Presentation #300.01 in the session “Henry Norris Russell Lecture”.
Comets have inspired awe since prehistoric times, but even today there are only a few thousand comets with well-determined orbits. Nevertheless, the analysis of this limited sample yields a compelling model for the formation, evolution and present distribution of comets. This model implies that the primary source of comets is the Oort cloud, containing over 1011 comets at 5,000 to 100,000 times the Earth-Sun distance. I will review our current understanding of the history and current structure of the Oort cloud, and what comets can tell us about possible undiscovered planets beyond Neptune.