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The Oort Cloud

The Oort cloud (sometimes called the Öpik-Oort Cloud) is a postulated spherical cloud of comets 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 eclipticfrom the Kuiper belt . Although no direct observations have been made of sucha 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 theoutermost edge of the solar system. In 1950 the idea was revived and proposed by Dutch astronomer Jan Oort toexplain an apparent contradiction: comets are destroyed by several passes through the inner solar system, yet if the comets weobserve had existed since the origin of the solar system, all would have been destroyed by now. According to the theory, the Oortcloud contains millions of comet nuclei, which are stable because the sun's radiation is very weak at their distance. The cloudprovides a continual supply of new comets, replacing those that are destroyed.

The Oort cloud is a remnant of the original nebula that collapsed to form the sunand planets five billion years ago , and is loosely bound to the solar system. The most widely-accepted theoryof its formation is that the Oort cloud's objects initially formed much closer to the Sun aspart of the same process that formed the planets and asteroids , but that gravitational interaction with young gas giants such as Jupiter ejected them into extremelylong elliptical or parabolic orbits. This process also served to scatter the objects out of the ecliptic plane, explaining thecloud's spherical distribution. While on the distant outer regions of these orbits, gravitational interaction with nearby starsfurther modified their orbits to make them more circular.

It is thought that other stars are likely to possess Oort clouds of their own, and thatthe outer edges of two nearby stars' Oort clouds may sometimes overlap, causing the occasional intrusion of a comet into theinner solar system.

So far, only one potential Oort cloud object has been discovered; 2003 VB12 (Sedna) . With an orbit that ranges from roughly 76 to 850 AU, it is much closer thanoriginally expected and may belong to an "inner" Oort cloud. If Sedna indeed belongs to the Oort cloud, this may mean that theOort cloud is both more dense and closer to the Sun as was previously thought. This has been proposed as possible evidence thatthe Sun initially formed as part of a dense cluster of stars; with closer neighbors during Oort cloud formation, objects ejectedby gas giants would have their orbits circularized closer to the Sun than was predicted for situations with more distantneighbors.

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