Nemesis (hypothetical star)
[5][6] 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 perceived cycle of mass extinctions.
[7] 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.
[9] However, in 2011, researchers analyzed the ages of known craters on Earth's surface and found strong evidence against periodic impacts, concluding that the earlier findings based on small samples were statistical artifacts.
The 2MASS astronomical survey, which ran from 1997 to 2001, failed to detect an additional star or brown dwarf in the Solar System.
[14][16][17][18] 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.
[19][20] 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.
[9] 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.
[19][20] This hypothesis proposes that the Sun may have an undetected companion star in a highly elliptical orbit that periodically disturbs comets in the Oort cloud, causing a large increase of the number of comets visiting the inner Solar System with a consequential increase of impact events on Earth.
[citation needed] Muller, referring to the date of a recent extinction at 11 million years before the present day, posits that Nemesis has a semi-major axis of about 1.5 light-years (95,000 AU)[21] and suggests it is located (supported by Yarris, 1987) near Hydra, based on a hypothetical orbit derived from original aphelia of a number of atypical long-period comets that describe an orbital arc meeting the specifications of Muller's hypothesis.
Brown has stated that it is more likely that one or more non-companion stars, passing near the Sun billions of years ago, could have pulled Sedna out into its current orbit.
[28] In 2004, Kenyon forwarded this explanation after analysis of Sedna's orbital data and computer modeling of possible ancient non-companion star passes.
