Blog

About two million years ago, the Earth was a very different place, with our early human ancestors living alongside sabre-toothed tigers, mastodons and huge rodents. And depending on where they were, they may have been cold: Earth went 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, shifting plate tectonics, volcanic eruptions, and atmospheric carbon dioxide levels. But what if drastic changes like these are not just the result of Earth’s environment, but also the location of the sun in the galaxy?

In a new paper published in natural astronomy, lead author and astrophysicist Merav Ofer—a professor of astronomy at Boston University and a fellow at Harvard’s 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. Offer and her co-authors believe this shows that the sun’s location in space may have shaped Earth’s history more than previously thought.

Our entire solar system is enveloped in a protective plasma shield that radiates from the sun known as the heliosphere. It’s made of a steady stream of charged particles called the solar wind that stretches well past Pluto, enveloping the planets in what NASA calls a “giant 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 the way it did. According to the latest paper, the cold cloud compressed the heliosphere in such a way that it briefly put Earth and the other planets in the solar system outside the influence of the heliosphere.

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

Her models have quite literally shaped our scientific understanding of the heliosphere and how the bubble is structured by the solar wind pushing against the interstellar medium—which is the space between the stars and beyond the heliosphere in our galaxy. Her theory is that the heliosphere is shaped like a fluffy croissant, an idea that shook the space physics community.

Now it sheds new light on how the heliosphere and where the sun moves through space can affect the chemistry of Earth’s atmosphere.

“Stars move, and now this paper shows not only that they move, but that they encounter drastic changes,” says Ofer. She first discovered and began working on this study during a one-year fellowship at Harvard’s Radcliffe Institute.

To study this phenomenon, Ofer and her collaborators essentially looked back in time, using sophisticated computer models to visualize where the sun was located two million years in the past—and with it the heliosphere and the rest of the solar system. They also charted the path of the Local Ribbon of Cold Clouds system, a series of large, dense, very cold clouds made mostly of hydrogen atoms.

Their simulations showed that one of the clouds near the end of this band, called the Local Lynx of Cold Cloud, may have collided with the heliosphere.

If that had happened, Ofer says, Earth would have been fully exposed to the interstellar medium, where gas and dust mix with the leftover atomic elements from 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 that shows uplift ⁶⁰Fe (iron 60) and ²⁴⁴Pu (plutonium 244) isotopes in the ocean, on the Moon, Antarctic snow and ice cores from the same time period. The timing also coincides with temperature records that indicate a cooling period.

“Only rarely does our cosmic neighborhood outside the solar system affect life on Earth,” said Avi Loeb, director of Harvard University’s Institute for Theory and Computation and co-author of the report.

“It is exciting to find that our passage through dense clouds several million years ago could have exposed Earth to a much greater flux of cosmic rays and hydrogen atoms. Our results open a new window into the connection between the evolution of life on Earth and our cosmic neighborhood.”

Outward pressure from the Cold Cloud’s local lynx may have continuously blocked the heliosphere for several hundred years to a million years, Offer says—depending on the size of the cloud. “But as soon as Earth moved away from the cold cloud, the heliosphere engulfed all the planets, including Earth,” she says. And so it is today.

It’s impossible to know the exact effect the cold clouds had on Earth – like they could have caused 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, Ofer says. And it will probably come across more in another million years or so.

Ofer and her collaborators are now working to trace where the sun was seven million years ago and even further back. 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 giving an unprecedented look at the speed stars are moving .

“This cloud was really in our past, and if we were to cross something this massive, we would be exposed to the interstellar medium,” Offer says. The effect of crossing paths with so much hydrogen and radioactive material is unclear, so Offer and her team at BU’s SHIELD (Solar Wind with Hydrogen Ion Exchange and Large-Scale Dynamics) DRIVE Science Center are now investigating the effect it could have on Earth radiation, as well as the atmosphere and climate.

“This is just the beginning,” says Ofer. She hopes that this paper will open the door to much more research into how the solar system was influenced by external forces in the deep past, and how those forces in turn shaped life on our planet.