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Over the last 500 million years or so, life on Earth has been threatened on many occasions; the fossil record is littered with extinction events. What's curious about these events is that they seem to occur with alarming regularity.
The periodicity is a matter of some controversy among paleobiologists but there is a growing consensus that something of enormous destructive power happens every 26 or 27 million years. The question is what?
In this blog, we've looked at various ideas such as the Sun's passage through the various spiral arms of the Milky Way galaxy (it turns out that this can't explain the extinctions because the motion doesn't have had the right periodicity).
But another idea first put forward in the 1980s is that the Sun has a distant dark companion called Nemesis that sweeps through the Oort cloud every 27 million years or so, sending a deadly shower of comets our way. It's this icy shower of death that causes the extinctions, or so the thinking goes.