First, it must be stated that the Oort Cloud has not been proven beyond a doubt to exist, but is considered very likely to exist by many astronomers, which would explain a lot of observations made by astronomers on the nature of comets, and their orbits and origins. The cloud is said to be composed of a large number of comet nuclei, mostly composed of ices of common gases such as water and methane, and to be the cause of comets reaching the Solar System. It should not be confused with the Kuiper Belt, just beyond the orbit of Neptune, which is the origin of short distance comets, whereas the Oort Cloud is said to be the origin of long distance comets.
Short distance comets have orbits of less than 200 years, whereas long distance comets orbits last for thousands of years, thus separating them into two categories.
The Oort Cloud is said to be a ring of comets which defines the outermost reaches of the Solar System according to some astronomers. It is thought to be a remnant of the protoplanetary disc formed at the creation of the Solar System. It is thought to be the origin of long period comets, short period comets being from the Kuiper Belt.
It is named after the Dutch astronomer Jan Hendrik Oort, who in 1950 revived an earlier theory by Estonian astronomer Ernst Opik made in 1932 that comets originated from a distant cloud encompassing the solar system. Oort thought that this would explain the behaviour of comets within the Solar System.
The cloud is thought to be vast in size and located about a light year from the Sun. Its outer region is thought to contain trillions of comet nuclei separated by tens of millions of kilometres from one another. It’s total mass is estimated at roughly five times that of the Earth. It is thought to be mainly composed of several varieties of ice, however a discovery in 1996 suggests it may also be home to rock like objects.
Comets are thought to reach distances close to the Sun by the tidal forces acting on the Milky Way which affect the gravitational distortion of the cloud, and pulls comets towards the center of the galaxy. It is thought that up to 90 percent of comets are formed as a result of the galactic tides effect on the Oort Cloud. The other cause for comets reaching distances close to the Sun is thought to be the interaction between the cloud and the gravitational forces of the nearest stars or nearby large molecular clouds.
A bizarre theory made in 1984 by the physicist Richard Muller suggests that a brown dwarf star following an elliptic orbit with the cloud, known as Nemesis, passes once every 26 million years through part of the cloud, causing a multitude of comets to pass through the Solar System. Astronomer John Matese in 2002 stated that an object with the mass of Jupiter must be passing in a distant orbit from the Sun to cause an anomaly in comet numbers.
Source: http://en.wikipedia.org/wiki/Oort_Cloud
