Nemesis is a hypothetical red dwarf to be orbiting the Sun at a distance of about 95,000 AU (1.5 light-years), Such a star would have separated from this binary system over four billion years ago, meaning it could not be responsible for the more recent cycles of mass extinctions.

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 (Shiva hypothesis), may be the cause of orbital perturbations of some outer Solar System objects. They do believe that this was driven by a terrestrial mechanism led by climate and oceanic circulation, without proposing an extraterrial cause.

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. claimed that the periodicity is not statistically relevant.

Development of the Nemesis hypotheses

Two teams of astronomers, Daniel P. Whitmire and Albert A. Jackson IV, and Marc Davis, Piet Hut, and Richard A. Muller, independently published similar hypotheses to explain Raup and Sepkoski's extinction periodicity in the same issue of the journal Nature.

Orbit of Sedna

thumb|300px|Sedna orbit compared to the Solar System and Oort cloud

The trans-Neptunian object Sedna has an extra-long and unusual elliptical orbit around the Sun, Tyche should be a gas giant with 3–4 Jupiter masses. In 2016, studying the orbit of Sedna and of 14 other extreme trans-Neptunian objects, Michael Brown and Konstantin Batygin postulated that a big super-Earth named Planet Nine would explain their peculiar features, but this hypothesis is not strictly related to that of the Nemesis star.

Past, current, and pending searches for Nemesis

Searches for Nemesis in the infrared are important because cooler stars comparatively shine brighter in infrared light. The University of California's Leuschner Observatory failed to discover Nemesis by 1986. 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 a star, or brown dwarf, in the Solar System.

Calculations in the 1980s suggested that a Nemesis object would have an irregular orbit due to perturbations from the galaxy and passing stars. The Melott and Bambach work

  • Richard A. Muller, Measurement of the lunar impact record for the past 3.5 billion years, and implications for the Nemesis theory, Geological Society of America Special Paper 356, pp 659–665 (2002). I
  • Richard A. Muller, Nemesis (Weidenfeld & Nicolson, 1988, OP)
  • Richard A. Muller, lecture where he describes Nemesis Theory
  • (Provides a very entertaining and readable review of the Nemesis extinction hypothesis, including dozens of references to scientific articles on the topic.)
  • SpaceDaily. Evidence Mounts For Companion Star To Our Sun. Apr 25, 2006
  • Lynn Yarris. "Does a Companion Star to the Sun Cause Earth's Periodic Mass Extinctions?" Science Beat. Spring 1987
  • Nemesis is a myth (Max Planck August 1, 2011)