Two million years ago, the solar system encountered a dense interstellar cloud that may have had a profound effect on Earth’s climate by compressing the heliosphere and exposing Earth to high levels of cosmic radiation and galactic rays. (Artist’s concept) Credit: SciTechDaily.com
New astrophysical research has uncovered a significant cosmic event that occurred two million years ago when our solar system passed through a dense interstellar cloud, exposing Earth to increased cosmic radiation and potentially altering its climate, as supported by increased isotopes found in the geological record.
About 2 million years ago, Earth was a very different place. Our human ancestors coexisted with sabre-toothed cats, mastodons, and giant rodents. It may have also been cold in places. The planet was frigid and experienced multiple ice ages up until about 12,000 years ago. Scientists theorize that ice ages occurred for a variety of reasons, including the Earth’s tilt and rotation, changes in plate tectonics, volcanic eruptions, and atmospheric carbon dioxide levels.
But what if these dramatic changes are caused not just by Earth’s environment, but also by the Sun’s position in the galaxy?
Effects of the Sun’s Galactic Journey
In a new paper published today (June 10) in Nature Astronomy, lead author astrophysicist Merab Ofer, a professor of astronomy at Boston University and a fellow at Harvard’s Radcliffe Institute, finds evidence that about 2 million years ago, our solar system encountered an interstellar cloud dense enough to disrupt the solar wind. Ofer and his coauthors think this indicates that the sun’s position in space may have influenced Earth’s history more than previously thought.
The role of the heliosphere in protecting the Earth
The entire solar system is enveloped in a protective plasma shield that radiates from the Sun, the so-called heliosphere. Made from a constant stream of charged particles called the solar wind, the heliosphere extends far beyond Pluto, enveloping the planet in what NASA calls a “giant bubble.” This bubble protects us from radiation and galactic rays that can alter our DNA, and scientists think this is part of the reason why life on Earth evolved.
A new paper suggests that the cold cloud compressed the heliosphere, temporarily placing Earth and the other planets in the solar system outside of its influence.
The impact of galactic encounters on Earth
“This paper is the first to quantitatively show that there was an encounter between the Sun and something outside the solar system that affected Earth’s climate,” said Ofer, an expert on the heliosphere.
Her models have literally shaped scientific understanding of the heliosphere and how bubbles form as the solar wind pushes up interstellar material (the space between the stars in our galaxy and outside the heliosphere). In her theory, the heliosphere is shaped like a fluffy croissant, an idea that shook the astrophysics community. Now she is shedding new light on how the heliosphere and where the Sun moves through space affect the chemistry of Earth’s atmosphere.
“Stars move, and this paper shows that not only do they move, but they also undergo dramatic changes,” says Ofer, who first discovered and began her research during a year-long research fellowship at Harvard’s Radcliffe Institute.
Simulation insights into cosmic interactions
To study this phenomenon, Ofer and his collaborators essentially went back in time and used advanced computer models to visualize where the sun was located two million years ago, along with the heliosphere and the rest of the solar system.
They also mapped the path of the Local Cold Cloud Ribbon System, a string of large, dense, and very cold clouds made mainly of atomic hydrogen. Their simulations showed that one of the clouds near the edge of the ribbon, the Local Cold Cloud Lynx, may have collided with the heliosphere.
Geological and cosmic evidence
If that had happened, Ofer says, Earth would have been fully exposed to interstellar matter, a mixture of gas and dust mixed with atomic elements left over from exploded stars, such as iron and plutonium. The heliosphere normally filters out most of these radioactive particles, but without protection, they could easily reach Earth.
This coincides with geological evidence showing increased levels of the isotopes 60Fe (iron-60) and 244Pu (plutonium-244) in the oceans, the Moon, Antarctic snow and ice cores from the same period, according to the paper, and also with temperature records indicating a cooler period.
Long-term galactic influences
“It’s rare for our cosmic neighbors outside our solar system to affect life on Earth,” said study co-author Avi Loeb, director of Harvard University’s Institute for Theoretical Computation. “It’s exciting to discover that our passage through a dense cloud millions of years ago could have exposed Earth to much larger amounts of cosmic rays and atomic hydrogen. Our findings open a new window into the evolution of life on Earth and its relationship to our cosmic neighbors.”
According to Offer, external pressure from the localized lynx of the cold cloud could have continuously blocked the heliosphere for hundreds to millions of years, depending on the size of the cloud. “But as soon as Earth moved away from the cold cloud, the heliosphere swallowed up all the planets, including Earth,” Offer said. And that’s the situation today.
Future research and its implications
It’s impossible to know exactly how cold clouds might have affected Earth, such as whether they caused ice ages, but Ofer says there are some cold clouds in the interstellar medium that the sun likely encountered in its first billion years, and we’ll probably encounter more in the next million years or so.
Ofer and his colleagues are currently working to trace where the sun was 7 million years ago and beyond. Pinpointing the sun’s position and cold cloud systems millions of years ago will be made possible by data collected by the European Space Agency’s Gaia mission, which has produced the largest 3-D map of the galaxy ever, showing in unprecedented detail how fast stars move.
Exploring the Sun’s past orbit
“This cloud certainly existed in the past, and if it passed through something this large, it would have been exposed to interstellar material,” Offer says. The effects of intersecting with so much hydrogen and radioactive material are unknown, so Offer and her team at the NASA-funded SHIELD (Solar Wind with Hydrogen Ion Exchange and Large-Scale Dynamics) DRIVE Science Center at Boston University are currently investigating its possible effects on Earth’s radiation, atmosphere, and climate.
“This is just the beginning,” says Ofer, who hopes the paper will inspire deeper investigations into how the solar system was influenced by outside forces in the ancient past, and how these forces may in turn have shaped life on Earth.
Reference: “Earth may have been directly exposed to cold, dense interstellar material 2-3 million years ago” by Merav Opher, Abraham Loeb and JEG Peek, 10 June 2024, Nature Astronomy.
Publication date: 10.1038/s41550-024-02279-8
This research was supported by NASA.
