Comets originate far out in the solar system – some from the Kuiper belt of icy bodies beyond the orbit of Neptune, and others from a more distant region known as the Oort Cloud.
Statistics imply that Oort Cloud may contain as many as a trillion comets and may account for a significant fraction of the mass of the solar system. However, since the individual comets are so small and so far away, we have no direct evidence about the actual existence of the Oort Cloud.
The Kuiper Belt is a disk-shaped region past the orbit of Neptune roughly 30 to 100 AU from the Sun. The Belt contains many icy bodies which can become comets. Occasionally the orbit of a Kuiper Belt object will be disturbed by gravitational interactions with the giant planets in such a way as to cause the object to take up an orbit that crosses into the inner solar system.
Although the Oort Cloud is much farther away from the Sun than the Kuiper Belt, it appears that the Oort Cloud objects were formed closer to the Sun than the Kuiper Belt objects. Small objects formed near the giant planets would have been ejected from the solar system by gravitational encounters. Those that didn’t escape entirely formed the distant Oort Cloud. Small objects that formed farther out had no such interactions, and remained as the Kuiper Belt objects.
Picture Credit : Google