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Earth hit by mass extinctions 'every 27m years'

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".
But the regularity of the extinctions and the timescale they occur over - the 500 million years examined by Dr Melott and Dr Bambach is almost double that earlier studies have looked at - seem to rule that out.

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Title: Nemesis Reconsidered
Authors: Adrian L. Melott (University of Kansas), Richard K. Bambach (Smithsonian Institution Museum of Natural History)

The hypothesis of a companion object (Nemesis) orbiting the Sun was motivated by the claim of a terrestrial extinction periodicity, thought to be mediated by comet showers. The orbit of a distant companion to the Sun is expected to be perturbed by the Galactic tidal field and encounters with passing stars, which will induce variation in the period. We examine the evidence for the previously proposed periodicity, using two modern, greatly improved paleontological datasets of fossil biodiversity. We find that there is a narrow peak at 27 My in the cross-spectrum of extinction intensity time series between these independent datasets. This periodicity extends over a time period nearly twice that for which it was originally noted. An excess of extinction events are associated with this periodicity at 99% confidence. In this sense we confirm the originally noted feature in the time series for extinction. However, we find that it displays extremely regular timing for about 0.5 Gy. The regularity of the timing compared with earlier calculations of orbital perturbation would seem to exclude the Nemesis hypothesis as a causal factor.

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