The Oort cloud, alternatively termed the Öpik-Oort cloud, is a hypothetical spherical cloud of comets situated about 50,000 to 100,000 astronomical unit (AU) from the Sun. This is approximately two thousand 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 a theory that periodic (every about 30 million years) disturbances of the Oort cloud by nearby star have been inducing massive falls of comets onto the Earth that caused mass extinction of life on Earth. If that is true, the Oort cloud is very significant in influencing the evolution of life on Earth (by supplying water and destroying life) as well as preserving primitive materials, from which planetary scientists can study the origin and evolution of the Solar System.
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 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 five and 100 Earth masses.
There is also a theory of a denser, inner part of the Oort cloud coined the Hills cloud; it would have a well-defined outer boundary at 20,000 to 30,000 AU, a less well defined inner boundary at 50 to 3,000 AU, and would be about 10 to 100 times denser than the remainder.
The most widely-accepted hypothesis of its formation is that objects in the Oort cloud 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 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 based on the records of heavy bombardments on the Earth that caused mass extinctions. 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.
|TNOs and similar bodies|
90377 Sedna's orbit that ranges from roughly 76 to 925 AU, 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. 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.
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.
|Perihelion (AU)||Aphelion (AU)||Date discovered||Discoverer||Diameter method|
|90377||Sedna||1180 - 1800 km||76 (±7)||975||2003||Michael E. Brown, Chad Trujillo, David L. Rabinowitz||thermal|
|—||2000 CR105||265 km||44.3||397||2000||Lowell Observatory||assumed albedo|
|87269||2000 OO67||28 - 87 km||20.8||1005.5||2000||Cerro Tololo telescope||assumed albedo|
All links retrieved September 20, 2007.
|Large trans-Neptunian objects|
|Kuiper belt: Orcus | Pluto | Ixion | 2002 UX25 | Varuna | 2002 TX300 | 2003 EL61 | Quaoar | 2005 FY9 | 2002 AW197|
|Scattered disc: 2002 TC302 | Eris | 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.
|The minor planetsedit|
|Vulcanoids | Near-Earth asteroids | Main belt | Jupiter Trojans | Centaurs | Damocloids | Comets | Trans-Neptunians (Kuiper belt · Scattered disc · Oort cloud)|
|For other objects and regions, see: asteroid groups and families, binary asteroids, asteroid moons and the Solar system
For a complete listing, see: List of asteroids. See also Pronunciation of asteroid names and Meanings of asteroid names.
|The Sun · Mercury · Venus · Earth · Mars · Ceres · Jupiter · Saturn · Uranus · Neptune · Pluto · Eris|
|Planets · Dwarf planets · Moons: Terran · Martian · Asteroidal · Jovian · Saturnian · Uranian · Neptunian · Plutonian · Eridian|
|SSSBs: Meteoroids · Asteroids (Asteroid belt) · Centaurs · TNOs (Kuiper belt/Scattered disc) · Comets (Oort cloud)|
|See also astronomical objects and the solar system's list of objects, sorted by radius or mass.|
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