Earth collided with an interstellar cloud millions of years ago

Around two million years ago, Earth was home to early human ancestors living alongside saber-toothed tigers, mastodons, and massive rodents. 

The planet experienced multiple ice ages, likely caused by factors such as Earth’s tilt, rotation, tectonics, volcanic activity, and atmospheric carbon dioxide levels. 

However, a new study led by Boston University (BU) has suggested that the sun’s location in the galaxy may have also played a significant role.

Interstellar cloud encounter

The experts found that around two million years ago, the solar system encountered an interstellar cloud dense enough to disrupt the sun’s solar wind, which forms the protective heliosphere around the solar system. 

The heliosphere shields Earth from harmful cosmic radiation and galactic rays, which can alter DNA. The dense cloud compressed the heliosphere, briefly exposing Earth to the interstellar medium.

“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 lead author Merav Opher, an astrophysicist at BU. 

Solar wind and the interstellar medium 

Opher’s models, which propose that the heliosphere is shaped like a puffy croissant, have significantly influenced the understanding of how the solar wind interacts with the interstellar medium.

To investigate these phenomena, Opher and her team used sophisticated computer models to trace the sun’s position two million years ago. 

They mapped the path of the Local Ribbon of Cold Clouds system, a series of large, dense, and cold clouds primarily composed of hydrogen atoms. 

Local Lynx of Cold Cloud

The simulations indicated that the Local Lynx of Cold Cloud, a part of this ribbon, could have collided with the heliosphere.

If such a collision occurred, Earth would have been exposed to the interstellar medium, which contains gas, dust, and radioactive particles from exploded stars. Normally, the heliosphere filters out these particles, but without it, they could reach Earth. 

This scenario aligns with geological evidence showing increased levels of iron-60 and plutonium-244 isotopes in ocean sediments, lunar samples, Antarctic snow, and ice cores from that period, coinciding with a cooling phase on Earth.

Iron 60 and plutonium 244 are very rare heavy metals that are created when massive stars explode.

“It is everywhere, in the deep ocean, on the moon, on ice in Antarctica,” said Opher. “These papers describe a global phenomenon. Something happened. And iron 60 is not produced on Earth. So I knew that somehow this iron 60 got trapped in dust, and somehow, 2 to 3 million years ago, we had more dust delivered to us.”

Dense cloud compressed the heliosphere 

“Only rarely does our cosmic neighborhood beyond the solar system affect life on Earth,” said co-author Avi Loeb, the director of Harvard University’s Institute for Theory and Computation. 

“It is exciting to discover that our passage through dense clouds a few million years ago could have exposed the Earth to a much larger flux of cosmic rays and hydrogen atoms.”

The pressure from the Local Lynx of Cold Cloud could have kept the heliosphere compressed for hundreds of years to a million years, depending on the cloud’s size. Once Earth moved away from the cloud, the heliosphere would have re-expanded to encompass all the planets.

Interstellar clouds and Earth’s climate 

While it is difficult to determine the exact impact of these cold clouds on Earth’s climate, Opher argued that the sun has likely encountered other cold clouds over billions of years and will do so again in the future. 

Her team is now working on mapping the sun’s location seven million years ago and further back, using data from the European Space Agency’s Gaia mission, which is creating the largest 3D map of the galaxy.

“This cloud was indeed in our past, and if we crossed something that massive, we were exposed to the interstellar medium,” noted Opher.

Her team at BU’s NASA-funded SHIELD (Solar wind with Hydrogen Ion Exchange and Large-scale Dynamics) DRIVE Science Center is now exploring the potential effects on Earth’s radiation, atmosphere, and climate.

“This is only the beginning,” Opher said. She hopes this research will lead to further exploration of how the solar system’s encounters with interstellar forces have shaped Earth’s history and life.

The study is published in the journal Nature Astronomy.

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