A massive extinction like the one that claimed the dinosaurs has hit the Earth like clockwork every 27 million years, a new fossil analysis confirms. But the study claims to rule out one controversial explanation: a dark stellar companion called Nemesis that sends a regular rain of deadly comets toward Earth.

"The main astronomical ideas you can come up with that could cause something like this just don't work," said physicist Adrian Melott of the University of Kansas, a co-author of the new study.

Nemesis was first suggested in 1984 as a way to explain an alarmingly regular series of extinctions in the marine fossil record, which was discovered by paleontologists David Raup and Jack Sepkoski. In light of the suggestion in 1980 that the dinosaurs were killed by a catastrophic impact, an invisible cosmic sniper lobbing comets at the inner solar system seemed like a plausible culprit.

Two independent groups of astronomers suggested that a dim brown dwarf or red dwarf star lying between one and two light-years from the sun could throw a shower of ice and rock from the Oort Cloud every 26 million or 27 million years to wreak havoc on Earth. Because the orbit of this "death star" would be tweaked by interactions with other stars and the Milky Way, the time between one impact and the next should vary by 15 to 30 percent.

But now, Melott and co-author Richard Bambach of the National Museum of Natural History in Washington, D.C., say that's not actually what happens. The extinctions come almost exactly every 27 million years, they say, to a confidence interval of 99 percent.

"It's really too good, it's too sharp and fixed," Melott said. "It's like a clock."

Melott and Bambach compared two huge data sets going back 500 million years, twice as far as the 1984 study looked. One dataset, the Sepkoski database, is a continuation of the original study. The other, the Paleobiology Database, was compiled between 2000 and 2008. Both sets include many fossils that have been found and cataloged since 1984.

The researchers searched mathematically for patterns that were common to both datasets, and found that both showed an excess of organisms disappearing every 27 million years, too regularly to be caused by a shiftable star.

"It was a slam dunk on finding exactly what you would expect to find if they [Raup and Sepkoski] were right, which surprised me," Melott said. "We have strong confirmation of this periodicity, it's exactly the same one that those guys found in '84, and we have no clue what's causing it."

Other astronomers think Nemesis is still out there, however. Richard A. Muller of the University of California at Berkeley, one of the authors of the 1984 paper proposing the dark star and the author of a popular book called Nemesis: The Death Star, thinks Melott is "coming to too strong a conclusion."

"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."

There is a way to check. Several ongoing astronomical survey telescopes, including NASA's Wide-Field Infrared Survey Explorer, the Large Synoptic Survey Telescope and the Pan-STARRS survey, are scanning the sky with enough sensitivity to find Nemesis if it exists. If they don't find the dark star, then it probably isn't there.

"That's the ultimate test," Muller said.

Image: Dallas1200am/Flickr

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