The Nemesis Star - Our Sun's Companion

Scientists have been puzzled by an apparent cycle of mass extinctions that happen roughly every 26 million years (though some argue that it is more likely a 63 million year cycle). If the evidence holds true, and there are cycles of mass extinction on Earth, what could be the cause? Well some have suggested that there is a heavenly explanation. Specifically, our Sun might be in a binary system. And our Sun's companion -- deemed the Nemesis star -- is responsible for destroying life on Earth.

History of the Nemesis Star Theory

Analysis of the fossil record indicates that there are periods of time in history where a massive number of species, including all types of living creatures, become instinct. In 1984 researchers David Raup and Jack Sepkoski identified 12 such mass extinctions with each occurring roughly 26 million years after the previous event.
It should be noted that the accuracy of these claims has been widely challenged in the literature. At the very least, the 26 million year cycle length is often called into question, with periods closer to 63 million years often proposed. In either case, the last 25 years has seen a multitude of theories to try and explain this phenomenon.
In 1984 after the popularization of the mass extinction models two independent research groups (the first of Daniel Whitmire and Albert Jackson, and the second of Marc Davis, Piet Hut, and Richard Muller) devised theories for the cause of these mass extinctions that involved the presence of a companion star to our Sun. Essentially, each theory outlined how a star, yet to be detected, could disrupt the comets in the Oort Cloud and send them hurtling into the inner part of the solar system.
The logical question is, why is this theory any better than others that have been proposed? The main reason is that it provides a convenient explanation to observational data of the Oort cloud that scientists don't have an explanation for. Our Oort cloud has sharp, well-defined edges like those of Oort clouds around stars in other binary systems. Where as Oort clouds of isolated stars are far more diffuse. Additionally, researchers have noticed that most of the comet which make their way into the inner part of the solar system come from the same region of the Oort cloud. This indicates that there is some sort of gravitational disruption in that specific direction.
There was also recently discovered a dwarf planet, known as Sedna, that has a very unusual, and highly elliptical, orbit. At its farthest point it is nearly 1000 times further away from the Sun than the Earth is from the Sun. Scientists are baffled by its very existence, claiming that it could not possibly remain in its current orbit unless it is being influenced by another, massive object. So it would seem that even though no companion has yet been confirmed, it would seem likely given the observational data we have thus far.

What is the Nemesis Star and How Do We Find It?

There are two competing theories as to what the Nemesis star actually is. The theory put forth by Whitmire and Jackson stipulates that it is not a star at all, but rather a brown dwarf -- a protostar that never accumulates enough mass to ignite nuclear burning and become a star -- orbiting out in the Oort cloud. The typical mass of a brown dwarf is somewhere between that of a planetary gas giant (like Jupiter) and the lowest mass stars (about 8% the mass of our Sun).
Brown dwarfs are very difficult to detect since they are non-luminous -- since they do not readily radiate energy due to a lack of nuclear fusion or other exothermic reaction. Though, energy can be radiated due to their slow gravitational contraction heating the gas molecules within the object. For this reason, brown dwarfs should radiate light in the infrared.
Luckily a new infrared observatory, WISE, has been launched that should have the capability to detect such an object if it does exist. After WISE performs two complete scans of the sky researchers will be able to compile the data and map the positions and motions of all the nearby infrared objects, including Nemesis if it exists. However, it will take until at least 2013 to complete the scans and compile all the data.
But Richard Muller doesn't think that the Nemesis star is a brown dwarf at all. He and his colleagues have proposed that it is rather a small, cool star known as a red dwarf. These objects are typically much smaller than our Sun (though can be as much as half the solar mass). Muller argues that such a red dwarf is currently orbiting our Sun about 1.5 light years away (the Oort cloud extends roughly 1 light year from the Sun). But critics argue that there are problems with this theory.
If the Nemesis is a red dwarf, it would be much closer than the next closest star, Proxima Centauri (4.2 light years away). So first of all, we should have seen it already. And secondly, at 1.5 light years away it could not possibly be in a stable orbit so it could not explain the extinction cycle.
The first concern is completely valid and completely correct. We should have seen it. And actually, we may already have. There are many stars that we have observed and catalogued that we are still learning about. And given its proximity and slow proper motion, it would be difficult to determine its distance. So it may turn out that we detected the source long ago, but have only yet to realize it.
And Muller is quick to answer his critics on the second point and retort that a non-stable orbit is not a problem. In fact Nemesis has been spiraling out from our Sun for billions of years and will continue to do so. In fact, he predicts that within the next billion years or so it will no longer be bound by the Sun's gravity.

What Does This Mean For Us?

This is a question of great debate as well. There have been those that speculate that gravitational disruptions of the Oort cloud could cause comets to reign down on the inner solar system. While others believe that the Nemesis is innocuous. But we won't know for sure until we either find it or eliminate it as a possibility.

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