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Regions Where Comets Are Found

Comets are found in two main regions of the cosmos: the Kuiper belt and the Oort cloud. Short-period comets -- comets that frequently return to the solar system -- probably originate from an area called the Kuiper belt. This belt is located within the solar system's ecliptic plane, beyond the orbit of Neptune. Astronomers found the first object in the Kuiper belt in 1992. Since that discovery many objects have been discovered within that region. These objects are usually small compared with planets. Their size ranges from 10 to 100 kilometers in diameter. Earth's diameter, for example, is 14,000 kilometers.

The Hubble Space Telescope may have detected a population of small comets dwelling in this region in space. Based upon the Hubble observations, astronomers estimate that this belt contains at least 200 million comets, which are thought to have remained essentially unchanged since the birth of the solar system 4.5 billion years ago. Long-period comets are thought to emanate from a vast, spherical cloud of frozen bodies called the Oort cloud, named for the Dutch astronomer Jan Hendrik Oort. This cloud of comets, which also orbits the Sun, resides in the farthest region of the solar system, beyond Neptune and Pluto.

The Oort cloud objects are made up of matter such as frozen ammonia (), methane (), cyanogen (), water ice (), and rock. Infrequently, a gravitational disturbance caused by a passing star or an interstellar cloud causes one of these bodies in the Oort cloud to begin a journey toward the inner solar system, where it makes a transient rendezvous with our Sun.