Nemesis is a hypothetical hard-to-detect red dwarf

Red dwarf

According to the Hertzsprung-Russell diagram, a red dwarf star is a small and relatively cool star, of the main sequence, either late K or M spectral type....

 star, white dwarf

White dwarf

A white dwarf, also called a degenerate dwarf, is a small star composed mostly of electron-degenerate matter. They are very dense; a white dwarf's mass is comparable to that of the Sun and its volume is comparable to that of the Earth. Its faint luminosity comes from the emission of stored...

 star or brown dwarf

Brown dwarf

Brown dwarfs are sub-stellar objects which are too low in mass to sustain hydrogen-1 fusion reactions in their cores, which is characteristic of stars on the main sequence. Brown dwarfs have fully convective surfaces and interiors, with no chemical differentiation by depth...

, originally postulated in 1984 to be orbiting the Sun

Sun

The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...

 at a distance of about 95,000 AU

Astronomical unit

An astronomical unit is a unit of length equal to about or approximately the mean Earth–Sun distance....

 (1.5 light-year

Light-year

A light-year, also light year or lightyear is a unit of length, equal to just under 10 trillion kilometres...

s), somewhat beyond the Oort cloud

Oort cloud

The Oort cloud , or the Öpik–Oort cloud , is a hypothesized spherical cloud of comets which may lie roughly 50,000 AU, or nearly a light-year, from the Sun. This places the cloud at nearly a quarter of the distance to Proxima Centauri, the nearest star to the Sun...

, to explain a perceived cycle of mass extinctions in the geological record

History of Earth

The history of the Earth describes the most important events and fundamental stages in the development of the planet Earth from its formation 4.578 billion years ago to the present day. Nearly all branches of natural science have contributed to the understanding of the main events of the Earth's...

, which seem to occur once per 26 million years. , over 1300 brown dwarfs have been identified and none of them are inside the Solar System.

More recent theories suggest that other forces, like close passings of other stars, or the angular effect of the galactic gravity plane working against the outer solar orbital plane, may be the cause of orbital perturbations of some outer solar system objects. In 2010, two astronomers, Melott and Bambach analysed earlier data on outer solar system object orbital patterns and announced that their findings suggest Nemesis couldn't possibly exist. In 2011, Coryn Bailer-Jones did an analysis of craters on the surface of the Earth and reached the conclusion that the earlier findings of simple periodic patterns (implying periodic comet showers dislodged by a hypothetical Nemesis star) to be statistical artifacts, and found that the crater record shows no evidence for Nemesis. The Infrared Astronomical Satellite (IRAS

IRAS

The Infrared Astronomical Satellite was the first-ever space-based observatory to perform a survey of the entire sky at infrared wavelengths....

) failed to discover Nemesis in the 1980s. The 2MASS

2MASS

Observations for the Two Micron All-Sky Survey began in 1997 and were completed in 2001 at two telescopes located one each in the northern and southern hemispheres to ensure coverage of the entire sky...

 astronomical survey, which ran from 1997 to 2001, failed to detect a star, or brown dwarf, in the solar system.

Using newer and more powerful infrared telescope technology, able to detect brown dwarfs as cool as 150 Kelvin

Kelvin

The kelvin is a unit of measurement for temperature. It is one of the seven base units in the International System of Units and is assigned the unit symbol K. The Kelvin scale is an absolute, thermodynamic temperature scale using as its null point absolute zero, the temperature at which all...

 out to a distance of 10 light-year

Light-year

A light-year, also light year or lightyear is a unit of length, equal to just under 10 trillion kilometres...

s from the sun, preliminary results from the Wide-field Infrared Survey Explorer

Wide-field Infrared Survey Explorer

Wide-field Infrared Survey Explorer is a NASA infrared-wavelength astronomical space telescope launched on December 14, 2009, and decommissioned/hibernated on February 17, 2011 when its transmitter was turned off...

