Oort cloud

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This diagram shows the presumed distance of the Oort cloud compared to the rest of the solar system.

The Oort cloud, alternatively termed 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 2000 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 confirmed, direct observations have been made of such a cloud, astronomers believe it 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 direct observations of the orbits of comets. The denser, inner part of the Oort cloud is also known as the Hills cloud;<ref>Hills, J. G. (November 1981). "Comet showers and the steady-state infall of comets from the Oort cloud". Astronomical Journal 86: 1730-1740. DOI:10.1086/113058.</ref> it has a well-defined outer boundary at 20-30 000 AU, a less well defined inner boundary at 50 to 3000 AU, and should be about 10 to 100 times denser than the remainder<ref>Planetary Sciences: American and Soviet Research, Proceedings from the U.S.-U.S.S.R. Workshop on Planetary Sciences, 1991, p. 251</ref>.

In 1932 Ernst Öpik , an Estonian astronomer, proposed<ref>Öpik, E., Note on Stellar Perturbations of Nearby Parabolic Orbits, Proceedings of the American Academy of Arts and Sciences, Vol. 67, pp. 169-182 (1932)</ref> that comets originate in an orbiting cloud situated at the outermost edge of the solar system. In 1950 the idea was revived and proposed<ref>Oort, J. H., The structure of the cloud of comets surrounding the Solar System and a hypothesis concerning its origin, Bull. Astron. Inst. Neth., 11, p. 91-110 (1950) Text at Harvard server (PDF)</ref> 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 really existed for billions of years (since the generally accepted 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 if the Oort cloud exists and supplies comets, in order for it to supply the necessary volume of comets, the total mass of comets in the Oort cloud must be many times that of Earth. Estimates range between 5 and 100 Earth masses.

The Oort cloud is thought to be a remnant of the original nebula that collapsed to form the Sun and planets approximately 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 perturbations in the comets' orbits and thereby increasing the number of comets that enter the inner solar system. The star with the greatest possibility of perturbing the Oort cloud in the next 10 million years is Gliese 710.

[edit] Oort cloud objects

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

So far, only two potential Oort cloud objects has been discovered, 90377 Sedna and 2000 CR₁₀₅.

90377 Sedna, with an orbit that ranges from roughly 76 to 928 AU, 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^{[citation needed]}; 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. 90377 Sedna is considered to be simply a Trans-Neptunian object; its orbit does not carry it completely out to the assumed position of the Oort Cloud and is too far out for it to be truly considered as a Kuiper Belt object.

Other astronomers may include the object 2000 CR₁₀₅ as part of the Oort cloud, this object has a perihelion of 45 AU, an aphelion of 415 AU, and an orbital period of 3420 years.

**Oort cloud objects (potential)**
Number	Name	Equatorial diameter (km)	Perihelion (AU)	Aphelion (AU)	Date discovered	Discoverer	Diameter method
90377	Sedna	<1800, >1180	76 (±7)	975	2003	Michael E. Brown, Chad Trujillo, David L. Rabinowitz	thermal
—	2000 CR₁₀₅	265 km	44,3	397	2000	Lowell Observatory	???
87269	2000 OO₆₇	28-87 km	20.8	1005.5	2000	Cerro Tololo telescope	???

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