Origin of Theory:

The Nemesis Theory which was originally published in "Nature" by Davis, Hut and Muller, was appropriately named after the Greek Goddess of retribution. "She whom none can escape." (Yarris 1987)  It offers an explanation for the seemingly periodicity of mass extinctions that have occurred throughout the history of the earth. It has been shown with the fossil record that mass extinctions have occurred on average, a cycle of every 26 to 34 million years. According to this theory, the sun has a companion star  called the Nemesis, which agitates the Oort cloud as it orbits around the sun. (Goldsmith 1985)  The Oort cloud is a comet belt that orbits the sun; extending tens of thousands A.U ( the distance from the earth to the sun). (Yarris 1987)  The gravitational field of the Nemesis causes some comets to stray from heir normally stable orbit. (Goldsmith) These comets would hit the earth; causing varying levels of global extinction with such devastating effects such as acid rain, deep freeze, and ozone loss. (Grossman 2001) (Morris 2001)  These comet showers would last anywhere from 100,000 years to 2 million years with approximately 10 impacts occurring in intervals of 50,000 years between each impact. (Yarris 1987)

The Nemesis Theory stems from the discovery of a crater 10 km in diameter which is believed to have caused the extinction of the dinosaurs 65 million years ago. (Muller 2001)  This period of extinction resulted in the death of 95% of all living creatures. (Yarris 1987) 

Evidence Supporting the Nemesis Theory:

For one to first consider the Nemesis Theory, one must first accept the notion that mass extinctions have occurred periodically. By examining the fossil record of marine fossils, Dave Raup and Jack Sepkoski have gathered data showing the statistical estimation of extinction as shown in the picture on the right. The arrows are plotted every 26 million years and as one can see, they correlate for the most part to the peaks of extinction. The dinosaur icon refers to the end of the "Age of Reptiles." This graph displays a periodicity of extinction, essential to the Nemesis Theory. This regularity of extinction has also been shown to exist from data collected from fossil genera of Raup and Sepkoski. (Muller 2001)

Evidence for the Nemesis Theory can also be found with the discovery of high concentrations of iridium which is only found in extraterrestrial objects such as comets. (Morris 2001)  These traces of iridium can be found in samples of limestone at more than 25 sites around the world. They provide a geological history of the end of the Cretaceous and the beginning of the Tertiary Period, which marks the extinction of the dinosaurs. (Yarris 1987)  Between the limestone of these two periods which is known as the K/T boundary, is found a layer of red clay. When tested, this layer of clay was found to be 600 times richer in iridium than that of the surrounding limestone layers. It has been shown that the iridium found at the 25 different sites come from the same source. By measuring the concentration of elements that are usually found with iridium such as gold and platinum, it can be determined through a comparison of the ratio of the elements that the iridium is a result from one unifying source such as a comet. (Goldsmith 1985) 

With a further contemplation of the iridium data collected from the limestone, it has been deduced that the presence of iridium was a result of a collision of a comet with a 6 mile diameter, with the earth. (Morris 2001)  An impact of such magnitude, would cause fine dust particles to rise into the stratosphere; blackening the sky and leaving the world in perpetual darkness. (Yarris 1987)  These conditions would create an uninhabitable environment and therefore the extinction of living creatures.  

Another line of evidence not based on terrestrial effects, is found by cosmic ray exposure.  Meteorites created by comets have been shown through cosmic ray exposure to have fallen to earth at approximately during the same time of the last three extinctions.  Cosmic ray exposure is determined by measuring certain types of isotopes, such as Neon 21 in meteorite samples. It is used to discover the time since formation, the meteorite spent orbiting the solar system.  The meteorites that would indicate an impact of a comet would be a high- iron or "H chrondites" which receive their high- iron content from the iron rich cores of asteroid or planets that were blasted by collisions with comets.  A correlation of these "H chrondites" have been found with the occurrences of mass extinctions; giving further evidence of the Nemesis Theory. (Yarris 1987)

With the discovery of paleomagnetism, the reversals of earth's magnetic field another piece of evidence was added to the Nemesis Theory. (Morris 2000)  It was hypothesized that when a comet hit the earth, water near the equator due to extreme temperatures, evaporated and turned into ice and snow on polar caps.  Due to conservation of angular momentum, a redistribution of mass would be a great enough agitation to alter the magnetic field.  These geomagnetic reversals, through the research of Dave Raup to have occurred 296 times and approximately every 30 million years; correlating with the theory of the death star. (Yarris 1987)  

A drop in sea level which would result from a bolide impact would also occur.  This  has been shown to exist at the K/T boundary. (Yarris 1987)

The probability of a companion star also gains support by the fact that more than 50% of stars in a galaxy are in a binary system, which would lead one to conclude that a "death" companion star to the sun to be a probable reality.  

