What killed the dinosaurs? No, it wasn’t the ice age, despite what Mr. Freeze would have you believe. Most scientists agree that a large impact event 66 million years ago was the cause, creating the famous Chicxulub impact crater in Mexico.

But a new study published in the journal Gondwana Research has proposed a rather more radical theory. A team of Japanese scientists has suggested that Earth may have passed through a dense interstellar cloud around the time the dinosaurs went extinct, leading to a “Nebula Winter” that caused a large amount of global cooling, ultimately wiping out most life on Earth.

Their evidence is based on an especially thick deposit of iridium the team found in the Pacific Ocean. According to New Scientist, Earth is covered in a layer of iridium 30 centimeters (12 inches) thick, which is thought to be of extraterrestrial origin – such as from asteroid impacts. But the team, led by Tokuhiro Nimura of the Japan Spaceguard Association and colleagues, found a bigger deposit 5 meters (16 feet) thick that they say cannot be explained by an asteroid.

Instead, they propose that a molecular cloud 330 light-years across, and more than a thousand times more dense than the surrounding space, could be the culprit. As Earth passed through this cloud, taking up to a million years, our planet would have experienced significant global cooling as the sky was darkened, ultimately leading to the extinction of the dinosaurs 66 million years ago, known as the K-Pg boundary. The iridium would have been picked up from this cloud as we passed through.

“The dark cloud would have caused global climate cooling in the last 8 million years of the Cretaceous period,” they write in the paper. “The resulting growth of the continental ice sheet also resulted in a regression of the sea level. The global cooling, which appears to be associated with a decrease in the diversity of fossils, eventually led to the mass extinction at the K-Pg boundary.”

The cloud would have been hundreds of light-years across. Vadim Sadovski/Shutterstock

The idea that the Solar System periodically passes through dense interstellar clouds is not new. Indeed, we know such clouds exist, so the theory is not entirely implausible. But it’s certainly a bit outlandish, especially considering the evidence we have for an asteroid impact being the cause. Occam’s razor, anyone?

That’s not to say the scientists are denying the Chicxulub impact. In fact, they’re saying both could have happened – it’s just the interstellar cloud was the primary cause of the extinction. David Kring from the Lunar and Planetary Institute in Houston, Texas, however, poured doubt on the theory. He told New Scientist that the extra deposit of iridium was “sharp, not broad,” supporting the asteroid impact theory.

Still, it’s an interesting idea nonetheless. The scientists say it could be further strengthened by finding other radioactive elements that may have originated from space such as plutonium on the sea floor, something they will surely now endeavor to do.