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Study suggests ancient interstellar cloud collision shrank heliosphere to 0.22 AU, substantially impacting Earth

Zoom-in of the LxCCs as seen in 21 cm data from the GALFA-HI survey f

A new study published in Nature Astronomy provides evidence that our solar system passed through a dense interstellar cloud 2 – 3 million years ago, exposing Earth to higher cosmic radiation and altering the climate. The discovery suggests that the Sun’s location in space might shape Earth’s history more than it was previously considered.

  • Researchers present evidence that Earth was exposed to cold, harsh interstellar clouds, 2 to 3 million years ago — a phenomenon not previously considered in geologic climate models.
  • The study shows that stars encounter drastic changes as they move through space, opening a new window into the relationship between the evolution of life on Earth and our cosmic neighborhood.

A collaborative effort by astronomers from Boston University, Harvard University, and Johns Hopkins University found evidence that some two to three million years ago, our solar system encountered a dense interstellar cloud that shrank the heliosphere to just 0.22 AU — or about 33 million km (20 million miles), causing drastic changes on Earth.

The heliosphere — made from a constant flow of charged particles from the Sun (solar wind) — normally stretches well past Pluto and acts as a plasma shield from cosmic radiation and galactic rays which can alter DNA.

Using computer models, authors of the study visualized where the Sun, heliosphere and the rest of our solar system were positioned 2 million years ago and then mapped the path of the Local Ribbon of Cold Clouds system — a string of large, dense, very cold clouds mostly made of hydrogen atoms.

Researchers propose 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, fully exposing Earth and other planets to the interstellar medium — a mix of dust, gas and leftover atomic elements of exploded stars.

Zoom-in of the LxCCs as seen in 21 cm data from the GALFA-HI survey
In this visualization, three 21 cm velocity channels, each 0.786 km s−1 wide, are mapped to red, blue and green. Red represents 8 km s−1, green 8.7 km s−1 and blue 9.5 km s−1, all in the local standard of rest (LSR) frame. The scale is logarithmic from 2 to 40 K brightness temperature. The visualization technique is designed to make the cold clouds stand out in colour (green and red for the left component and iridescent blue for the right component) by taking advantage of the narrowness of their velocity profiles compared to the warmer background gas much farther away. GALFA-HI survey. Credit: Authors

According to the paper, this aligns with geological evidence that shows increased 60Fe (iron 60) and 244Pu (plutonium 244) isotopes in the ocean, Antarctic snow, and ice cores—and on the moon—from the same time period.

The timing also matches with temperature records that indicate a cooling period.

“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,” said Merav Opher, Boston University space physicist, an expert on the heliosphere and lead author of the paper.

Opher said the heliosphere again engulfed the solar system as soon it was away from the cold cloud, and that’s how it is today.

“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,” Opher said.

“Stars move, and now this paper is showing not only that they move, but they encounter drastic changes.”

In an article released by Boston University on June 10, Jessica Colarossi said Opher’s models have quite literally shaped our scientific understanding of the heliosphere, and how it is structured by the solar wind pushing up against the interstellar medium.

“Her theory is that the heliosphere is shaped like a puffy croissant, an idea that shook the space physics community. Now, she’s shedding new light on how the heliosphere, and where the Sun moves through space, could affect Earth’s atmospheric chemistry,” Colarossi said.

While it’s impossible to know the exact effect the cold cloud had on Earth 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, Opher says. And it will probably stumble across more in another million years or so.


1 The Solar System May Have Passed through Dense Interstellar Cloud 2 Million Years Ago, Altering Earth’s Climate – The Brink/Boston University – June 10, 2024

2 A possible direct exposure of the Earth to the cold dense interstellar medium 2–3 Myr ago. – Opher, M., Loeb, A. & Peek, J.E.G. Nature Astron (2024) – DOI: https://doi.org/10.1038/s41550-024-02279-8 – OPEN ACCESS


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