Rival Hypotheses:

Volcanoes were first though of as an explanation for the mass extinctions that have occurred throughout earth's history, however they do not explain the periodicity of extinction periods.  They also do no explain the existence of high concentrations of iridium. (Yarris 1987) 

Meteorites could also account for extinction, yet due to the randomness of their nature, they can not explain the recurring cycles of extinction either. (Goldsmith 1985)

With an acceptance of the bolide impacts originating fro  the Oort cloud, varying sources of gravitational pull that would result in a comet shower also arise.  Molecular dust clouds found in the Milky Way were considered, yet the dust clouds would be too thinly dispersed to cause enough gravitational force.  Another source comes from a hypothesized 10th planet called "Planet X".  This theory however would require an inner disk in the Oort cloud which could not remain stable. (Yarris 1987)  

Arguments Against the Nemesis Theory:

Some argue that the extinction of the dinosaurs would not happen in 1 to 2 years, yet would occur gradually with the decline of species and would use this information as a negation of the Nemesis Theory.  However the Nemesis Theory does not require an immediate extinction of life, for some forms of life could have survived the first the impact and later be killed by another impact that would hit at later time. (Goldsmith 1985)

Others argue that the proposed elliptical orbit of the Nemesis star would not be able to be maintained and that the companion star would have long left our solar system. Yet the developers of the Nemesis theory state that the orbit is not constant with time and that the orbit was once closer to that of the sun.  Objects such as passing stars would have; through their gravitational field, caused the orbit of the Nemesis to become larger and less stable. (Muller 2001)

Also because the Nemesis Theory is not precise; extinctions occurring within a range of 26 to 30 million years, that the theory is invalid. (Muller 2001) However, this apparent problem in the theory can be explained by passing stars which would cause a deviation in the orbit of the death star, resulting in a change of a few million years, without disrupting its orbit. (Goldsmith 1985)

The Search for Nemesis:

The ultimate evidence, supporting the Nemesis Theory, would of course be the actual discovery of the Nemesis star.  It is predicted that the Nemesis is probably a red dwarf star with a magnitude between 7 and 12.  Its size is believed to be a third of the sun's and 1/1000 as bright. (Yarris 1987)  It is thought that the Nemesis star is right under our noses; being able to be seen with a pair of binoculars.  The reason it has not been discovered is that though all stars have been catalogued, only a few of their distances have been measured; making it difficult for scientists to pin point which star could be the Nemesis. (Muller 2001)

The search for the Nemesis in underway at the Leuschner Observatory in Layfette California with an automated telescope.  To determine which star is the Sun's companion star, photographs of 5,000 red stars have been taken along with the measurement of the apparent shift in position determined by a second photograph picture taken 2 to 6 months later.  A little change in position indicate that the stars are far away.  However, stars that are close enough to possibly be the Nemesis will show a significant shift in position. (Muller 2001)   It is guessed that the Nemesis star will be discovered in the Hydra constellation. (Yarris 1987)  

Conclusion:

It has been shown through evidence such as iridium, that comets are the probable cause for the mass extinction that have occurred on Earth.  This information coupled with the evidence for the periodicity of mass extinctions, give validity to the Nemesis Theory which should lead to its greater acceptance.  Hopefully however, future generations of humans will not be around to see the actualization of the theory into law.

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Works Cited:

Goldsmith, Donald. Nemesis. New York: Walker and Company, 1985.

Grossman, Daniel. "One Disaster After Another." Scientific American. Feb. 2001

Morris, Mark. , et al. GE70A; Origin and Evolution of the Cosmos and Life. Course Reader Material, 2000

Morris, Mark. , et al. GE70B; Origin and Evolution of the Cosmos and Life. Course Reader Material, 2001

Muller, Richard. "Nemesis." http://www-muller.lbl.gov/pages/lbl-nem.htm

Yarris, Lynn. "Does a Companion Star to the Sun Cause Earth's Periodic Mass Extinctions?" Science Beat. Spring 1987