A Giant Interstellar Cloud May Have Once Enveloped Earth, Potentially Causing Ice Ages

Astronomers suggest this cold, dense cloud compressed our sun’s protective field between two and three million years ago, leaving the Earth exposed to cosmic material

orange dust surrounds a bright star
Captured by the Hubble Space Telescope, the heliosphere of a young star, LL Orionis, collides with an interstellar medium of dust. A similar event with our own sun, researchers say, might have occurred between two and three million years ago. NASA / STScI / AURA

Some two million years ago, our human ancestors shared the Earth with the likes of mastodons, saber-toothed tigers and giant sloths. At that time, survival was anything but easy. For a long period ending only about 12,000 years ago, our planet withstood a series of harsh ice ages that saw global temperatures fluctuate, sea levels rise and fall as glaciers melted and re-froze, and plant and animal life shift from the poles toward the warmer equator.

The causes behind this tumultuous epoch have been debated for years. Scientists have considered volcanic eruptions, varying levels of atmospheric carbon dioxide, Earth’s tilt and volatile plate tectonics. Now, research published this week in the journal Nature Astronomy offers another hypothesis of cosmic proportions: that our solar system passed through a massive, interstellar cloud of dust and gas.

Typically, Earth and all our solar system’s planets are enveloped in a protective, giant bubble of plasma from the sun known as the heliosphere. Within this perhaps croissant-shaped region, the sun’s charged particles, known as solar wind, shield the planets from galactic radiation and the extreme weather of outer space.

But between two million and three million years ago, the researchers posit that an interstellar cloud compressed the heliosphere, temporarily leaving much of the solar system, including Earth, vulnerable.

A three-dimensional rendering of the heliosphere. Image A shows the heliosphere from the side. Image B shows the helisophere, as seen from above. The red line depicts Earth's orbit around the sun.
A three-dimensional rendering of the heliosphere. Image A shows the heliosphere from the side. Image B shows the helisophere, as seen from above. The red line depicts Earth's orbit around the sun. Opher et al., Nature Astronomy, 2024

“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,” Merav Opher, a space physicist at Boston University and lead author of the paper, says in a statement.

Opher and her team built and analyzed a series of models based on data collected by the European Space Agency’s Gaia satellite, the “billion star surveyor” creating a 3D map of more than a billion stars, both within our galaxy and beyond it.

The astronomers found that roughly around the Pleistocene era, which began some 2.6 million years ago, the solar system may have crossed paths with the Local Ribbon of Cold Clouds, a system of cool, dense clouds made primarily of hydrogen atoms. Passage through one cloud in particular, located in the Lynx constellation, might have sent the heliosphere out of whack. Based on their models, its reach would have shrunk to 0.22 astronomical units—or just about a fifth of the distance between the Earth and sun—leaving our planet outside the star’s protective field.

The galaxy known as NGC 2337, located 25 million light-years away from Earth and inside the Lynx constellation.
The galaxy known as NGC 2337, located about 26 million light-years from Earth, is inside the Lynx constellation. ESA / Hubble & NASA

Without the sheltering benefit of solar wind, interstellar iron and plutonium would have battered the Earth. Geological evidence supports this idea: Isotopic samples of these elements—plutonium-244 and iron-60—have been found in the planet’s oceans, Antarctic snow and in lunar samples. These isotopes were dated to the same era, around 1.7 to 3.2 million years ago.

The cloud could have compressed the heliosphere, though the researchers don’t know for sure that it would have caused ice ages. But either way, its effects would have been temporary.

“As soon as the Earth was away from the cold cloud, the heliosphere engulfed all the planets, including Earth,” Opher says in the statement.

The map of the night sky, colorized, as captured by the European Space Agency's Gaia Satellite.
A map of the night sky, colorized, as captured by the European Space Agency's Gaia satellite. ESA / Gaia / DPAC; CC BY-SA 3.0 IGO. (A. Moitinho)

As for next steps, the team is hoping to model the exact position the sun held in the sky about seven million years ago, a time when other research has spotted another peak in the plutonium and iron isotopes associated with interstellar clouds. They are also hoping to analyze additional atmospheric and climate data from approximately the Pleistocene.

“Understanding this helps us learn about the effects of the interstellar medium on life on Earth in the past,” Sarah Spitzer, an astronomer at the University of Michigan who was not involved in the research, tells New Scientist’s James Woodford. “But it also helps us better understand the current effects of the heliosphere on life on Earth, what might happen if the Earth is exposed to the interstellar medium again in the future, and when that might happen.”

Editor’s Note, June 17, 2024: This article has been corrected to state that our solar system, not the entire Milky Way, is proposed to have interacted with an interstellar cloud between two and three million years ago.

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