Nemesis (star)

From Freepedia

Nemesis is the name given to a hypothetical red dwarf star or brown dwarf, orbiting the Sun at a distance of about 50,000 to 100,000 AU, somewhat beyond the Oort cloud. Richard A. Muller suggests that the most likely object is a red dwarf with magnitude between 7 and 12 [1]. Such a bright object would undoubtedly already be in existing star catalogs, but its true nature would only be detectable by measuring its parallax; due to orbiting the Sun it would have a very low proper motion and would escape detection by proper motion surveys that have found stars like the 9th magnitude Barnard's star.

Nemesis' existence was proposed by Muller in 1984 (Nature, vol 308, pp 715-717, 1984) to explain an apparent 26-million year cycle in the occurrence of mass extinctions on Earth as noted by Raup and Sepkoski.

According to the theory, Nemesis periodically (roughly every 26 million years), passes through a denser region of the Oort cloud, disrupting the orbits of comets, and sending millions into the inner solar system and potential collision with the Earth. It was initially nicknamed the "death star" (after the fictional Star Wars weapon), although this nickname has fallen out of usage.

No direct evidence for Nemesis has been found, however, and the existence of a periodicity in the Earth's series of mass extinctions is disputed.

It is also worth noting that Matese and Whitman have suggested that the cycle might be caused by the solar system oscillating across the galactic plane. These oscillations may lead to gravitational disturbances in the Oort cloud with the same proposed consequences as the orbit of "Nemesis". However, the period of oscillation is not well-constrained observationally, and may differ from the needed 26 million years by as much as 40%.