Interstellar invader: the cosmic event that rewrote Earth’s climate history

New astrophysical research highlights a major cosmic event two million years ago, when the solar system passed through a dense interstellar cloud. This possibly changed Earth’s climate by exposing it to increased cosmic radiation, supported by an increase in the number of isotopes in geological records.

About two million years ago, the Earth looked very different, where our early human ancestors lived alongside sabre-tooth tigers, mastodons and enormous rodents. And depending on where they were, it may have been cold: the Earth was deeply frozen, with multiple ice ages coming and going until about 12,000 years ago. Scientists theorize that ice ages happen for a number of reasons, including the tilt and rotation of the planet, shifting plate tectonics, volcanic eruptions and the carbon dioxide levels in the atmosphere.

But what if these kinds of drastic changes are due not only to Earth’s environment, but also to the Sun’s location in the Milky Way?

Impact of the Sun’s Galactic Journey

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

The role of the heliosphere in protecting the Earth

Our entire solar system is shrouded in a layer of protection plasma shield emanating from the sun, known as the heliosphere. It is made of a constant stream of charged particles, called solar wind, that extend far beyond Earth Pluto, enveloping the planets in what NASA calls a “giant bubble.” It protects us from radiation and galactic radiation that could change DNAand scientists believe this is one of the reasons why life on Earth evolved the way it did.

According to the last article, the cold cloud compressed the heliosphere so much that Earth and the other planets in the solar system were briefly placed outside the heliosphere’s influence.

The consequences of the galactic encounter on Earth

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

Her models literally shaped our scientific understanding of the heliosphere, and how the bubble is structured by the solar wind pushing up against the interstellar medium – the space between the stars and beyond the heliosphere in our Milky Way. 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, can influence Earth’s atmospheric chemistry.

“Stars move, and now this paper not only shows that they move, but that they undergo drastic changes,” says Opher. She first discovered this research and began working on it 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 two million years ago — 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 series of large, dense, very cold clouds composed largely of hydrogen atoms. Their simulations showed that one of the clouds close to the end of that ribbon, called the Local Lynx or Cold Cloud, could 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 article, this is consistent with geological evidence showing an increase in 60Fe (iron 60) and 244Pu (plutonium 244) isotopes in the ocean, on the moon, Antarctic snow and ice cores from the same period. The timing also matches temperature data indicating a cooling period.

Long-term galactic influences

“Only rarely does our cosmic environment outside the solar system impact life on Earth,” said Avi Loeb, director of Harvard University’s Institute for Theory and Computation and co-author of the paper. “It is exciting to discover that our passage through dense clouds a few million years ago could have exposed Earth to a much greater flow 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 environment.”

Outside pressure from Cold Cloud’s local Lynx could have continuously blocked the heliosphere for a few hundred to a million years, Opher says, depending on the size of the cloud. “But once Earth moved away from the cold cloud, the heliosphere engulfed all the planets, including Earth,” she says. And so it is today.

Future research and implications

It is impossible to know exactly what effect the cold clouds had on Earth, as if it could have caused an ice age. But there are a few other cold clouds in the interstellar medium that the Sun has likely encountered in the billions of years since its birth, Opher says. And in a million years it will probably encounter even more.

Opher and her collaborators are now trying to find out where the sun was seven million years ago, and even further back. Pinpointing the location of the Sun millions of years ago, 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 Milky Way and providing an unprecedented view of the speed stars move.

Research into the sun’s past

“This cloud was indeed in our past, and crossing something that big exposed us to the interstellar medium,” Opher says. The effect of crossing paths with so much hydrogen and radioactive material is unclear, so Opher and her team at BU’s NASAThe SHIELD-funded DRIVE Science Center (Solar Wind with Hydrogen Ion Exchange and Large-Scale Dynamics) is now investigating the effect this could have had on Earth’s radiation, as well as the atmosphere and climate.

“This is just the beginning,” says Opher. She hopes that this paper will open the door for much more research into how the solar system was affected by outside forces in the deep past and how these forces in turn shaped life on our planet.

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

This research was supported by NASA.