 (WISE survey) have not, to date, detected Nemesis, although the analysis of the full survey is not yet complete. In 2011, David Morrison

David Morrison (astrophysicist)

Dr. David Morrison is director of the Carl Sagan Center for Study of Life in the Universe at the SETI Institute, former director of the NASA Lunar Science Institute, and senior scientist at the NASA Astrobiology Institute, at NASA Ames Research Center in Mountain View, California. He is the past...

, a senior scientist at NASA known for his work in risk assessment of near Earth objects, has written that confidence in the existence of an object like Nemesis has drastically diminished, since it is expected it should have been detected in infrared sky surveys before now. However other scientists have observed that there are hundreds of red stars recorded in sky surveys with no parallax information on their actual distance from the solar system. Until parallax and proper motion and measurements are completed, there is no way to ascertain the distance of these red stars (whether they are nearby red dwarfs or distant red giants).

Claimed periodicity of mass extinctions

In 1984, paleontologists David Raup

David M. Raup

David M. Raup is a University of Chicago paleontologist. Raup studied the fossil record and the diversity of life on Earth. Raup contributed to the knowledge of extinction events along with his colleague Jack Sepkoski...

 and Jack Sepkoski

Jack Sepkoski

J. John Sepkoski Jr., , was a University of Chicago paleontologist. Sepkoski studied the fossil record and the diversity of life on Earth. Sepkoski and David Raup contributed to the knowledge of extinction events...

 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

Time series

In statistics, signal processing, econometrics and mathematical finance, a time series is a sequence of data points, measured typically at successive times spaced at uniform time intervals. Examples of time series are the daily closing value of the Dow Jones index or the annual flow volume of the...

. They focused on the extinction intensity of fossil

Fossil

Fossils are the preserved remains or traces of animals , plants, and other organisms from the remote past...

 families of marine vertebrates, invertebrates, and protozoans, identifying 12 extinction event

Extinction event

An extinction event is a sharp decrease in the diversity and abundance of macroscopic life. They occur when the rate of extinction increases with respect to the rate of speciation...

s 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

Cretaceous-Tertiary extinction event

The Cretaceous–Paleogene extinction event, formerly named and still commonly referred to as the Cretaceous-Tertiary extinction event, occurred approximately 65.5 million years ago at the end of the Maastrichtian age of the Cretaceous period. It was a large-scale mass extinction of animal and plant...

 and Late Eocene

Eocene-Oligocene extinction event

The transition between the end of the Eocene and the beginning of the Oligocene, called the Grande Coupure in Europe, occurring 33.9 ± 0.1 Ma, is marked by large-scale extinction and floral and faunal turnover .Most of the affected organisms were marine or aquatic in nature...

) could be shown to coincide with large impact events. Although Raup and Sepkoski could not identify the cause of their supposed periodicity, they suggested a possible 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

Nature (journal)

Nature, first published on 4 November 1869, is ranked the world's most cited interdisciplinary scientific journal by the Science Edition of the 2010 Journal Citation Reports...

. This hypothesis proposes that the Sun may have an undetected companion star

Binary star

A binary star is a star system consisting of two stars orbiting around their common center of mass. The brighter star is called the primary and the other is its companion star, comes, or secondary...

 in a highly elliptical orbit that periodically disturbs comet

Comet

A comet is an icy small Solar System body that, when close enough to the Sun, displays a visible coma and sometimes also a tail. These phenomena are both due to the effects of solar radiation and the solar wind upon the nucleus of the comet...

s in the Oort cloud

Oort cloud

The Oort cloud , or the Öpik–Oort cloud , is a hypothesized spherical cloud of comets which may lie roughly 50,000 AU, or nearly a light-year, from the Sun. This places the cloud at nearly a quarter of the distance to Proxima Centauri, the nearest star to the Sun...

