About two million years ago, Earth was a place where our early human ancestors lived alongside saber-toothed tigers, mastodons, and giant rodents. At that time, Earth was in a deep ice age, with more ice ages alternating until about 12,000 years ago. Scientists theorize that ice ages occur for multiple reasons, including the tilt and rotation of the planet, tectonic plate shifts, volcanic eruptions, and carbon dioxide levels in the atmosphere. But what if these drastic changes are not only the result of Earth's environment but also the sun's position in the galaxy?

New Discovery
In a new paper published in the journal Nature Astronomy, researchers led by Boston University have found evidence that about two million years ago, the solar system encountered an interstellar cloud so dense that it could disrupt the solar wind. They believe this shows how the sun's position in space can shape Earth's history more than previously thought.

Our entire solar system is protected by a plasma shield that comes from the sun, known as the heliosphere. It consists of a constant stream of charged particles, called the solar wind, which extends far beyond Pluto, enveloping the planets in what NASA calls a "giant bubble." The heliosphere protects us from radiation and galactic rays that could alter DNA, and scientists believe it is one of the reasons life on Earth developed 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 other planets in the solar system outside its influence.

Quantitative Research
"This work quantitatively shows for the first time that there was an encounter between the sun and something outside the solar system that could affect Earth's climate," says Merav Opher, an astrophysicist at Boston University and the paper's lead author.

Her models have literally shaped our scientific understanding of the heliosphere and how the structure of that bubble is shaped by the solar wind colliding with the interstellar medium—the space in our galaxy between stars and outside the heliosphere. Her theory is that the heliosphere is shaped like an inflated croissant, an idea that has shaken the space physics community. Now, she sheds new light on how the heliosphere, and where the sun moves through space, can affect Earth's atmosphere.

"Stars move, and now this work shows not only that they move, but they encounter drastic changes," says Opher, a professor of astronomy in the College of Arts and Sciences at Boston University and a member of the university's Center for Space Physics. She worked on the study during a one-year fellowship at the Harvard Radcliffe Institute.

Looking Back Through Time
Opher and her colleagues have literally looked back through time, using sophisticated computer models to visualize where the sun was positioned two million years ago—and, with it, the heliosphere and the rest of the solar system. They also mapped the trajectory of the Local String of Cold Clouds, a series of large, dense, very cold clouds mainly composed of hydrogen atoms. Their simulations showed that one of the clouds near the end of that string, called the Local Ribbon Cold Cloud, could have collided with the heliosphere.

If that happened, says Opher, Earth would be completely exposed to the interstellar medium, where gas and dust mix with leftover atomic elements from exploded stars, including iron and plutonium. Normally, the heliosphere filters most of these radioactive particles. But without protection, they can easily reach Earth. According to the paper, this coincides with geological evidence showing increased isotopes of 60Fe (iron 60) and 244Pu (plutonium 244) in the ocean, Antarctic snow, and ice cores—and on the moon—from the same time period. The timing also matches temperature records indicating a cooling period.

"Our cosmic neighborhood outside the solar system rarely affects life on Earth," says Avi Loeb, director of the Institute for Theory and Computation at Harvard and co-author of the paper. "It's exciting to discover that our passage through dense clouds a few million years ago could have exposed Earth to a much higher flux of cosmic rays and hydrogen atoms. Our findings open a new window on the relationship between the evolution of life on Earth and our cosmic neighborhood."

The external pressure from the Local Ribbon Cold Cloud could have continuously blocked the heliosphere for several hundred to a million years, says Opher—depending on the cloud's size. "But as soon as Earth was away from the cold cloud, the heliosphere enveloped all the planets, including Earth," she says. And so it is today.

It is impossible to know the exact effect the cold cloud had on Earth—like whether it could have triggered an ice age. But there are several more cold clouds in the interstellar medium that the sun has likely encountered in the billions of years since it was born, says Opher. And it will likely encounter more in the next million years or so.

Opher and her colleagues are now working on tracking where the sun was seven million years ago, and even further back. Determining the sun's position millions of years in the past, as well as the cold cloud systems, is possible with data collected by the European Space Agency's Gaia mission, which is building the largest 3D map of the galaxy and providing an unprecedented view of the speed at which stars are moving.

"That cloud was indeed in our past, and if we passed through something so 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 the NASA-funded SHIELD (Solar wind with Hydrogen Ion Exchange and Large-scale Dynamics) DRIVE Science Center are now investigating the effect it could have had on Earth's radiation, as well as on the atmosphere and climate.

"This is just the beginning," says Opher. She hopes this work will open the door to much more research on how the solar system has been influenced by external forces in the distant past.

Source: Boston University