For at least the last 500 million years, say Adrian Melott, an astrophysicist at the University of Kansas, and Richard Bambach, a palaeontologist at the Smithsonian Institute, there has been a burst of extinctions every 27 million years.

Periodic mass extinctions have been posited before, and it has been suggested that it means the Sun has a huge, dark neighbour which orbits it every 27 million years, each time knocking a shower of comets out of the Oort Cloud at the fringes of the solar system and sending them crashing into Earth. This hypothetical dark satellite was called "Nemesis".

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