, causing a large increase of the number of comets visiting the inner Solar System with a consequential increase of impact event

Impact event

An impact event is the collision of a large meteorite, asteroid, comet, or other celestial object with the Earth or another planet. Throughout recorded history, hundreds of minor impact events have been reported, with some occurrences causing deaths, injuries, property damage or other significant...

s on Earth. This became known as the "Nemesis

Nemesis

Nemesis may refer to:* Nemesis , in Greek mythology, a spirit of divine retribution against those who succumb to hubris* Archenemy, the principal enemy of a character in a work of fiction-Literature:...

" or "Death Star" hypothesis.

If it does exist, the exact nature of Nemesis is uncertain. Richard A. Muller

Richard A. Muller

Richard A. Muller is a noted American professor of physics at the University of California, Berkeley. He is also a faculty senior scientist at the Lawrence Berkeley National Laboratory.-Career:...

 suggests that the most likely object is a red dwarf

Red dwarf

According to the Hertzsprung-Russell diagram, a red dwarf star is a small and relatively cool star, of the main sequence, either late K or M spectral type....

 with an apparent magnitude

Apparent magnitude

The apparent magnitude of a celestial body is a measure of its brightness as seen by an observer on Earth, adjusted to the value it would have in the absence of the atmosphere...

 between 7 and 12, while Daniel P. Whitmire and Albert A. Jackson argue for a brown dwarf

Brown dwarf

Brown dwarfs are sub-stellar objects which are too low in mass to sustain hydrogen-1 fusion reactions in their cores, which is characteristic of stars on the main sequence. Brown dwarfs have fully convective surfaces and interiors, with no chemical differentiation by depth...

. If a red dwarf, it would exist in star catalogs, but it would only be confirmed by measuring its parallax

Parallax

Parallax is a displacement or difference in the apparent position of an object viewed along two different lines of sight, and is measured by the angle or semi-angle of inclination between those two lines. The term is derived from the Greek παράλλαξις , meaning "alteration"...

; due to orbiting the Sun it would have a low proper motion

Proper motion

The proper motion of a star is its angular change in position over time as seen from the center of mass of the solar system. It is measured in seconds of arc per year, arcsec/yr, where 3600 arcseconds equal one degree. This contrasts with radial velocity, which is the time rate of change in...

 and would escape detection by older proper motion surveys that have found stars like the 9th-magnitude Barnard's star

Barnard's star

Barnard's Star, also known occasionally as Barnard's "Runaway" Star, is a very low-mass red dwarf star approximately six light-years away from Earth in the constellation of Ophiuchus . In 1916, the American astronomer E.E...

. (The proper motion of Barnard's star was detected in 1916.)

Muller, referring to the date of a recent extinction at 5 million years before the present day, posits that Nemesis has a semi-major axis

Semi-major axis

The major axis of an ellipse is its longest diameter, a line that runs through the centre and both foci, its ends being at the widest points of the shape...

 of about 1.5 ly and suggests it is located (supported by Yarris, 1987) near Hydra

Hydra (constellation)

Hydra is the largest of the 88 modern constellations, measuring 1303 square degrees. It has a long history, having been included among the 48 constellations listed by the 2nd century astronomer Ptolemy. It is commonly represented as a water snake...

, 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

Perturbation (astronomy)

Perturbation is a term used in astronomy in connection with descriptions of the complex motion of a massive body which is subject to appreciable gravitational effects from more than one other massive body....

 400 million years ago by a passing star from a circular orbit into an orbit with an eccentricity

Orbital eccentricity

The orbital eccentricity of an astronomical body is the amount by which its orbit deviates from a perfect circle, where 0 is perfectly circular, and 1.0 is a parabola, and no longer a closed orbit...

 of 0.7.

Orbit of Sedna

The planetoid Sedna

90377 Sedna

90377 Sedna is a trans-Neptunian object discovered in 2003, which was about three times as far from the Sun as Neptune. For most of its orbit it is even further from the Sun, with its aphelion estimated at 960 astronomical units , making it one of the most distant known objects in the Solar System...

 has an extra-long and unusual elliptical orbit around the Sun, ranging between 76 and 975 AU. Sedna’s orbit is estimated to last between 10.5 and 12 thousand years. Its discoverer, Michael Brown

Michael E. Brown

Michael E. Brown has been a professor of planetary astronomy at the California Institute of Technology since 2003....

 of Caltech, noted in a Discover magazine article that Sedna’s location seemed to defy reasoning: "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 therefore postulated that a massive unseen object may be responsible for Sedna’s anomalous orbit.

