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Oort cloud

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.

There is also a theory of a denser, inner part of the Oort cloud coined the Hills cloud;[1] it would have a well-defined outer boundary at 20-30 000 AU, a less well defined inner boundary at 50 to 3000 AU, and would be about 10 to 100 times denser than the remainder.[2]

In 1932 Ernst Öpik , an Estonian astronomer, proposed[3] that comets originate in an orbiting cloud situated at the outermost edge of the solar system. In 1950 the idea was revived and proposed[4] 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.

Origin

The Oort cloud is thought to be a remnant of the original solar nebula that collapsed to form the Sun and planets approximately 4.6 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.

A recent alternative hypothesis for the origin of the Oort cloud is that the comets were already present in the original solar nebula, even before the protosun and the protoplanetary disk was formed. Regarding the current distribution of the comets, this alternative hypothesis makes the same predictions as does the protoplanetary disk formation of comets.

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.

Star Perturbations and Nemesis Theory

The known star with the greatest possibility of perturbing the Oort cloud in the next 10 million years is Gliese 710. However, physicist Richard A. Muller and others have postulated that the Sun has a heretofore undetected companion star in an elliptical orbit beyond the Oort cloud. This star, known as Nemesis, is theorized to pass through a portion of the Oort cloud approximately every 26 million years, bombarding the inner solar system with comets. Although the theory has many proponents, no direct proof of the existence of Nemesis has been found.

Oort Cloud Objects (OCO)

So far, only three potential Oort cloud objects have been discovered, 90377 Sedna, 2000 OO67 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; 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.

Some astronomers include the objects 2000 CR105 and 2000 OO67 as part of the Oort cloud. The object 2000 CR105 has a perihelion of 45 AU, an aphelion of 415 AU and an orbital period of 3,241 years while the object 2000 OO67 has a perihelion of 21 AU, an aphelion of 1,000 AU and an orbital period of 12,705 years.

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