Nemesis is a hypothetical companion star to the Sun. Whether or not it actually exists, however, is a controversial matter.

The concept of Nemesis arose following the development by David Raup and Jack Sepkoski of the theory that periodic mass-extinctions have occurred throughout the Earth's history (for example, the interval sixty-five million years ago when dinosaurs became extinct). An increasingly-accepted explanation of such an occurrence suggests that an asteroid or comet slammed into the Earth, sending a cloud of debris into the atmosphere that blocked sunlight for decades. A lengthy "winter" then ensued, killing plants and decimating the food chain. Examination of sediment layers showed regular gaps in the fossil record, and Raup and Sepkoski suggested a regular cycle of 26 million years in which various life-forms died. Marc Davis, Piet Hut and Richard Muller believe a companion star to the Sun might be the catalyst for such an...