The Pleistocene Epoch—with its glaciers, woolly mammoths, and Neanderthals—still looms large in Earth’s rearview mirror, having ended a mere 12,000 years ago. Now, a team of researchers posit that those hundreds of thousands of years of our planet’s history may have been chilly due to a cloud in space that briefly removed Earth from the safety of the Sun’s warm glow.

The researchers propose that, about two million years ago, an interstellar cloud interfered with the solar system in such a way that Earth and other planets were briefly outside of the Sun’s heliosphere, the bubble of charged particles from our host star that today forms an amorphous envelope around the system. Their research was published today in Nature Astronomy.

“This paper is the first to quantitatively show there was an encounter between the sun and something outside of the solar system that would have affected Earth’s climate,” said Merav Opher, an astrophysicist at Boston University and lead author of the study, in an email to Gizmodo. Opher added that the team is “still trying to quantify it with modern climate models” but with an increase of hydrogen and dust “Earth would have entered in an Ice Age.” 

Opher’s team modeled data from the HI4PI survey and found that our solar system may have passed through the Local Ribbon of Cold Clouds in the constellation Lynx between 2 million and 3 million years ago. The Pleistocene began about 2.6 million years ago. It’s not possible to say for certain whether such cold clouds could’ve catalyzed an ice age, the release noted, but more evidence of clouds tampering with the heliosphere may clarify the kind of impacts it would have on Earth.

The team’s model revealed that, in such a passage, the heliosphere that ensconces Earth and its neighboring planets would shrink to about 0.22 astronomical units, or less than a quarter the size of the Earth’s distance to the Sun. To put that in perspective, ESA estimates that the closest boundary of the heliosphere today is about 100 AU out from the Sun, about twice as far as the Kuiper Belt.

Outside of the heliosphere, the Earth would’ve been exposed to iron and plutonium in the interstellar medium, the team posited. Their timeline aligns with an uptick in the amount of plutonium-244 and iron-60, two isotopes of the respective elements which are known to occur from events in space, in Antarctic snow, deep-sea sediments, and samples from the Moon. And as Opher added, samples from Mars, if tested in the same way as the lunar and terrestrial samples, could reveal a similar spike in the iron isotope around 2 to 3 million years ago.

The heliosphere could’ve been blocked out for anywhere from just a couple hundred years to one million years, Opher said in a Boston University release. The moment Earth and the other planets moved away from the cloud, the heliosphere returned.

To vet their results, the team is now trying to figure out the position of the Sun some seven million years ago, where there is evidence for another peak in the ratios of plutonium-244 and iron-60 in earthly ice and sediments. They are trying to create a digital twin—basically, a high-tech model—of the heliosphere to better model the sorts of conditions our solar system may have been subject to. Lastly, additional data from ESA’s Gaia mission could further help the team place the Sun’s exact position at that moment in the ancient past.

According to the Utah Geological Survey, at least five major ice ages have occurred on Earth. The first occurred over 2 billion years ago and the most recent began around 3 million years ago. According to NASA, ice ages can kick off due to a combination of factors, including changes in Earth’s orbit, low amounts of energy from the Sun, the composition of the atmosphere, changes in ocean currents, and even volcanoes, which were responsible for the year without a summer. In other words, we’re not wanting for theories explaining Earth’s various cold moments, and the jury is out on exactly how Earth being outside the heliosphere may have catalyzed such a frigid period.

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