The Kuiper Belt In 1951, Astronomer Gerard Peter Kuiper proposed the existence of a disc shaped region of space outside the orbit of Neptune as a source for short-period comets (comets that make complete orbits around the sun in less than 200 years). The Kuiper Belt went from theory to fact in 1992 when a 150 mile wide object called 1992QB1 was detected in the vicinity of the belt predicted by Kuiper. Since then, other objects have been detected giving final proof that the belt actually exists. Pluto which was discovered in 1930 is now considered to be a Kuiper Belt Object (KBO) and so far seems to be the largest object in the belt. Neptune’s satellites Triton and Nereid and one of Saturn’s satellites Phoebe have unusual orbits and are thought to perhaps be captured KBO’s. The Kuiper Belt is thought to extend sparsely outwards to the Oort Cloud.

The Oort Cloud In 1950, Dutch astronomer Jan Hendrik Oort proposed that comets might originate in a cloud of material, now called the Oort Cloud. Orbiting the sun at great distances, the orbits of these comets are occasionally disrupted by the gravitational influence of nearby stars sending them hurtling towards the inner solar system. So great is the distance of the Oort cloud from the Sun that some of these long period comets can take 30 million years to finish one trip around the Sun. Spherical in nature, the Oort Cloud is believed to exist about halfway between the Sun and the Heliopause. The Heliopause is the imaginary edge that marks the end of the Sun's influence in our solar system.