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(Image credit: JWST/NASA)
In June 2024, Avi Loeb co-authored a groundbreaking paper in Nature Astronomy with Merav Opher and Josh Peek. Their research suggested that Earth might have encountered a dense cloud of interstellar gas several million years ago. This cloud, estimated to be thousands of times denser than the local interstellar medium, would have significantly compressed the heliosphere—the protective bubble formed by the Solar Wind, which shields the inner Solar System from cosmic radiation.
The Impact on the Heliosphere
Normally, the heliosphere extends about 120 times the distance from the Earth to the Sun, protecting us from energetic cosmic rays. However, the simulations in their study indicated that if Earth passed through such a dense cloud, the heliosphere could have shrunk to less than a quarter of its usual size. This compression would have exposed the inner Solar System, including Earth, to a significantly higher flux of cosmic rays.
Evidence from Beryllium-10 Production
A study by Dominik Koll and colleagues, published in Nature Communications, offers compelling evidence for this theory. They detected an unusual 70% increase in beryllium-10 production between 9 and 11.5 million years ago, recorded in several deep-ocean ferromanganese crusts in the Central and Northern Pacific. Beryllium-10 is produced when cosmic rays collide with cosmic dust particles in the atmosphere, and this significant spike correlates with the proposed compression of the heliosphere.
Ruling Out Other Possible Causes
Nearby supernovae or gamma-ray bursts, while capable of producing cosmic rays, are not frequent enough or occur in a manner to account for the prolonged increase in beryllium-10 over the 2.5 million-year period. These sources either produce too short-lived pulses or do not occur at a high enough rate. Instead, the authors suggest that a passage through a dense molecular cloud aligns best with the data.
The Size and Characteristics of the Cloud
Assuming the Solar System passed through a dense cloud, the duration of the enhanced beryllium-10 production indicates that the cloud must have been about 50 light-years in diameter and contained a few hundred thousand solar masses of molecular gas. This aligns with the characteristics of a giant molecular cloud, common structures in the Milky Way galaxy where new stars are born.
At the Solar System’s speed through the local standard of rest in the Milky Way (about 12 kilometers per second), the duration of the cloud passage implies a cloud located approximately 400 light years away, consistent with the concept discussed by the researchers.
Implications for Human History
While the exact effects of this cosmic event on Earth’s ecosystems and ancient life remain unclear, Avi Loeb posits an intriguing possibility: could the exposure to increased cosmic rays have influenced the emergence of humanity? Given that recorded human history is only about 8,600 years old, the impact might be discernible only through geological and isotopic records.
The metaphorical departure from childhood home, where past experiences are remembered through photos and memories, is akin to reconstructing Earth’s ancient history. The only tangible remnants of this ancient cosmic event are the imprints on the isotope abundances in the deep ocean.