The Oort Cloud
The Oort cloud is an immense spherical cloud of icy bodies around the solar system, together with some bodies close to the plane of the solar system, extending to the boundary of the Sun's influence, about 3 light years or 200,000 Astronomical Units. The Oort cloud is the origin of many comets, with period from 200 – a few million years.
Bodies in the Oort cloud are weakly bound to the Sun and can be disturbed by passing objects – stars or giant clouds of dust and gas - sending them into the solar system, or interstellar space. Such disturbance happen every few hundred million years. In addition, the Oort cloud is subject to tidal forces because of the rotation of the Milky Way, and many bodies are lost to interstellar space this way.
The total mass of comets in the Oort cloud is estimated to be 40 times that of Earth. This matter is believed to have originated at different distances and therefore temperatures from the sun, which explains the compositional diversity observed in comets.