, the Solar System keeps going. First comes the Kuiper belt, a disc of icy bodies stretching from Neptune's orbit at 30 AU to about 50 AU from the Sun. It is similar to the asteroid belt but 20 times as wide and 20 to 200 times as massive, and its objects are made largely of frozen methane, ammonia and water. Pluto lives here, along with most of the objects accepted as dwarf planets.

Much farther out, the Oort cloud is a theorized cloud of billions of icy planetesimals surrounding the Sun at distances from 2,000 to 200,000 AU. It is a thousand times more distant than the Kuiper belt and mostly spherical. Nobody has seen it: no direct observation of the Oort cloud is possible with present imaging technology.

The Kuiper belt was found by two people who would not let the empty sky go. In 1987 astronomer David Jewitt was puzzled by the apparent emptiness of the outer Solar System and asked graduate student Jane Luu to help him look beyond Pluto: "If we don't, nobody will." After five years of searching, on 30 August 1992 they announced 1992 QB1. Thousands of Kuiper belt objects are now known.

The Oort cloud exists on the strength of an argument. In 1950 Jan Oort reasoned that comets that approach the Sun cannot have been doing so for 4.5 billion years, so they must be stored in a distant reservoir. Meanwhile Voyager 1, the most distant spacecraft, will not reach the cloud for about 300 years.