Most of the comets that enter the solar system originate in either the Oort Cloud or the Kuiper Belt, zones containing trillions of icy and rocky bodies orbiting in the furthest reaches of the sun's influence. Gravitational perturbations or collisions occasionally cause these comets to approach Earth.
The Oort Cloud is a large spherical cloud surrounding the solar system, and it is the source of most long-period comets that take more than 200 years to make a full orbit. The Kuiper Belt is a disc-shaped area of space beyond Neptune where most short-period comets originate. Both zones are believed to be remnants from the creation of the solar system around 4.6 billion years ago.