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Nemesis is a hypothetical red dwarf〔 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.〔〔 , over 1800 brown dwarfs have been identified and none of them are inside the Solar System.〔 There are actually fewer brown dwarfs in our cosmic neighborhood than previously thought. Rather than one star for every brown dwarf, there may be as many as six stars for every brown dwarf.〔 The majority of solar-type stars are single.〔 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, may be the cause of orbital perturbations of some outer Solar System objects.〔 In 2011, Coryn Bailer-Jones analysed 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, A.L. Melott and R.K. Bambach found 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.〔〔〔〔 ==Claimed periodicity of mass extinctions== In 1984, paleontologists David Raup and Jack Sepkoski published a paper claiming that they had identified a statistical periodicity in extinction rates over the last 250 million years using various forms of time series analysis.〔 They focused on the extinction intensity of fossil families of marine vertebrates, invertebrates, and protozoans, identifying 12 extinction events over the time period in question. The average time interval between extinction events was determined as 26 million years. At the time, two of the identified extinction events (Cretaceous–Paleogene and Eocene–Oligocene) could be shown to coincide with large impact events. Although Raup and Sepkoski could not identify the cause of their supposed periodicity, they suggested a possible non-terrestrial connection. The challenge to propose a mechanism was quickly addressed by several teams of astronomers.〔〔 In 2010, Melott & Bambach re-examined the fossil data, including the now-improved dating, and using a second independent database in addition to that Raup & Sepkoski had used. They found evidence for a signal showing an excess extinction rate with a 27-million-year periodicity, now going back 500 million years, and at a much higher statistical significance than in the older work. They also determined that this periodicity is inconsistent with the Nemesis hypothesis. The change from 26 to 27 million years is expected based on a 3% "stretch" in the geological timescale since the 1980s.〔 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Nemesis (hypothetical star)」の詳細全文を読む スポンサード リンク
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