Nemesis comets. Artwork of comets passing close to the Earth. The Nemesis star, which caused them, is seen in a dark lane of the Milky Way at upper right. Nemesis is a hypothetical companion star of the Sun. It is claimed to be a low-mass, dim red or brown dwarf star which has an eccentric orbit. It is claimed Nemesis perturbs the orbits of lumps of ice in the Oort Cloud at the edge of our solar system. These perturbed comets enter the inner solar system and become comets. The comets' tails are formed from gas and dust which has evaporated from their surface due to the Sun's heat. Impacts from Nemesis comets could be the cause of the periodic mass extinctions in Earth's history.