comet: The Oort Cloud

The origin of the solar system's comets is still uncertain. They were once thought to have originated outside the solar system, but more recent theories suggest they were formed during the formation of the solar system and are permanent members of it. According to the storage-cloud hypothesis proposed by J. H. Oort and since modified, a spherical shell of more than 100 billion comets surrounds the solar system at a distance of 20,000 AU to 50,000 AU or greater (1 AU, or astronomical unit, being the mean distance from the earth to the sun); some astronomers have suggested an inner Oort cloud exists beginning at 2,000 to 5,000 AU and extending to 20,000 AU. While the comets move very slowly in this huge storage cloud, a passing star may change the orbits of bodies in the outer reaches of the Oort cloud enough to force some of them into the inner part of the solar system. The mechanism for the Oort cloud's creation, however, is unclear; it has been suggested that the Oort cloud may include a significant amount of material that originated outside the solar system and was gravitationally captured by the sun.

Sections in this article:

• Introduction
• The Kuiper Belt
• The Oort Cloud
• Structure of Comets
• The Orbits of Comets
• Bibliography

The Columbia Electronic Encyclopedia, 6th ed. Copyright © 2024, Columbia University Press. All rights reserved.

See more Encyclopedia articles on: Astronomy: General