Reassessing the Source of Long-Period Comets

Inside Oort Long-period comets are thought to come from the outermost region of the solar system, the Oort Cloud, where a large number of icy bodies orbit the Sun. Outer Oort Cloud bodies are more likely to penetrate the inner planetary region of the solar system as comets, because they experience stronger external gravitational perturbations. Inner Oort Cloud bodies, by contrast, are thought to be ejected before they reach observable orbits. Using numerical simulations that followed the orbital histories of bodies in the inner Oort Cloud, Kaib and Quinn (p. 1234 , published online 30 July 2009; see the Perspective by Duncan ) identified a dynamical pathway that allows comets in the inner Oort Cloud to move to the outer Oort Cloud, where they can be perturbed and enter the visible region in great numbers. According to this mechanism, the amount of mass in this region is consistent with the material needed to form the giant planets.