Ancient astrologers believed that stars and planets traced dizzying spirals across the sky, even going backward at times. Part of the reason Renaissance astronomers decided that the Earth revolves around the sun and not vice versa was to throw out the complicated mess of epicycles and retrograde motion that enthralled an earlier era.
But it turns out there was something in that “wheels within wheels” model if you look on a larger scale. Earth both spins on its axis and orbits the sun, and astronomers now know that the sun in turn rotates around the center of the galaxy and rises and sinks as it does so. And if a recent astronomy paper is correct, our convoluted path through the universe also explains why Earth seems so prone to mass extinctions.
William Napier, an astronomer at Cardiff University in Wales and primary author of the paper, explains that we live in a flat galaxy where most stars lie in the same plane. But our sun bobs up and down through that galactic plane “like a carousel,” he says. Every time the sun hits a peak, its immense gravity pulls comets toward it from a cloud around our solar system, the Oort Cloud.
“We’re dealing with a big cloud of comets, about 500-million of them,” Napier says. Just as the moon exerts a tide on Earth, the sun exerts a tide on the cloud, “and a small fraction of [the comets] will get moved into the solar system.” A small fraction of those comets will in turn strike Earth, with calamitous consequences.
Other scientists had seen evidence for periodic bombardments, but the paper by Napier and coauthor J.T. Wickrmasinghe, which appeared in the Monthly Notes of the Royal Astronomical Society, did two important things. First, by looking at impact craters across the globe, it nailed down how often the bombardments take place—approximately every 36 million years. More important, the paper linked specific craters to specific extinctions. Paleontologists still debate the point, but many now agree that major extinctions occurred in evenly spaced bunches throughout Earth’s history, eras when half or even three-quarters of all species died out. Napier’s paper finds a historical match between those extinction events and bombardment peaks. Overall, simple gravitational tides in space may be as deadly to earthlings as riptides to unwary swimmers.
What’s more, this work might force scientists to revisit the most famous extinction event ever, the death of dinosaurs. The problem with the prevailing theory, where one giant collision wiped out most known life 65 million years ago (known as the “T. rex and the crater of doom” theory), is that the fossil record shows the dinosaurs started to decline before the crater of doom, and they took a long time to succumb after it.
Napier and Wickrmasinghe can explain those facts neatly. They promote the idea of “not a single bang, but a multiplicity of bangs,” Napier says. In other words, dinosaurs weren’t killed by one huge mortar shell but something more like buckshot, as a number of comets pelted Earth. The reason we don’t see evidence for this is that many comets were small and many probably landed in oceans, leaving no permanent record in either case.
Napier calculates that Earth is, in astronomical terms at least, “pretty close to the peak of bombardment now, a few millions years out.” But even this might not be so bad. As comets and asteroids bombard Earth, they tend to throw up rocks and boulders that become new space debris. If any of that debris makes its way to new, barren planets, it could bring Earth-based microbes with them—and start evolution all over again.
Sam Kean is the associate editor of Science & Spirit.
