About two million years ago there was an encounter between the Sun and something outside the solar system that would have affected the Earth’s climate, new research concludes.
Scientists theorize that ice ages occur for several reasons, including the tilt and rotation of the planet, shifting tectonic plates, volcanic eruptions, and carbon dioxide levels in the atmosphere. But what if drastic changes like these are not just a result of Earth’s environment, but also the sun’s location in the galaxy?
In a new paper published in Nature Astronomy, lead author and astrophysicist Merav Opher, a professor of astronomy at Boston University and a member of 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 sun’s solar wind. Opher and his co-authors believe this shows that the Sun’s location in space could shape Earth’s history more than previously believed.
Our entire solar system is enveloped in a protective shield of plasma emanating from the Sun, known as the heliosphere. It’s made of a constant stream of charged particles, called the solar wind, that extend far beyond 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 is part of the reason life evolved on Earth as it did. According to the latest article, 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.
“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,” says Opher, who is an expert on the heliosphere.
Their models have 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. His theory is that the heliosphere is shaped like an inflated croissant, an idea that shook the space physics community.
Now, it’s shedding new light on how the heliosphere and where the sun moves through space could affect Earth’s atmospheric chemistry.
“Stars move and now this paper shows not only that they move, but that they encounter drastic changes,” says Opher. She discovered and began working on this study during a year-long fellowship at Harvard’s Radcliffe Institute.
To study this phenomenon, Opher and his 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 trajectory of the Local Cold Cloud Belt system, a chain of large, dense, very cold clouds made up primarily of hydrogen atoms.
Their simulations showed that one of the clouds near the end of that chain, called the Local Ribbon of Cold Clouds in the Lynx constellation, could have collided with the heliosphere.
If that had happened, Opher says, Earth would have been completely exposed to the interstellar medium, where gas and dust mix with atomic elements left over 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 coincides with geological evidence showing an increase in the isotopes 60Fe (iron 60) and 244Pu (plutonium 244) in the ocean, on the Moon, Antarctic snow and ice cores from the same time period. The timing also coincides with temperature records indicating a cooling period.
“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 Computing 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 flux 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.
It’s impossible to know the exact effect the cold clouds had on Earth, as if they might have spurred an ice age. But there are a couple more cold clouds in the interstellar medium that the Sun has probably encountered in the billions of years since it was born, Opher says. And it’s likely to encounter more in another million years or so.
