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article imageMass extinctions take place on Earth every 27 million years

By Igor I. Solar     Jul 16, 2010 in Science
Washington - The fossil record shows that during the past 500 million years, Earth has suffered mass extinction events on many occasions which seem to occur regularly at 27 million year intervals.
There is some controversy among paleo-biologists on the precise periodicity of these destructive events, but there is little doubt that cosmic phenomena of catastrophic magnitude happen on Earth every 26 or 27 million years. There have been some ideas to explain the record of mass extinctions such as the Sun's passage through the various spiral arms of the Milky Way galaxy, however this concept has not been deemed defensible to explain the extinctions because it lacks of the right periodicity.
Another idea came up in the 1980s based on the concept that the Sun could have a distant companion called Nemesis that sweeps through the Oort Cloud every 27 million years or so, sending towards our planet a deadly shower of comets. The Oort cloud is a huge belt of dust and ice that is believed to be located at about one light year away from the Sun and is may be the source of many of the comets that pass through our solar system. Supposedly, this icy shower of cosmic dust is what causes the mass extinctions.
Researchers Adrian Melott, of the University of Kansas and Richard Bambach, from the Smithsonian Institution Museum of Natural History, Washington DC, published July 2 on Monthly Notices of the Royal Astronomical Society a report where they re-examine the paleontological datasets of fossil biodiversity to see if they could get a more precise estimate of the orbit of Nemesis and its effect on the periodicity of mass extinctions on Earth.
They collected a massive set of paleontological records of mass extinctions from the last 500 million years, which is about double the time period previously studied. Their analysis shows that extinctions occur every 27 million years, with a confidence level of 99%. This would seem to support the idea that a distant object orbits the Sun every 27 million years. However, the accuracy and regularity of these events actually constitute evidence against the Nemesis' hypothesis according to Melott and Bambuch.
The reason is that Nemesis' orbit and its relation with our Sun would certainly have been affected by the many close encounters that the Sun has had with other stars in the past 500 million years. These encounters would have caused, in such an extended period, a variation of Nemesis' orbit and the regularity of its periodicity in relation to our Sun. The variation could be expressed in two ways. First, the orbit could have changed so that it would not show a single peak of extinctions, but perhaps two or more peaks. Or second, it could have changed gradually, in which case the peak would spread and become blurred over time. But the data indicates that the extinctions occur with amazing regularity every 27 million years.
"The regularity of the timing compared with earlier calculations of orbital perturbation would seem to exclude the Nemesis hypothesis as a causal factor." say Melott and Bambach.
According to the researchers, the last event which caused the extinction of about 10% of life forms on Earth took place about 11 million years ago. Thus, it seems that at this point there is not a chance that some disaster of cosmic proportions may happen to our planet until about 16 million years from now. There seems to be plenty of time for our astrophysics scientists to continue their work in search of the cause of the regular 27 million year cycle of mass extinctions.
More about Planet earth, Extinctions, Biodiversity, Sun, Nemesis
 
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