Oort cloudcomets situated about 50,000 to 100,000 AU from the sun (approximately 1000 times the distance from the Sun to Pluto); with an inner disk at the ecliptic from the Kuiper belt. Although no direct observations have been made of such a cloud, it is believed to be the source of most or all comets entering the inner solar system (some short-period comets may come from the Kuiper belt), based on observations of the orbits of comets.
In 1932 Ernst Öpik, an Estonian astronomer, proposed that comets originate in an orbiting cloud situated at the outermost edge of the solar system. In 1950 the idea was revived and proposed by Dutch astronomer Jan Oort to explain an apparent contradiction: comets are destroyed by several passes through the inner solar system, yet if the comets we observe had existed since the origin of the solar system, all would have been destroyed by now. According to the theory, the Oort cloud contains millions of comet nuclei, which are stable because the sun's radiation is weak at their distance. The cloud provides a continual supply of new comets, replacing those that are destroyed.
This diagram shows the presumed distance of the Oort cloud compared to the rest of the [[solar system
It is thought that other stars are likely to possess Oort clouds of their own, and that the outer edges of two nearby stars' Oort clouds may sometimes overlap, causing the occasional intrusion of a comet into the inner solar system.
So far, only one potential Oort cloud object has been discovered; Sedna. With an orbit that ranges from roughly 76 to 850 AU, it is much closer than originally expected and may belong to an "inner" Oort cloud. If Sedna indeed belongs to the Oort cloud, this may mean that the Oort cloud is both more dense and closer to the Sun as was previously thought. This has been proposed as possible evidence that the Sun initially formed as part of a dense cluster of stars; with closer neighbors during Oort cloud formation, objects ejected by gas giants would have their orbits circularized closer to the Sun than was predicted for situations with more distant neighbors.
|Perihelion (in AU)||Aphelion (in AU)||Date discovered||Discoverer||Diameter method|
|Sedna (2003 VB12)||<1800, >1250||76 (ñ7)||~850||2003||Mike Brown, Chad Trujillo, David Rabinowitz||thermal|
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