In the 1980s, fossil record research showed a curious cycle: Every 27 million years, Earth hosted a mass extinction. Some scientists blamed a dim star dubbed Nemesis.
They suggested it was in a deadly dance with our sun, periodically kicking comets out of the distant Oort Cloud to shower our planet with destruction. Morbidly fascinating as it may be, the authors of a new study argue that this "death star" theory doesn't hold up.
The cyclical extinctions do make a solid pattern, say Adrian Melott of the University of Kansas and Richard Bambach of Smithsonian Institution Museum of Natural History, whose paper is available through arXiv.org. The two have gone back in the record to 500 million years ago, further than any other researchers, and have confirmed the 27 million year cycle at a 99 percent confidence.
According to Bambach, there's no doubt at all that every 27 million years-odd, huge numbers of species suddenly become extinct. He says this is confirmed by "two modern, greatly improved paleontological datasets of fossil biodiversity" and that "an excess of extinction events are associated with this periodicity at 99% confidence". This regular mass slaughter has apparently taken place around 18 times, back into the remote past of half a billion years ago. [The Register]
The problem, Nemesis fans, is that the cycle is too precise, the researchers say. If these extinctions result from a dance between our sun and Nemesis, the researchers note, the period of these mass extinctions would change as other stars buffeted the pair and changed the courses of Nemesis's orbit around the sun.
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. [Technology Review]
Some scientists say that the sharply-defined periodicity isn't enough to rule out Nemesis. Richard Muller, an author of the original Nemesis paper, told Wired.com that there is still hope for a dark star.
"I would agree with most of what he says, but I think he is overestimating the accuracy of the geologic timescale," he said. The geological record gives only an approximate sense of when major extinctions happened. "You get them in the right order, but it's really difficult to get an actual date," he said. In light of that uncertainty, "I would say the Nemesis hypothesis is still alive." [Wired]
Luckily, given the precision of this death cycle, we can count on having time (i.e. 16 million years) to settle the debate.
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. [Technology Review]
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