The Oort cloud, (IPA: BrE /ɔːt klaʊd/, AmE /ɔɹt klaʊd/), alternatively termed the Öpik-Oort Cloud (/ˈøpik/-, like /ˈepik/ with a rounded /e/), is a postulated spherical cloud of comets situated about 50 to 50,000 AU from the Sun. This is approximately 1000 times the distance from the Sun to Pluto or nearly a light year. The outer extent of the Oort cloud places the boundary of our Solar System at nearly a quarter of the distance to Proxima Centauri, the nearest star to the Sun.
Although no confirmed direct observations of the Oort cloud have been made, astronomers believe it to be the source of all long period and Halley-type comets entering the inner solar system (some short-period comets, based on their orbits, may come from the Kuiper belt).
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 independently by Dutch astronomer Jan Hendrik 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 origin of the solar system), all would have been destroyed by now. According to the hypothesis, the Oort cloud contains trillions 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. 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.
 Structure and composition
The Oort cloud is thought to occupy a vast space from the outer boundary of the Kuiper belt at 50 AU to as far as 50,000 AU from the Sun. It can be subdivided into spherical outer Oort cloud (20,000-50,000 AU) and doughnut-shaped inner Oort cloud (50-20,000 AU). The outer cloud is only weakly bound to the Sun and supplies the long period (and possibly Halley-type) comets to the inner part of the Solar System. The inner Oort cloud is also known as the Hills cloud, and may be the source of Halley-type comets.. Some scientists think that the Hills cloud may contain much more material than the outer cloud. This hypothesis is employed to explain the continued existence of the Oort cloud over the course of billions of years.
The outer Oort cloud is commonly thought to contain several trillion individual comet nuclei larger than ~1.3 km, each tens of millions of kilometers apart. Its mass is not known with certainty, but is unlikely to be more than a few Earth masses . Earlier it was thought to be more massive (up to 380 Earth masses). However the improved knowledge about the size distribution of the long period comets led to much lower values. The mass of the inner Oort cloud is not currently known.
The vast majority of Oort cloud objects are believed to consist of various ices, but the discovery of the object 1996 PW suggests that it may also be home to rocky objects.
The Oort cloud is thought to be a remnant of the original protoplanetary disc that formed around the Sun approximately 4.6 billion years ago. Its outer part is loosely bound to the solar system, and thus easily affected by the motions of passing stars or other forces.
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. The current mass of the cloud (about 3 Earth masses) is only a small part of the mass of ejected material (50-100 Earth masses). While on the distant outer regions of these orbits, gravitational interaction with nearby stars and galactical tides further modified their orbits to make them more circular. This explains a near spherical shape of the outer Oort cloud. On the other hand the Hills cloud, being bound stronger to the Sun, hasn't acquired spherical shape yet. Recent studies have shown that the formation of the Oort cloud is broadly compatible with the hypothesis that the Solar System formed as part of an embedded cluster among between 200 and 400 stars. These early stars likely played a role in the cloud's formation.
 Star perturbations and Nemesis theory
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 interactions of the Oort cloud with those of neighboring stars, and its deformation by the galactic tide are thought to be the main triggers which send the long-period comets into the inner Solar System. This process also serves to scatter the objects out of the ecliptic plane, explaining the cloud's spherical distribution.
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 (brown dwarf or gaseous giant planet) in an elliptical orbit beyond the Oort cloud. This object, 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. Furthermore, many argue that a companion star at such a great distance could not have a stable orbit, as it would probably be ejected by perturbations from other stars.
 Oort cloud objects (OCO)
|TNOs and similar bodies|
So far, only two objects with orbits which suggest that they may belong to the Oort Cloud have been discovered: 90377 Sedna and 2000 CR105. Unlike scattered disk objects, their orbits cannot be explained by perturbations of the main known planets and may thus belong to the inner Oort cloud. Their orbits can then be explained by one of two theories. Either these objects were Oort cloud bodies disrupted by the passage of a neaby star close to the solar system, or else their orbits were disrupted by an as-yet-unknown planet-sized body within the Oort Cloud.
|Perihelion (AU)||Aphelion (AU)||Year discovered||Discoverer||Diameter method|
|90377||Sedna||1180 - 1800 km||76.1||892||2003||Brown, Trujillo, Rabinowitz||thermal|
|148209||2000 CR105||265 km||44.3||397||2000||Lowell Observatory||???|