<Ul> <Li> Over the course of the Solar System's existence the orbits of comets are unstable and eventually dynamics dictate that a comet must either collide with the Sun or a planet or else be ejected from the Solar System by planetary perturbations . </Li> <Li> Moreover, their volatile composition means that as they repeatedly approach the Sun, radiation gradually boils the volatiles off until the comet splits or develops an insulating crust that prevents further outgassing . </Li> </Ul> <Li> Over the course of the Solar System's existence the orbits of comets are unstable and eventually dynamics dictate that a comet must either collide with the Sun or a planet or else be ejected from the Solar System by planetary perturbations . </Li> <Li> Moreover, their volatile composition means that as they repeatedly approach the Sun, radiation gradually boils the volatiles off until the comet splits or develops an insulating crust that prevents further outgassing . </Li> <P> Thus, Oort reasoned, a comet could not have formed while in its current orbit and must have been held in an outer reservoir for almost all of its existence . He noted that there was a peak in numbers of long - period comets with aphelia (their farthest distance from the Sun) of roughly 20,000 AU, which suggested a reservoir at that distance with a spherical, isotropic distribution . Those relatively rare comets with orbits of about 10,000 AU have probably gone through one or more orbits through the Solar System and have had their orbits drawn inward by the gravity of the planets . </P>

Where did comets that are now in the kuiper belt originally come from