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.
Lesson plans, interactive activities, and other resources to help students learn about and explore our solar system