Nemesis is a hypothetical hard-to-see red dwarf star or brown dwarf, orbiting the Sun at a distance of about 50,000 to 100,000 AU (about 1-2 light years), somewhat beyond the Oort cloud. This star was originally postulated to exist as part of a hypothesis to explain a perceived cycle of mass extinctions in the geological record, which seem to occur once every 27 million years or so. In addition, observations by astronomers of the sharp edges of Oort clouds, similar to that of the Solar System, around various binary (double) star systems, in contrast to the diffuse edges of the Oort clouds around single-star systems, has prompted some scientists to also postulate that a dwarf star may be co-orbiting the Sun. Counter-theories also exist that other forces (like the angular effect of the galactic gravity plane) may be the cause of the sharp-edged Oort cloud pattern around the Sun. To date the issue remains unsettled in the scientific community.

Claimed periodicity of mass extinctions

Artist's conception of Nemesis as a red dwarf seen from a nearby debris field with the Sun visible in the center.

Artist’s conception of Nemesis as a red dwarf seen from a nearby debris field with the Sun visible in the center.

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. They focused on the extinction intensity of fossil families of marine vertebrates, invertebrates, and protozoans, identifying 12 extinction events over the time period in question. The average time interval between extinction events was determined as 26 million years. At the time, two of the identified extinction events (Cretaceous-Tertiary and Late Eocene) could be shown to coincide with large impact events. Although Raup and Sepkoski could not identify the cause of their supposed periodicity, they suggested that there might be a non-terrestrial connection. The challenge to propose a mechanism was quickly addressed by several teams of astronomers.

Development of the Nemesis hypotheses

Two teams of astronomers, Whitmire and Jackson, and Davis, Hut, and 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 as yet undetected companion star in a highly elliptical orbit that periodically disturbs comets in the Oort cloud, causing a large increase in the number of comets visiting the inner solar system with a consequential increase in impact events on Earth. This became known as the Nemesis (or, more colorfully, Death Star) hypothesis.

If it does exist, the exact nature of Nemesis is uncertain. Richard A. Muller suggests that the most likely object is a red dwarf with magnitude between 7 and 12, while Daniel P. Whitmire and Albert A. Jackson argue for a brown dwarf. If a red dwarf, it would undoubtedly already exist in 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.

The last major extinction event was about 5 million years ago, so Muller posits that Nemesis is likely 1 to 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 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.

Other possible evidence: questions about orbital path of Sedna

The extremely distant planetoid Sedna has an extra-long and unusual elliptical orbit around the Sun, well beyond Pluto, ranging between 76 and 975 AU (where 1 AU is the distance between the Earth and the Sun). Sedna’s orbit is estimated to last between 10.5 and 12 thousand years. Its discoverer, Mike Brown of Caltech, noted in a Discover magazine article that Sedna’s location doesn’t make sense:

“Sedna shouldn’t be there,” said Brown. “There’s no way to put Sedna where it is. It never comes close enough to be affected by the Sun, but it never goes far enough away from the Sun to be affected by other stars.”

Brown postulates that perhaps a massive unseen object is responsible for Sedna’s mystifying orbit, its gravitational influence keeping Sedna fixed in that far-distant portion of space.

Current and pending scientific searches for Nemesis

If Nemesis exists, then it may be detected by the planned Pan-STARRS or LSST astronomical surveys.

In particular, if Nemesis is a red dwarf star or a brown dwarf, then the WISE mission (an infrared sky survey, currently underway, that will finish covering most of our solar neighborhood in movement-verifying parallax measurements by 2013) is expected to be able to find it, if it exists.

Alternate name proposed by some scientists

Some of the scientists who have proposed the brown dwarf or super-planet theories have proposed that this hypothetical body instead be named “Tyche”. They have stated that this is due to the desire to sidestep the negative connotations of the word “Nemesis” in mythology. “Tyche” also appears in Nemesis mythology, but as a more positive character.

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