Nemesis is the name given to a theoretical companion star or brown dwarf orbiting the Sun at a great distance beyond the Oort cloud, which periodically (roughly every 24 million years), passes through a denser region of the Oort cloud, disrupting the orbits of comets, sending them into collision with the Earth and the other planets.

Nemesis was proposed by Richard Muller to explain the recurring mass extinctions on Earth, and the primary motivation for the existence of Nemesis was the evidence from Raup and Sepkoski that mass extinction events occurred periodically. More careful examination has caused doubt that extinction events are in fact periodic. This and the total lack of observational evidence for Nemesis has caused the hypothesis to be largely discredited.

Nemesis is also the title of a science fiction novel written by Isaac Asimov.\n