Technology Review: The Death of Nemesis: The Sun's Distant, Dark Companion →
Apparently, it is almost certainly the case that mass extinctions occur every 26 to 27 million years (as per paleobiologists who have studied fossil records over the past 500 million years). One of the theories suggested to explain this peculiar periodicity was that the Sun has a twin, nicknamed Nemesis, that cuts through the Oort Cloud every 26-27 million years, thereby sending a flood of matter raining towards the inner solar system.
This article says the Nemesis theory is wrong. But, still, the idea is pretty cool.
