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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. This is approximately 1000 times the distance from the Sun to Pluto or roughly one light year, almost a quarter of the distance from the Sun to Proxima Centauri, the star nearest the Sun.

The Oort cloud would have its 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^[1] by Dutch astronomer Jan Hendrick 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 hypothesis, the Oort cloud contains millions of comet nuclei, which are stable because the sun's radiation is very weak at their distance. The cloud provides a continual supply of new comets, replacing those that are destroyed. It is believed that the total mass of comets in the Oort cloud is many times that of Earth, and estimates range between five and 100 Earth masses.

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

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. The star with the greatest possibility of perturbing the Oort cloud in the next 10 million years is Gliese 710.

Oort cloud objects

TNOs and similar bodies
cis-Neptunian objects centaurs trans-Neptunian objects (TNOs) Kuiper belt objects (KBOs) cubewanos (classical KBOs) plutinos (3:2 resonant KBOs) twotinos (2:1 resonant KBOs) scattered disc objects (SDOs) Oort cloud objects (OCOs)

So far, only one potential Oort cloud object has been discovered; (90377) Sedna. With an orbit that ranges from roughly 76 to 928 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 denser and closer to the Sun than 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.

Oort cloud objects

Number	Name	Equatorial diameter (km)	Perihelion (in AU)	Aphelion (in AU)	Date discovered	Discoverer	Diameter method
90377	Sedna	<1800, >1180	76 (±7)	928	2003	Michael E. Brown, Chad Trujillo, David L. Rabinowitz	thermal

References

1. ^ Oort, J. H., The structure of the cloud of comets surrounding the Solar System and a hypothesis concerning its origin, Bull. Astron. Inst. Neth., vol. 11, p. 91-110 (1950) Text at Harvard server (PDF)

External links

• Representation, Southwest Research Institute
• The Kuiper Belt and The Oort Cloud

Large trans-Neptunian objectsedit

Kuiper belt: Pluto (Charon) | Orcus | Ixion | 2002 UX25 | Varuna 2002 TX300 | 2003 EL61 | Quaoar | 2005 FY9 | 2002 AW197

Scattered disc: 2003 UB313 | 2004 XR190 | Sedna†

See also Triton, astronomical objects and the solar system's list of objects, sorted by radius or mass.  For pronunciation, see: Centaur and TNO pronunciation. † Current MPC classification. Some consider Sedna an Oort cloud object.

The minor planetsedit

Vulcanoids | Main belt | Groups and families | Near-Earth asteroids | Jupiter Trojans

Centaurs | Damocloids | Comets | Trans-Neptunians (Kuiper belt · Scattered disc · Oort cloud)

For other objects and regions, see: Binary asteroids, Asteroid moons and the Solar system For a complete listing, see: List of asteroids. For pronunciation, see: Pronunciation of asteroid names.

The Solar System
Sun - Mercury - Venus - Earth (Moon) - Mars - Asteroid belt - Jupiter
Saturn - Uranus - Neptune - Pluto - Kuiper belt - Scattered disc - Oort cloud
See also astronomical objects and the solar system's list of objects, sorted by radius or mass.

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