The Sun's Evil Twin
In 1983, a scientist named Richard Muller came up with an interesting theory to explain the almost regular interavals between mass extinctions on Earth.
Roughly every 26 million years, the Earth suffers a massive extinction event in which whole species and ecosystems disappear. It's widely believed and accepted by the scientific community that the extinction events are precipitated by the impacts of comets and asteroid impacts, but what Muller devised to explain the almost regular event was rather scandalous - what if the sun had an evil twin brother?
Muller theorized that, orbiting the sun at a great distance, is another star - possibly a red or brown dwarf which orbits the sun once every 26 million years at a distance of 1 to 3 light years (this is very distant when you consider that the closest known star, Proxima Centauri, is only 4.2 light years away!) When the star, which Muller named Nemesis after the Greek goddess of divine retribution, gets close to the sun thanks to an irregular orbit, it disturbs the Oort Cloud, the grouping of icy comets and bodies at the edge of our solar system, and sends them on a collision course towards the inner planets.
But wait a minute, if Nemesis really is out there, why can't we see it? According to scientist who support the theory, Nemesis is a brown dwarf star which is a fancy way of saying it's a failed star. It would be too small to sustain nuclear fusion and would simply be nothing more than a big ball of gas making it dark and very hard to find.
However, Muller's theory does fall under some scrutiny. No where in the known galaxy have astronomers found a star that orbits at the distance that he described. For this and other reasons, support for the Nemesis theory has waned, but it hasn't died.
Who knows... future surveys may detect the star, but until then, whenever you look up at the night sky, think of the possible evil twin of our sun that may exist in a distant orbit, waiting to unleash yet another flurry of extinction upon our world.