Scientists still think a space rock slammed into Earth 65 million years ago in a catastrophic collision that wiped out the dinosaurs and most other species. But a new theory presented this week suggests that the space rock was a speedy comet, not a hulking asteroid as previously believed, reports the BBC. Dartmouth researchers came up with the theory after determining that the amount of sediment—specifically extraterrestrial iridium and osmium—that got thrown into the atmosphere at the time of the collision was less than is commonly believed.
Then they factored in the size of the 120-mile-wide crater that the collision created in the Yucatan Peninsula. A massive, slow-moving asteroid didn't compute. "So we said, 'How do we get something that has enough energy to generate that size of crater, but has much less rocky material?" explains one of the Dartmouth researchers. It had to have been something relatively small and fast. "That brings us to comets." Not all scientists are convinced just yet, however, notes the Science Recorder. It's possible that an unusually fast asteroid could have done the trick. (Read more comet stories.)