Oort Cloud

Graphical Representation of Oort Cloud (obviously, not to scale)

The Oort Cloud is a spherical region of the Solar System that can effectively thought of as the boundary between our Solar System and interstellar space. Unlike the Kuiper Belt, the Oort Cloud does not lie mainly in the plane of the Solar System, but it does, however, contain objects similar to the Kuiper Belt. The Oort Cloud objects are mainly water ice and ammonia ice bodies.

The Kuiper Belt is home to short-period comets, which generally are comets with elliptical orbits and periods of less than 200 years. Oort Cloud comets have much longer periods (if they are periodic) of more than 200 years, and may have elliptical, parabolic, or hyperbolic orbits. The difference between elliptical orbits and parabolic and hyperbolic orbits, is that elliptical orbits are closed (comets with these orbits return periodically), while parabolic and hyperbolic orbits are open (comets with these orbits come towards the Sun once, never to return). When the eccentricity of an orbit equals 1, that is what defines a parabolic orbit, while eccentricities greater than one have hyperbolic orbits.

Different Orbits

There is a theory that the objects in the Oort Cloud formed closer to the Sun, but were bumped into the region of the Oort Cloud by gravitational interactions with the outer planets. It is about 100,000 AU in diameter with two main regions: the spherical outer Oort Cloud and a disk-like inner Oort Cloud (also called Hills Cloud). The cloud is named after Dutch astronomer Jan Oort and is sometimes referred to the Opik-Oort Cloud in conjection with Estonian astronomer Ernst Opik who postulated in 1932 that the long-period comets came from a cloud on the outer edges of the Solar System. Jan Oort revived this theory to help resolve a paradox on the stability of comets. Comets could not have formed in their orbits because the Sun's radiation would cause them to boil away.