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. 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. This became known as the "Nemesis" or "Death Star" hypothesis.
If it does exist, the exact nature of Nemesis is uncertain. Muller suggests that the most likely object is a red dwarf with an apparent magnitude between 7 and 12, while Daniel P. Whitmire and Albert A. Jackson argue for a brown dwarf. If a red dwarf, it would exist in star catalogs, but it would only be confirmed by measuring its parallax; due to orbiting the Sun it would have a low proper motion and would escape detection by older proper motion surveys that have found stars like the 9th-magnitude Barnard's star. (The proper motion of Barnard's star was detected in 1916.) Muller expects Nemesis to be discovered by the time parallax surveys reach the 10th magnitude.
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) and suggests it is located (supported by Yarris, 1987) near Hydra, based on a hypothetical orbit derived from original apogees of a number of atypical long-period comets that describe an orbital arc meeting the specifications of Muller's hypothesis. Richard Muller's most recent paper relevant to the Nemesis theory was published in 2002. In 2002, Muller speculated that Nemesis was perturbed 400 million years ago by a passing star from a circular orbit into an orbit with an eccentricity of 0.7.
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