Oort cloud


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Related to Oort cloud: Kuiper belt, Sedna

Oort cloud:

see cometcomet
[Gr.,=longhaired], a small celestial body consisting mostly of dust and gases that moves in an elongated elliptical or nearly parabolic orbit around the sun or another star. Comets visible from the earth can be seen for periods ranging from a few days to several months.
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Oort cloud

(oort, ort) (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.

Collins Dictionary of Astronomy © Market House Books Ltd, 2006
References in periodicals archive ?
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Now, technically, most scientists won't consider the probe to have really left the solar system until it passes through a sheath of comets known as the Oort cloud and is no longer influenced the by sun's gravity, but it will take Voyager another 300 years just to get there and another 30,000 to get through.
The chapters take up only a bit over 200 pages, and the bulk of the volume is devoted to appendices of technical information on such matters as virial theory, the formation of commensurate planetary orbits, perturbations of the Oort Cloud, global warming, and physical constants and useful data.
Well Volvo may be one step ahead of the Oort Cloud - check it out online - on that one.
NASA scientists estimate that comet Siding Spring has been on a more than a million-year journey, arriving from our solar system's distant Oort cloud.
The extracts which appear at regular intervals involve a number of fantastic and colourfully named characters like Starfish, Green Lady, Red Herring and the Oort Cloud's Wife.
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