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Definition: Oort cloud from Merriam-Webster's Collegiate(R) Dictionary

(1974) : a spherical shell of cometary bodies believed to surround the sun far beyond the orbits of the outermost planets and from which some are dislodged when perturbed to fall toward the sun compare kuiper belt

Summary Article: Oort cloud from Collins Dictionary of Astronomy

(Öpik–Oort cloud)

A cloud of comets that move in orbits round the Sun with perihelia in the range 5–50 AU and aphelia in a zone with heliocentric distances between 30 000 and 100 000 AU, i.e. in a zone far beyond Pluto's orbit. Since the nearest star to the Sun – Proxima Centauri – is 270 000 AU away these comets can be perturbed by passing stars; this changes their orbits and send them in closer toward our Sun (or out toward the perturbing star). These perturbed comets have periods of hundreds of thousands of years. The Oort cloud is so distant that it acts as a refrigerated cometary reservoir.

The existence of a distant long-lived cloud of comets was first suggested in 1932 by Ernst Öpik. The same idea was proposed in 1950 by J.H. Oort who thought that comets originated from the disintegration of a planet some time in the early history of the Solar System. After this the major planets, Jupiter in particular, drove the majority of the comets either into the inner Solar System, where they decayed, or into the outer regions. Of this latter group 98% would have been lost from the Solar System, only 2% remaining in the Oort cloud. The number of comets in the cloud is uncertain but it could be somewhere in the range 1011 to 1012 with a total mass in the region of 1028 grams. The orbits are oriented at random. A cometary cloud of radius 100 000 AU has a half life of at least 1.1 × 109 years. The existence of the cloud shows up very clearly when the number distribution of comets is considered as a function of the reciprocal of the semimajor axes of the orbits. See also Kuiper belt.

© Market House Books Ltd, 2006

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