Oort Cloud of comets. Artwork showing the Oort Cloud (orange), a postulated cloud of comet nuclei surrounding the Solar System. It is thought to extend over a light year from the Sun (centre), up to a distance of about 100000 astronomical units (AU). One AU is the distance from the Earth to the Sun, some 150 million kilometres. Near the Solar System planets it has the form of a disc in the plane of the planets' orbits (ecliptic). Further out, it forms a spherical shell. The existence of the Oort Cloud is inferred from the orbits of long-period comets. These tend to orbit the Sun on elongated paths (some seen here) with their most distant point around 50000 AU from the Sun.