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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.

Today, Adrian Melott at the University of Kansas and Richard Bambach at the Smithsonian Institute in Washington DC re-examine the paleo-record to see if they can get a more accurate estimate of the orbit of Nemesis.

Their work throws up a surprise. They have brought together a massive set of extinction data from the last 500 million years, a period that is twice as long as anybody else has studied. And their analysis shows an excess of extinctions every 27 million years, with a confidence level of 99%.

That’s a clear, sharp signal over a huge length of time. At first glance, you’d think it clearly backs the idea that a distant dark object orbits the Sun every 27 million years.

But ironically, the accuracy and regularity of these events is actually evidence against Nemesis’ existence, say Melott and Bambuch.

That’s because Nemesis’ orbit would certainly have been influenced by the many close encounters we know the Sun has had with other starsin the last 500 million years.

These encounters would have caused Nemesis’ orbit to vary in one of two ways. First, the orbit could have changed suddenly so that instead of showing as a single the peak, the periodicity would have two or more peaks. Or second, it could have changed gradually by up 20 per cent, in which case the peak would be smeared out in time.

But the data indicates that the extinctions occur every 27 million years, as regular as clockwork. “Fossil data, which motivated the idea of Nemesis, now militate against it,” say Melott and Bambuch.

That means something else must be responsible. It’s not easy to imagine a process in our chaotic interstellar environment that could have such a regular heart beat; perhaps the answer is closer to home.

There is a smidgeon of good news. The last extinction event in this chain happened 11 million years ago so, in theory at least, we have plenty of time to work out where the next catastrophe is coming from.

Either way, the origin of the 27 million year extinction cycle is hotting up to become one of the great scientific mysteries of our time. Suggestions, if you have any, in the comments section please.

Ref: arxiv.org/abs/1007.0437: Nemesis Reconsidered

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