Scientists seem to have found evidence that your buddy, the Sun, has a wee baby brother. It is said to be a dark star whose eccentric orbit is responsible for periodically showering the Earth with comets and meteorites. This weird "unseen crazy aunt in the attic" has been named Nemesis by astronomers. It's thought to be a brown dwarf that spins around the Sun in an orbit so large it is measured in light years equivalent to about 6,000 billion miles.
The research suggests that every 26 million years the star's orbit brings it within one light year of the solar system. When this occurs, it causes havoc in the Oort Cloud, a huge region surrounding the solar system that contains billions of bits of cosmic rubble left over from the formation of planets.
Of the millions of rocks it throws out of orbit at each visit, some hurtle Earthwards. The scientists believe that this has several times nearly wiped out life on Earth.
Other scientists have already noted that mass extinctions of life on Earth seem to occur in a pattern with gaps equivalent to multiples of 26 million years, suggesting some regular event is causing the comets to come Earth's way.
Update 8/21/00: Tem42 alerts me to possibility that this theory has been disproven. I'm looking for the documentation.