Interstellar interstellar: The cosmic event that rewrote Earth’s climate history

New astrophysical research highlights an important cosmic event two million years ago, when the solar system passed through a dense interstellar cloud. This probably changed Earth’s climate by exposing it to enhanced cosmic radiation, supported by increased isotopes found in the geological record.

Earth was a very different place about two million years ago, with our early human ancestors living alongside saber-toothed tigers, mastodons and large rodents. And, depending on where they were, they may have been cold: Earth had fallen into a deep freeze, with multiple ice ages coming and going until about 12,000 years ago. Scientists theorize that ice ages occur for a number of reasons, including the tilt and rotation of the planet, changing plate tectonics, volcanic eruptions, and carbon dioxide levels in the atmosphere.

But what if drastic changes like these are not just the result of Earth’s environment, but also of the sun’s location in the galaxy?

Impact of the Sun’s Galactic Voyage

In a new paper published today (June 10) in Astronomy of Nature, Lead author and astrophysicist Merav Opher – a professor of astronomy at Boston University and fellow at the Harvard Radcliffe Institute – found evidence that about two million years ago, the solar system encountered an interstellar cloud so dense that it could have interfered with the solar wind of the sun. Opher and her co-authors believe this shows that the sun’s location in space may shape Earth’s history more than previously considered.

The role of the heliosphere in protecting the Earth

Our entire solar system is covered with a shield plasma shield emanating from the sun, known as the heliosphere. It’s made of a continuous stream of charged particles, called the solar wind, stretching well ahead Pluto, enveloping the planets in what NASA calls “a giant bubble.” It protects us from radiation and galactic rays that can change DNAand scientists believe this is part of the reason why life evolved on Earth the way it did.

According to the latest paper, the cold cloud compressed the heliosphere in such a way that it briefly placed Earth and the other planets in the solar system outside the influence of the heliosphere.

Effects of the Galactic Encounter on Earth

“This paper is the first to show quantitatively that there was an encounter between the sun and something outside the solar system that would have affected Earth’s climate,” says Opher, who is an expert on the heliosphere.

Her models have literally shaped our scientific understanding of the heliosphere and how the bubble is structured by the solar wind pushing up against the interstellar medium—that is, the space between stars and beyond the heliosphere in our galaxy. Her theory is that the heliosphere is shaped like a puffy croissant, an idea that shocked the space physics community. Now, it’s shedding new light on how the heliosphere, and where the sun moves through space, can affect Earth’s atmospheric chemistry.

“Stars move, and now this paper is showing not only that they move, but that they experience drastic changes,” says Opher. She first discovered and began working on this study during a yearlong fellowship at the Harvard Radcliffe Institute.

Simulation Insights into Cosmic Interactions

To study this phenomenon, Opher and her collaborators 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 made mostly of hydrogen atoms. Their simulations showed that one of the clouds near the end of that strip, called the Local Cold Cloud Lynx, may have collided with the heliosphere.

Geological and cosmic evidence

If that had happened, Opher says, Earth would have been completely 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 is consistent with geological evidence showing increased isotopes 60Fe (iron 60) and 244Pu (plutonium 244) in the ocean, moon, Antarctic snow and ice cores from the same time period. The timing also coincides with temperature data indicating a cooling period.

Long-term galactic influences

“Only rarely does our cosmic neighborhood beyond the solar system affect life on Earth,” says Avi Loeb, director of Harvard University’s Institute for Theory and Computation and co-author on the paper. “It is exciting to discover that our passage through dense clouds a few million years ago may have exposed Earth to a much greater influx of cosmic rays and hydrogen atoms. Our results open a new window on the relationship between the evolution of life on Earth and our cosmic neighborhood.”

The outward pressure from the Local Lynx Cold Cloud could have continuously trapped the heliosphere for several hundred years to a million years, Opher says—depending on the size of the cloud. “But once Earth left the cold cloud, the heliosphere engulfed all the planets, including Earth,” she says. And so it is today.

Future Research and Implications

It’s impossible to know the exact effect the cold clouds had on Earth—like whether it might have triggered an ice age. But there are several 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 is likely to encounter more in a million years or so.

Opher and her collaborators are now working to trace where the sun was seven million years ago, even further. Accurately determining the location of the sun millions of years in the past, as well as the cold cloud system, is possible with data collected by the European Space Agency’s Gaia mission, which is building the largest 3D map of the galaxy and gives an unprecedented view of the speed stars move.

Exploring the past trajectory of the Sun

“This cloud was really in our past, and if we passed something that massive, we were exposed to the interstellar medium,” says Opher. The effect of crossing paths with so much hydrogen and radioactive material is unclear, so Opher and her team at BU NASAThe SHIELD-funded DRIVE (Solar Wind with Hydrogen Ion Exchange and Large-Scale Dynamics) science center is now exploring the effect it could have on Earth’s radiation, as well as the atmosphere and climate.

“This is just the beginning,” says Opher. She hopes this paper will open the door to a greater exploration of how the solar system was affected by external forces in the deep past and how these forces have shaped life on our planet.

Reference: “A possible direct exposure of Earth to the cold dense interstellar medium 2–3 Myr ago” June 10, 2024 Astronomy of Nature.
DOI: 10.1038/s41550-024-02279-8

This research was supported by NASA.