From Academic Kids
Nemesis' existence was proposed by Richard A. 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" for this property, although this nickname has fallen out of usage due to its fictional use in the Star Wars universe.
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%.
- Richard A. Muller´s Nemesis Theory (http://muller.lbl.gov/pages/lbl-nem.htm)
- Nemesis: Does the Sun Have a 'Companion'? (http://www.space.com/scienceastronomy/solarsystem/nemesis_010320-1.html) (Space.com, 3 April 2001)
- Exit Mundi (http://www.exitmundi.nl/Nemesis.htm), Nemesis an End-Of-World scenario.de:Nemesis (Stern)