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. In 2004, Kenyon forwarded this explanation after analysis of Sedna's orbital data and computer modeling of possible ancient non-companion star passes.

Past, current and pending searches for Nemesis

Searches for Nemesis in the infrared are important because cooler stars shine in infrared light. The Infrared Astronomical Satellite (IRAS

IRAS

The Infrared Astronomical Satellite was the first-ever space-based observatory to perform a survey of the entire sky at infrared wavelengths....

) failed to discover Nemesis in the 1980s. The 2MASS

2MASS

Observations for the Two Micron All-Sky Survey began in 1997 and were completed in 2001 at two telescopes located one each in the northern and southern hemispheres to ensure coverage of the entire sky...

 astronomical survey, which ran from 1997 to 2001, failed to detect a star, or brown dwarf, in the solar system. If Nemesis exists, it may be detected by Pan-STARRS

Pan-STARRS

The Panoramic Survey Telescope and Rapid Response System is a planned array of astronomical cameras and telescopes and computing facility that will survey the sky on a continual basis, including accurate astrometry and photometry of detected objects...

 or the planned LSST

Large Synoptic Survey Telescope

The Large Synoptic Survey Telescope is a planned wide-field "survey" reflecting telescope that will photograph the available sky every three nights. The LSST is currently in its design and development phase and will achieve engineering first light four years after construction starts...

 astronomical surveys.

In particular, if Nemesis is a red dwarf

Red dwarf

According to the Hertzsprung-Russell diagram, a red dwarf star is a small and relatively cool star, of the main sequence, either late K or M spectral type....

 star or a brown dwarf

Brown dwarf

Brown dwarfs are sub-stellar objects which are too low in mass to sustain hydrogen-1 fusion reactions in their cores, which is characteristic of stars on the main sequence. Brown dwarfs have fully convective surfaces and interiors, with no chemical differentiation by depth...

, the WISE mission (an infrared sky survey that covered most of our solar neighborhood in movement-verifying parallax

Parallax

Parallax is a displacement or difference in the apparent position of an object viewed along two different lines of sight, and is measured by the angle or semi-angle of inclination between those two lines. The term is derived from the Greek παράλλαξις , meaning "alteration"...

 measurements) is expected to be able to find it. WISE can detect 150 Kelvin brown dwarfs out to 10 light-year

Light-year

A light-year, also light year or lightyear is a unit of length, equal to just under 10 trillion kilometres...

s. But the closer a brown dwarf is the easier it is to detect. Preliminary results of the WISE survey were released on 14 April 2011. The final release of analyzed results is to be released in March 2012. Recent scientific analysis no longer supports the idea that extinctions on Earth happen at regular, repeating intervals. Thus, the Nemesis hypothesis is no longer needed.

Cultural references

• Nemesis, a book about a civilization affected by an unknown additional star, by Isaac Asimov.

External links

• "Astrobiology Magazine", "Cosmic Evolution" Section, "Getting WISE about Nemesis" 03/11/10, Author: Leslie Mullen, Article about Nemesis and Tyche theory, and how the WISE Sky Survey Mission may prove or disprove the theories.
• Robert Roy Britt, Nemesis: Does the Sun Have a 'Companion'?, Space.com, 3 April 2001.
• R. Foot, Z. K. Silagadze, Do mirror planets exist in our solar system? Acta Physica Polononica B32 (2001) pp. 2271–2278.
• 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 and Nicholson, 1988, OP)
• Richard A. Muller, lecture where he describes Nemesis Theory
• Z.K. Silagadze, TeV scale gravity, mirror universe, and ... dinosaurs, Acta Physica Polonica B32 (2001) 99-128. (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)