Nemesis

Nemesis
Diameter	300,000 km
Mass of Planet	Unknown

Nemesis is a hypotethical brown dwarf star thought to be found beyond the Oort Cloud. Nemesis might be causing the mass extinctions every few million years by crossing the Oort Cloud and sending Long-period comets bombarding the inner solar system. The existence of Nemesis is nearly unaccepted due to nearly no evidence of our solar system being a binary star system, and the fact that binary star system stars are usually a lot closer to each other.