A roughly spherical halo of comet nuclei surrounding the Sun out to perhaps 100 000 AU (over one-third of the distance to the nearest star). Its existence was proposed in 1950 by J. H. Oort to account for the fact that new comets approach the Sun on highly elliptical orbits at all inclinations. The Oort Cloud remains a theoretical concept, since we cannot currently detect inert comets at such great distances. The cloud is estimated to contain some 1012 comets remaining from the formation of the Solar System. The most distant members are fairly loosely bound by the Sun's gravity. There may be a greater concentration of comets relatively close to the ecliptic, at 10 000–20 000 AU from the Sun, extending inwards to join the Kuiper Belt. Oort Cloud comets are affected by the gravitational influence of passing stars, occasionally being perturbed into orbits which take them through the inner Solar System.
Subjects: Astronomy and Astrophysics.