Nemesis (star)

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For other uses, see Nemesis.

Nemesis is 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], while Daniel P. Whitmire and Albert A. Jackson argue for a brown dwarf. Such a bright red dwarf 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.

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[edit] History of the hypotheses

Nemesis' existence was proposed by Whitmire and Jackson in Nature in 1984 (308 : 713-715) and also by Davis, Hut & Muller in 1984 (Nature 308 : 715-717) to explain an apparent 26-million year cycle in the occurrence of mass extinctions on Earth as noted by Raup and Sepkoski. In Whitmire and Jackson's model Nemesis is a brown dwarf while in the model of Davis et al. Nemesis is assumed to be a normal M-type star.

According to the hypothesis, Nemesis periodically (roughly every 26 million years) passes through a denser region of the Oort cloud, disrupting the orbits of comets, and sending millions of comets into the inner solar system and potential collision with the Earth. The last major extinction period was about 5 million years ago, so Muller posits that Nemesis is likely 1-1.5 light years away at present, and even has ideas of what area of the sky it might be in (supported by Yarris, 1987), near Hydra, based on a theoretical orbit derived from original apogees of a number of atypical long period comets that describe an orbital arc meeting the specifications of Muller's theory. It was initially nicknamed the "death star", after the fictional Star Wars weapon, although this nickname has fallen out of usage.

No direct proof of the existence of Nemesis has been found, however, and the existence of a periodicity in the Earth's series of mass extinctions is disputed. Muller has proposed that, based on analysis of lunar rocks that indicate the impact history of the Moon, a major uptick in lunar impacts 400 million years ago (mya) represents a major shift in the orbit of Nemesis into its present rather eccentric orbit, which according to Piet Hut will only be stable for another billion years.

Matese and Whitman have suggested that the cycle might be caused by the solar system oscillating across the galactic plane of the Milky Way. 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%.

If Nemesis exists, it may be detected by the planned Pan-STARRS or LSST astronomical surveys, or similar future projects. If Nemesis is a brown dwarf, as proposed by Whitmire and Jackson, then the upcoming WISE mission (scheduled for June 2009) should easily find it.

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