Millions of years ago, Earth might have temporarily moved out of the sun’s protective shield, known as the heliosphere. This shield, depicted as a dark gray bubble against the backdrop of interstellar space, plays a crucial role in shielding our planet from harmful radiation and cosmic rays. According to recent research published in Nature Astronomy, this event could have exposed Earth to high levels of radiation, significantly impacting its climate. The study, led by Merav Opher, a space physicist from Boston University, suggests that about two million years ago, our solar system traveled through a dense interstellar cloud, potentially disrupting the sun’s solar wind and altering Earth’s environment.
The heliosphere is a protective bubble of plasma created by the sun’s solar wind, extending beyond Pluto and enveloping the planets. This shield is essential for protecting life on Earth from radiation that could alter DNA. However, Opher’s research proposes that a cold, dense interstellar cloud compressed the heliosphere, leaving Earth and other planets exposed to the interstellar medium for a brief period. This interstellar medium consists of gas and dust mixed with remnants of exploded stars, including elements like iron and plutonium.
Opher’s models have reshaped our understanding of the heliosphere, suggesting it is structured by the solar wind pushing against the interstellar medium—the space between stars in our galaxy. Her groundbreaking theory that the heliosphere is shaped like a puffy croissant has significantly impacted the field of space physics. Now, her research highlights how the heliosphere, along with the sun’s movement through space, could influence Earth’s atmosphere and climate. During her fellowship at Harvard’s Radcliffe Institute, Opher and her team used advanced computer models to visualize the sun’s position two million years ago, along with the rest of the solar system and the Local Ribbon of Cold Clouds—a group of large, dense, cold clouds mostly made of hydrogen. Their simulations indicate that one of these clouds, known as the Local Lynx of Cold Cloud, could have collided with the heliosphere.
If such a collision occurred, Earth would have been fully exposed to the interstellar medium. Normally, the heliosphere filters out most radioactive particles, but without its protection, these particles could have easily reached Earth. Geological evidence supports this theory, showing increased levels of iron-60 and plutonium-244 isotopes in ocean sediments, Antarctic snow, ice cores, and even on the moon from that time period. This exposure coincides with a cooling period recorded in temperature data, suggesting a potential link between the interstellar encounter and climatic changes on Earth.
While it’s challenging to determine the exact impact of the cold cloud on Earth, such as whether it triggered an ice age, Opher explains that the pressure from the Local Lynx of Cold Cloud could have blocked the heliosphere for hundreds of years to a million years, depending on the cloud’s size. Once Earth moved away from the cold cloud, the heliosphere would have re-enveloped the planets, including Earth, restoring its protective shield.
This study opens a new window into the relationship between the evolution of life on Earth and our cosmic environment. Avi Loeb, coauthor of the study and director of Harvard University’s Institute for Theory and Computation, emphasizes the significance of discovering that our passage through dense clouds a few million years ago could have exposed Earth to a much larger flux of cosmic rays and hydrogen atoms. This rare event suggests that our cosmic neighborhood beyond the solar system can indeed affect life on Earth, although such occurrences are infrequent.
Opher and her team are now working to trace the sun’s position seven million years ago and even further back. Identifying the location of the sun millions of years in the past, along with the cold cloud system, is possible using data from the European Space Agency’s Gaia mission, which is creating a 3D map of the galaxy and providing unprecedented insights into star movements. This ongoing research is supported by NASA, and Opher hopes it will lead to further exploration of how external forces have influenced our solar system throughout its history.
This is just the beginning, Opher says, expressing her hope that this study will encourage more research into the ways in which the solar system was influenced by external forces in the deep past, shedding light on the complex interplay between our planet and the broader cosmos.
SOURCES
Opher, M., Loeb, A. & Peek, J.E.G. A possible direct exposure of the Earth to the cold dense interstellar medium 2–3 Myr ago. Nat Astron (2024). doi.org/10.1038/s41550-024-02279-8
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