A supposed spherical shell, surrounding the solar system at a distance of about 1 light year (50,000 AU), containing billions of comets with a total mass about that of the Earth.
The cloud is invoked as a source for the 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. àpik in 1932, and developed by Jan Oort in the 1950s. (The term àpik-Oort cloud is sometimes used.) There is no direct evidence for the existence of the cloud, other than the need to explain an origin for comets. If it exists, it is unclear how it formed, though a popular theory is that the incipient comets were formed near the present location of the outer planets and ejected to a much greater distance later.
