Cold, dense clouds in the interstellar medium of our Milky Way Galaxy are around four-five orders of magnitude denser than their diffuse counterparts. A team of astronomers from Boston University, Harvard University and Johns Hopkins University has now found evidence that between two and three million years ago, our Solar System encountered one of these dense clouds, which was so dense that it could have interfered with the solar wind.
Opher et al. show that in the interstellar medium that the Sun has traversed for the last couple of million years, there are cold, compact clouds that could have drastically affected the heliosphere. Image credit: NASA / JPL-Caltech.
Most stars generate winds and move through the interstellar medium that surrounds them.
This motion creates a cocoon that protects planets from the interstellar medium. The Sun’s cocoon is the heliosphere.
It’s made from a constant flow of charged particles, called solar wind, that stretch well past Pluto, wrapping the planets in what astronomers call the Local Bubble.
It protects us from radiation and galactic rays that could alter DNA, and scientists believe it’s part of the reason life evolved on Earth as it did.
According to the new study, the cold interstellar cloud compressed the heliosphere in such a way that it briefly placed Earth and the other planets in the Solar System outside of the heliosphere’s influence.
“Our 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 Boston University’s Professor Merav Opher.
“Stars move, and now this paper is showing not only that they move, but they encounter drastic changes.”
To study this phenomenon, Professor Opher and colleagues essentially looked back in time, using sophisticated computer models to visualize where the sun was positioned two million years in the past — and, with it, the heliosphere, and the rest of the Solar System.
They also mapped the path of the Local Ribbon of Cold Clouds system, a string of large, dense, very cold clouds mostly made of hydrogen atoms.
Their simulations showed that one of the clouds close to the end of that ribbon, named the Local Lynx of Cold Cloud, could have collided with the heliosphere.
If that had happened, Earth would have been fully exposed to the interstellar medium, where gas and dust mix with the leftover atomic elements of exploded stars, including iron and plutonium.
Normally, the heliosphere filters out most of these radioactive particles. But without protection, they can easily reach Earth.
According to the paper, this aligns with geological evidence that shows increased iron-60 and plutonium-244 isotopes in the ocean, on the Moon, Antarctic snow, and ice cores from the same time period.
The timing also matches with temperature records that indicate a cooling period.
“Only rarely does our cosmic neighborhood beyond the Solar System affect life on Earth,” said Harvard University’s Professor Avi Loeb.
“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.”
“Our results open a new window into the relationship between the evolution of life on Earth and our cosmic neighborhood.”
“The outside pressure from the Local Lynx of Cold Cloud could have continually blocked out the heliosphere for a couple of hundred years to a million years, depending on the size of the cloud.”
“But as soon as the Earth was away from the cold cloud, the heliosphere engulfed all the planets, including Earth.”
“It’s impossible to know the exact effect the cold clouds had on Earth — like if it could have spurred an ice age.”
“But there are a couple of other cold clouds in the interstellar medium that the Sun has likely encountered in the billions of years since it was born.”
“And it will likely stumble across more in another million years or so.”
The authors are now working to trace where the Sun was seven million years ago, and even further back.
Pinpointing the location of the Sun millions of years in the past, as well as the cold cloud system, is possible with data collected by ESA’s Gaia mission, which is building the largest 3D map of the galaxy and giving an unprecedented look at the speed stars move.
“This cloud was indeed in our past, and if we crossed something that massive, we were exposed to the interstellar medium,” Professor Opher said.
“This is only the beginning. We hope that this paper will open the door to much more exploration of how the Solar System was influenced by outside forces in the deep past and how these forces have in turn shaped life on our planet.”
The paper appears today in the journal Nature Astronomy.
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M. Opher et al. A possible direct exposure of the Earth to the cold dense interstellar medium 2-3 Myr ago. Nat Astron, published online June 10, 2024; doi: 10.1038/s41550-024-02279-8