Around two million years ago, Earth was a very different place. Our early human ancestors lived alongside animals like saber-toothed tigers and mastodons.
The planet experienced multiple ice ages, with long periods of cold temperatures that came and went until about 12,000 years ago.
Scientists have various theories about why ice ages happen. They believe factors like the tilt and rotation of Earth, moving tectonic plates, volcanic eruptions, and carbon dioxide levels in the atmosphere play a role.
But now, they are considering another possibility: the sun’s location in the galaxy.
A new study published in Nature Astronomy by astrophysicist Merav Opher from Boston University suggests that two million years ago, our solar system passed through a dense interstellar cloud.
This cloud might have affected the sun’s solar wind and, as a result, Earth’s climate.
The solar system is surrounded by a protective bubble called the heliosphere, created by the solar wind— a constant flow of charged particles from the sun. The heliosphere shields us from harmful radiation and cosmic rays that could damage DNA. Scientists think this protection helped life evolve on Earth.
Opher and her team used computer models to see where the sun and the heliosphere were positioned two million years ago. They mapped the path of the Local Ribbon of Cold Clouds, a series of large, cold clouds made mostly of hydrogen. Their models showed that one of these clouds, called the Local Lynx of Cold Cloud, might have collided with the heliosphere.
If this collision happened, Earth would have been exposed to the interstellar medium, a mix of gas and dust from exploded stars. Normally, the heliosphere filters out most of these particles, but without it, they could reach Earth. Geological evidence supports this idea, showing increased levels of certain isotopes like iron-60 and plutonium-244 in ocean sediments, Antarctic snow, and moon samples from that time period. These isotopes suggest that Earth experienced higher levels of cosmic rays and other particles.
This exposure might have contributed to the cooling period that led to ice ages. The cold cloud could have blocked the heliosphere for hundreds of thousands of years. Once the Earth moved away from the cloud, the heliosphere would have reformed, protecting the planets again.
While it’s hard to know the exact effects of these cold clouds on Earth, Opher’s research opens a new window into how our planet’s climate could be influenced by the sun’s journey through the galaxy. Opher and her team are now trying to trace the sun’s path even further back in time using data from the European Space Agency’s Gaia mission, which is mapping the galaxy in 3D.
“This is just the beginning,” Opher says. She hopes this study will lead to more research on how interstellar clouds have influenced Earth’s history and the evolution of life. Understanding these cosmic interactions could help us better comprehend the forces that shaped our planet and continue to affect it today.
