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 event. (Unknown to them, Daniel Whitmire and Albert Jackson were proposing the same thing.) Muller named the hypothetical star after the Greek goddess who ensured that no mortal ever successfully challenged the gods. Other names he considered were Kali, the Hindu goddess of destruction, and Indra, vedic god of storms and war.
According to the Nemesis theory, a red dwarf or a brown dwarf star with a 26,000 year period orbits the Sun. When farthest from the Sun, the star is 14 trillion miles (22 trillion km), or 2.4 light-years, away; at its closest it is about 3 trillion miles (5 trillion km), which is over one half light-year away and still beyond the orbit of Pluto. Astronomer Peter van de Kamp claims a star at this distance would be virtually undetectable; yet it could be inferred by its gravitational effect on other objects. When Nemesis is closest to the Sun, it should have a considerable effect on the Oort cloud, which, according to astronomer Jan Oort, may be the origin of comets. The gravitational pull of Nemesis would disturb the material in the cloud and send it falling toward the inner solar system. Some of the in-fall of these dozens, if not hundreds, of meteors and meteorites would undoubtedly hit the Earth, Moon and other planets, with devastating effect.
Many craters on the Earth have an age of 26 million years, corresponding to the mass-extinction cycle and the period of the orbit Muller has proposed for Nemesis. In addition, it has long been known that the orbits of the outer planets do not exactly match their predicted positions. The existence of a Planet X had been proposed as causing perturbations, but could Nemesis be responsible for that as well?
No, say some scientists who raise numerous objections to Nemesis. First, they claim, no real periodicity to mass-extinctions can be found. Second, even if a cycle exists, there are too few craters on the planet to account for a rain of cometary debris hitting every 26,000 years. The best way to settle the argument would be to find Nemesis, but attempts to do so far have failed. A search for a star with a large parallax, indicating its proximity to the solar system, has not yielded a candidate for Nemesis, and analysis of the spectral red shift of nearby stars has not found any that appear to be linked in orbit around the Sun. Even NASA's Infrared Astronomical Satellite (IRAS) has been unable to spot a low-luminosity star. On the other hand, it is very difficult to prove a negative; not detecting an object does not necessarily mean that it does not exist.
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