Swarm of billions of comets thought to surround the Solar System between 2000 and 20 000 AU from the Sun. First proposed by E. Öpik in 1932 and later developed by J. Oort in the 1950s. Its existence is based on studies of long-period comet orbits, which seem to have their aphelia in this zone.
A Dutch astronomer, Jan Oort, observed in 1950 that no comet had ever been seen with an orbit indicating that it came from interstellar space, nor did comets seem to have a preferred direction of origin. He therefore hypothesized that the Sun is surrounded by an immense cloud of comets - the Oort Cloud - extending from about 1200 times Pluto's distance from the Sun out to about 3 light years. (The nearest star, Proxima Centauri, is 4.25 light years away.) So distant that its members are only weakly bound to the Sun, the Oort Cloud is occasionally disturbed by passing stars that fling bits of it either into the inner solar system as comets or out into interstellar space. The Oort Cloud is thought to contain as many as a trillion comets, but since they are so small and at such a great distance, there is no direct evidence that it actually exists at all.