Nemesis (hypothetical Star)
Nemesis is a hypothetical red dwarf star or brown dwarf, originally postulated in 1984 to be orbiting the Sun at a distance of about 95,000 AU (1.5 light-years), 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. As of 2011, over 1300 brown dwarfs have been identified and none of them are inside the Solar System.
More recent theories suggest that other forces, like close passings of other stars, or the angular effect of the galactic gravity plane working against the outer solar orbital plane, may be the cause of orbital perturbations of some outer Solar System objects. In 2011, Coryn Bailer-Jones did an analysis of craters on the surface of the Earth and reached the conclusion that the earlier findings of simple periodic patterns (implying periodic comet showers dislodged by a hypothetical Nemesis star) to be statistical artifacts, and found that the crater record shows no evidence for Nemesis. However, in 2010, Melott & Bambach found strong evidence in the fossil record confirming the extinction event periodicity originally claimed by Raup & Sepkoski in 1984, but at a higher confidence level and over a time period nearly twice as long. 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.
Using newer and more powerful infrared telescope technology, able to detect brown dwarfs as cool as 150 kelvins out to a distance of 10 light-years from the Sun, results from the Wide-field Infrared Survey Explorer (WISE survey) have not detected Nemesis. 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.
Read more about Nemesis (hypothetical Star): Claimed Periodicity of Mass Extinctions, Development of The Nemesis Hypotheses, Orbit of Sedna, Past, Current, and Pending Searches For Nemesis, Cultural References