Oort cloud

The Oort cloud (ort, , alternatively the Öpik-Oort Cloud (: as with a rounded /e/)), is a postulated  spherical cloud of comets situated about 50,000 AU  from the Sun. This is approximately 1000 times the distance from the Sun to Pluto or nearly a light year. The outer extent of the Oort cloud places the boundary of our Solar System at nearly a quarter of the distance to Proxima Centauri, the nearest star to the Sun.
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Noun
1. (astronomy) a hypothetical huge collection of comets orbiting the sun far beyond the orbit of Pluto; perturbations (as by other stars) can upset a comet's orbit and may send it tumbling toward the sun
(hypernym) collection, aggregation, accumulation, assemblage
(classification) astronomy, uranology

The spherical region theorized to surround the solar system which contains a vast number of comets . These comets are disturbed by the passage of near-by stars and fall in towards the inner solar system. Comets coming from the Oort cloud are known as long period comets since they often have orbital periods measured in thousands of years. Some even have orbits which will eject them from the solar system altogether. The Oort cloud is thought to exist between the distances of 30,000 to 100,000 astronomical units. As yet there is no positive proof of its existence.