Centaurs Have Been Trashing the Earth for Eons, Study Says

While humans are often (and justifiably) painted as the biggest threat to life on Earth, ​centaurs may have us beat by a longshot. No, we're not talking about mythical human-horse hybrids here, but rather, a collection of wayward space rocks careening about the boondocks of our solar system. According to one astronomer,these centaurs have been showering their garbage down on us and wreaking havoc for eons.

"I'd expect giant centaur comets and their associated bombardment episodes to have occurred all the way back to the early days of the solar system," William Napier, a professor of astronomy at the University of Buckingham, told me in an email.

That belief—highly controversial in the astronomical community—is espoused Napier's newest paper, which appears this week in Monthly Notices of the Royal Astronomical Society. Napier postulates that mass extinctions throughout geologic history are the result of giant, centaurian comets hurling themselves into our planet's orbit, shattering apart, and raining death upon our biosphere for thousands of years. Neat.

It's no secret that massive impacts can spell disaster for life on Earth. We've all heard of the infamous collision that probably ended the reign of the dinosaurs some 65 million years ago. The idea of a life-ending space rock captivates us, as evidenced by the popularity of disaster movies such as Deep Impact and Armageddon. But the K-T boundary aside, no other mass extinction event has been strongly tied to a single, monster impact.

While massive collisions may be rare, some astronomers, including Napier, hold that "episodes of bombardment" caused by a large comet or asteroid breaking up in Earth's vicinity are much more frequent. But where such cosmic shrapnel bombs might originate isn't clear. Two likely suspects—the Oort cloud and the main asteroid belt— don't appear to be slinging enough space debris our way to fit the bill.

Another possible perpetrator is the centaur population—a bizarre clan of comet-asteroid hybrids that whiz around the outer solar system between Jupiter and Neptune. These conglomerates of rocks and ice are known for their erratic orbits, and for occasionally getting thrust into our neighborhood. Indeed, the first centaur ever discovered, Chiron, is a 233 kilometer behemoth that's thought to have crossed our path several times over the last hundred thousand years. If a comet like Chiron were to break up in the inner solar system, the fallout on Earth could be disastrous.

To assess whether large centaurs could indeed be responsible for bombardment episodes, Napier performed computer simulations on 100 hypothetical, Chiron-like comets, tracing their path over the course of a million years. On average, Napier finds that one Chiron-sized comet crosses our path every half a million years. But centaurs that are somewhat smaller—say, 100 kilometers across—may be zipping by the Earth as frequently as every 30 thousand years, according to Napier's simulations.

Napier then turned to the Earth Impact Database, an apocalypse catalog that details all known impact craters. Focusing on the past 500 million years, he identified nine bombardment episodes which could be the result of a large centaurian comet breaking up in orbit. What's more, each bombardment episode is tightly correlated with widespread disappearances of marine life in the fossil record.

"It's only over the most recent 500 million years or so that we have enough evidence to show both that these bombardment episodes take place and also that they are associated with mass extinctions of life," Napier told me. "But I'd expect giant comets and their associated bombardment episodes to have occurred all the way back to the early days of the solar system."

Napier's views on the relationship between mass extinctions and comet showers have been met with strong criticism in the past. Alan Harris, a researcher at the Space Science Institute in Colorado who studies impact hazards, sees the claim of periodic bombardment events as tenuous, at best.

"The clustering [of impact craters] has always remained at just about the threshold of any significance, and in my mind is about as significant as seeing the form of a dinosaur in the sky—patterns are easy to see, even when they are actually random," Harris told me over email.

What's more, Harris says, there's no strong reason to believe that centaur comets are disintegrating during the brief period that their orbits cross the Earth's.

"There is no mechanism or real expectation that such a thing happens, certainly not so exquisitely timed," he told me. (For his part, Napier posits that centaurs entering the inner solar system can crumble apart as intense solar radiation causes their ice to fizzle away.)

Other researchers who claim to note periodicity in the mass extinction record have also faced harsh criticism from their peers. These include biologist Michael Rampino of New York University, who recently published a controversial study suggesting that our solar system's forays across the galactic disc bring us into repeated contact with blobs of dark matter, which can influence the rate of meteor bombardment and geologic upheaval here on Earth.

Still, many revolutionary discoveries have come from voices at the fringes of science. Let's not forget that Galileo's notion of the Earth orbiting the Sun was once heretical, and in some circles, Darwin's theory of evolution still is. As for Napier's pet theory, time, it seems, will tell whether centaurs are indeed our planet's worst enemy.

Or, we'll get hit with Earth-sized asteroid first, and that'll be the end of that debate.