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Nemesis is a hypothetical star orbiting the Sun at a distance of about 50,000 to 100,000 AU, somewhat beyond a hypothetical cloud of proto-comets at a great distance from the Sun ( the Oort cloud ).The "Nemesis Theory" was an outgrowth of the discovery that the impact of a large comet or asteroid was responsible for the great mass extinction on Earth that took place 65 million years ago.

Nemesis' existence was proposed in 1984 by American physicists Daniel Whitmire and Albert Jackson (The University of Southern Louisiana) and also by Richard Muller, Piet Hut & Davis M. in 1984 in order to explain an apparent 26-million year cycle in the occurrence of mass extinctions on Earth, like the one that killed the dinosaurs 65 million years ago, as noted by Raup and Sepkoski.

This hypothetical "death star" or "death companion" of the Sun has received a name: Nemesis. In the Greek mythology Nemesis was the spirit of divine retribution against those who succumb to hubris, vengeful fate personified as a remorseless goddess.

According to the hypothesis, Nemesis periodically (approximately 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.

But, many geologists are convinced that mass extinctions on Earth are not periodic, so they see no need for such a star. Nevertheless, Richard Muller and his colleagues have embarked on the difficult search for a possible, dim companion to the Sun - Nemesis.

According to the hypothesis Nemesis moves in an elliptical orbit, its solar distance varying between 90,000 AU (1.4 light years) and 20,000 AU, with a period of 30 million years. It is dark or at least very faint, so we haven't noticed it yet.

Once every 30 million years that companion star of the Sun (Nemesis) would pass through a hypothetical cloud of proto-comets at a great distance from the Sun (the Oort cloud). During such a passage, the proto-comets in the Oort cloud would be stirred around. Some tens of thousands of years later, here on Earth we would notice a dramatic increase in the the number of comets passing the inner solar system. If the number of comets increases significantly, so does the risk of the Earth colliding with the nucleus of one of those comets.

When examining the Earth's geological record, it appears that about once every 30 million years a mass extinction of life on Earth has occurred. The most well-known of those mass extinctions is the dinosaur extinction some 65 million years ago. According to this hypothesis about 25 million years from now it's time for the next mass extinction on Earth.

Richard Muller concluded 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.

One of the criticisms of the Nemesis hypothesis was that passing molecular clouds might already have depleted the Oort comet cloud to such an extent that Nemesis would not have enough comets left to modulate their impacts. This argument was shown to be wrong in the article "Have Interstellar Clouds Disrupted the Oort Comet Cloud?" by Hut & Tremaine.

A more serious criticism concerned the lack of stability of an orbit around the Sun with a period of tens of millions of years. Such an orbit would not be far from the sphere of influence of nearby stars, which would constantly perturb the orbit. Not only would these perturbations threaten to unbound Nemesis, they would also preclude a precise periodicity, long before they would disrupt its orbit.

This problem was addressed through a large number of simulations of different candidate Nemesis orbits in the article "How stable is an astronomical clock which can trigger mass extinctions on earth?" by Piet Hut where he showed that all is well: even though the orbital periodicity will become noisy through the perturbations of other stars, in most cases the periodicity is still well preserved over the period of 250 million years for which Raup and Sepkoski originally fitted the timing of the last ten major mass extinctions on Earth.

One awkward fact of the Nemesis hypothesis is that there is no evidence whatever of a companion star of the Sun. It need not be very bright or very massive, a star much smaller and dimmer than the Sun would suffice, even a brown or a black dwarf (a planet-like body insufficiently massive to start "burning hydrogen" like a star). It is possible that this star already exists in one of the catalogues of dim stars without anyone having noted something peculiar, namely the enormous apparent motion of that star against the background of more distant stars (i.e. its parallax).

If it should be found, few will doubt that it is the primary cause of periodic mass extinctions on Earth.

If an anthropologist of a previous generation had heard such a story, the resulting scholarly tome would doubtless use words like 'primitive' or 'pre-scientific'.

There is another Sun in the sky, a Demon Sun we cannot see. Long ago, the Demon Sun attacked our Sun. Comets fell, and a terrible winter overtook the Earth. Almost all life on Earth was destroyed. The Demon Sun has periodically attacked many times before. It will attack again.

This is why some scientists thought this Nemesis theory was a joke when they first heard of it: an invisible Sun attacking the Earth with comets sounds like myth. It deserves an additional dollop of skepticism for that reason: we are always in danger of deceiving ourselves. But even if the theory is speculative, it's serious, because its main idea is testable: you find the star and examine its properties.

But, no direct proof of the existence of Nemesis has been found yet. Also, the existence of a periodicity in the Earth's series of mass extinctions is still disputed. Richard 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 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%.

Richard Muller has been engaged in searches for Nemesis and so far, these searches have not turned up any solar companion star. A complication in these searches is the fact that we have no idea where in the sky Nemesis might reside: given the strong perturbations of nearby stars, there is no reason to believe that the star will be anywhere close to the ecliptic.

The Binary Research Institute (BRI) has found that orbital characteristics of the recently discovered planetoid, "Sedna", demonstrated the possibility that the sun might be part of a binary star system. A binary star system consists of two stars gravitationally bound orbiting a common center of mass. Once thought to be highly unusual, such systems are now considered to be common in the Milky Way.

However, the question as to the existence of Nemesis will not remain open indefinitely: as soon as a repeated infrared all-sky survey will reach a positional sensitivity of an arc-second, even a dim companion to the Sun can be discovered because of its lack of proper motion.

Nemesis 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, should find it.

WISE is a NASA-funded scientific research project that will provide a vast amounts of information about the solar system, the Milky Way and the Universe. Among the objects WISE will study are asteroids, the coolest and dimmest stars, and the most luminous galaxies. WISE is an unmanned satellite carrying an infrared-sensitive telescope that will image the entire sky.