Nemesis (hypothetical star)

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 (on a scale of millions of 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 to explain the recurring mass extinctions on Earth, and the primary motivation for the existence of Nemesis was the belief that mass extinction events occured periodically. More careful examination has cast 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.