Another possibility that has been suggested is that the Solar System passes through the Galactic Plane every 30 million years or so. Passage through the Galactic Plane might allow some relatively nearby star to disturb some outlying comets in the Solar System. These comets could plunge inward, and perhaps one of them might hit the Earth.

That might just seem unlucky ...

http://www.boulder.swri.edu/clark/chance.html

... but with enough comets, this could of course happen.

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