Oort cloud. Artwork of the Oort cloud of comet nuclei thought to form a spherical halo around the solar system, with outer (spherical) and inner (toroidal) regions. The Sun and planets are not visible at this scale. The outer edge of the Oort cloud may be up to 100,000 astronomical units (AU) from the Sun, or around 1.5 light years (one AU is the distance from the Earth to the Sun). The Oort cloud comprises billions of comet nuclei, small lumps of rock and ice. Some of these fall into the solar system to become visible as comets. The bright horizontal band is the Kuiper Belt, a disc of asteroid-like bodies in the plane of the solar system.