Astronomy Colloquium

Comets have inspired awe since prehistoric times, but their nature and origin have only been investigated in the last few decades. The
statistical analysis of a few thousand comets with well-determined orbits implies that there are two distinct sources of comets: the Oort cloud,
containing over 100 billion comets at 5,000 to 50,000 times the Earth-Sun distance; and the Kuiper belt outside Neptune's orbit. I will review our
current understanding of the formation of the Oort cloud and Kuiper belt, our successes and failures in explaining the properties of comets and their orbits, and the relation between comets and possible undiscovered planets in the outer solar system.