This would mean that once every 30 million years that hypothetical companion star of the Sun would pass through the Oort cloud (a hypothetical cloud of proto-comets at a great distance from the Sun). 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 number of comets passing the inner solar system. If the number of comets increases dramatically, 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 events is, of course, the dinosaur extinction some 65 million years ago. The theory predicts there will be another mass extinction in 15 million years.
This hypothetical "death companion" of the Sun was suggested in 1985 by Daniel P. Whitmire and John J. Matese of the University of Southern Lousiana. It has even received a name, Nemesis. 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 if it was 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 Nemesis should be found, few will doubt that it is the primary cause of periodic mass extinctions on Earth.
In 1987, Whitmire and Matese suggested a tenth planet at 80 AU with a period of 700 years and an inclination of perhaps 45°, as an alternative to their "Nemesis" hypothesis. However, according to Eugene M. Shoemaker, this planet could not have caused those meteor showers that Whitmire and Matese suggested.
Nemesis is also a notion of mythical power. If an anthropologist of a previous generation had heard such a story from his informants, the resulting scholarly tome would doubtless use words like 'primitive' or 'pre-scientific.' Consider this story:
There is another Sun in the sky, a Demon Sun we cannot see. Long ago, even before great grandmother's time, the Demon Sun attacked our Sun. Comets fell, and a terrible winter overtook the Earth. Almost all life was destroyed. The Demon Sun has attacked many times before. It will attack again.This is why some scientists thought this theory was a joke when they first heard of it - an invisible Sun attacking the Earth with comets sounds like delusion or 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 and respectable, because its main idea is testable: you find the star and examine its properties.
However, the existence of Nemesis is not very likely. The Infrared Astronomical Satellite (IRAS) examined the entire sky in the far infrared (IR) spectrum. However, it did not find any evidence of a star that would fit the description of "Nemesis."
Nemesis, goddess who gives to each what they deserve - Wikipedia
