Comets are sometimes spectacular objects from the outer regions of the solar system, as far away as a substantial fraction of the distance to the nearest star. They appear to be typically a few kilometers in radius and are composed largely of icy substances. Their chemistry is, however, clearly complex. As a comet enters the inner solar system, it emits large amounts of volatile materials that are transformed by the energy of sunlight and of the solar wind into a variety of individual atoms, molecules, and ions, mostly of the common materials carbon, nitrogen, oxygen, and hydrogen and combinations that include these. Many complex molecules have been detected by spectroscopic analysis of comet tails. Comets also emit a large number of tiny dust particles.

The Dutch astronomer Jan H. Oort recognized (about 1950) that most of the apparently fresh comets coming into the inner solar system started from initial distances beyond 50,000 astronomical units. Furthermore, he recognized that the ease with which planetary perturbations can change the orbits of the comets meant that typical comets were unlikely to endure many orbital passages through the inner solar system. Because several comets are observed each year, this means that there must be a very large reservoir of them in the outer solar system. Oort suggested that a thick shell of cometary material surrounds the Sun very far beyond the orbits of Neptune and Pluto. This shell has not yet actually been observed. Any disturbances of the Oort cloud and the much nearer Kuiper belt could send material plunging into the solar system to be observed as a comet. Chiron is now conjectured to have had such an origin.