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Nemesis (hypothetical star)
Nemesis is a hypothetical red dwarf[1] or brown dwarf,[2] originally postulated in 1984[3] to be orbiting the Sun at a distance of about 95,000 AU (1.5 light-years),[2] somewhat beyond the Oort cloud, to explain a perceived cycle of mass extinctions in the geological record, which seem to occur more often at intervals of 26 million years.[2][4][citation needed] In a 2017 paper, Sarah Sadavoy and Steven Stahler argued that the Sun was likely part of a binary system at the time of its formation, leading them to suggest "there probably was a Nemesis, a long time ago".[5][6] Such a star would have separated from this binary system over four billion years ago, meaning it could not be responsible for the more recent perceived cycle of mass extinctions.[7]
More recent theories suggest that other forces, like close passage of other stars, or the angular effect of the galactic gravity plane working against the outer solar orbital plane (Shiva Hypothesis), may be the cause of orbital perturbations of some outer Solar System objects.[8] In 2010, A. L. Melott and R. K. Bambach found evidence in the fossil record confirming the extinction event periodicity originally identified in 1984, but at a higher confidence level and over a time period nearly twice as long.[9] However, in 2011, Coryn Bailer-Jones analyzed craters on the surface of the Earth and reached the conclusion that the earlier findings were statistical artifacts, and found that the crater record shows no evidence for Nemesis.[10] The Infrared Astronomical Satellite (IRAS) failed to discover Nemesis in the 1980s. The 2MASS astronomical survey, which ran from 1997 to 2001, failed to detect an additional star or brown dwarf in the Solar System.[11]
Using newer and more powerful infrared telescope technology which is able to detect brown dwarfs as cool as 150 kelvins out to a distance of 10 light-years from the Sun,[12] the Wide-field Infrared Survey Explorer (WISE survey) has not detected Nemesis.[13][14] In 2011, David Morrison, a senior scientist at NASA known for his work in risk assessment of near Earth objects, has written that there is no confidence in the existence of an object like Nemesis, since it should have been detected in infrared sky surveys.[13][15][16][17]
