Oort cloud (Oort-Öpik cloud)

A spherical shell, surrounding the solar system between about 2,000 and 20,000 AU from the Sun, which contains billions of comets with a total mass about that of the Earth. The cloud is invoked as a source for long-period comets, observed in the inner solar system, which could be deflected there by the gravitational influence of a passing star. The idea was first put forward by E. Öpik in 1932 and developed by J. Oort in the 1950s. (The term Öpik-Oort cloud is sometimes used.) There is no observational evidence for the existence of the cloud but the orbits of long-period comets, and dynamical studies of the formation of the solar system provide strong circumstantial evidence. It is argued that incipient comets were formed near the present location of the outer planets and ejected to a much greater distance later.