No. 57: May-Jun 1988
Some 589 long-period comets are known. They ply orbits around the sun that may take millions of years to complete. Astronomers are generally agreed that these bodies originate in a very distant (100,000 A.U.*) halo of cometary material surrounding the entire solar system. J. Oort proposed this cloud, and it is named after him. Of course, we anomalists become wary when scientists "generally agree" on a hypothetical entity that no one can see. The Oort Cloud of comets, like the unseeable black holes, are given substance only by the effects they have on other solar-system denizens and seeable cosmic objects.
But there may be another cloud of comets that we can view directly. It is called the Kuiper Cloud (after G. Kui per). It is concentrated in the plane of the ecliptic just beyond the orbit of Neptune. Like the Oort Cloud, the Kuiper Cloud has not been seen yet, but we just might be able to with today's equipment! Its existence is hypothesized from the parameters of a different group of comets -- the so-called "short-period" comets, as exemplified by 76-year Halley's Comet. About 120 short-period comets have been discerned so far; and our computers now tell us that they cannot have originated in the Oort Cloud. Something closer and concentrated on the ecliptic is required. Thus the Kuiper Cloud or Belt was born. It is thought to be composed of debris left over after the formation of the solar system. (Kerr, Richard A.; "Comet Source: Close to Neptune," Science, 239:1372, 1988.)
Just before the referenced Science article appeared, a piece on comet origins was printed in the New Scientist. Curiously, the Kuiper Belt was not even mentioned. Instead, we find:
"Astronomers have discovered about 200 of these 'short'period' comets, with Halley's Comet the best known, but their orbits are of little use to astronomers studying the origin of comets."
(Theokas, Andrew; "The Origin of Comets," New Scientist, p. 42, February 11, 1988.)
* A.U. = Astronomical Unit = the distance from the sun to the earth.
Reference. We direct the reader to Category ACB15 in our catalog: The Sun and Solar System Debris, where the existence of the Oort Cloud is questioned. Details on the book here.
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