Did the Solar System Pass Through a Giant Molecular Cloud Ten Million Years Ago?
In June 2024, I co-authored a Nature Astronomy paper with the brilliant Merav Opher and Josh Peek. The paper suggested that Earth might have passed through a dense cloud of interstellar gas a few million years ago. A passage through a cloud that is a few thousand times denser than the diffuse interstellar medium locally, would have dramatically compressed the heliosphere — the region filled by the Solar wind which is bounded by the pressure of the surrounding interstellar gas. Using a state-of-the-art computer simulation of the heliosphere, our paper showed that during such a passage, the heliosphere would shrink from its current extent of about 120 times the Earth-Sun separation to less than a quarter of the Earth-Sun separation.
The magnetic field carried by the Solar wind blocks energetic cosmic-rays from entering into the inner Solar system from their origin in the Milky-Way galaxy. The current magnetized heliosphere acts as a womb that protects the Earth from these Galactic cosmic-rays. This protection was witnessed by NASA’s Voyager 1 and 2 spacecraft, which measured a sharp increase by a factor of ~2 in the intensity of cosmic-rays as they crossed during the years 2018–2019 the heliopause — the transition boundary between the Solar wind and the interstellar medium.
The shrinkage of the heliopause to a scale smaller than the Earth’s orbit around the Sun implies that the Earth could have been temporarily exposed to an enhanced flux of cosmic-rays. Is there any evidence for such an effect?
In a new Nature Communication paper, a team led by Dominik Koll reported an anomalous 70% enhancement in the production rate of the isotope of beryllium-10 by cosmic-rays between 9 and 11.5 million years ago. The increase is recorded in several Central and Northern Pacific deep-ocean ferromanganese crusts. The new paper discusses possible origins of this temporal anomaly in the production rate of beryllium-10 in the context of geological, climatic, solar and astrophysical events, and favors an association with a compression event of the heliosphere by a dense interstellar cloud.
Nearby stellar explosions or supernovae are plausible sources of cosmic rays that could have enhanced the beryllium-10 production rate on Earth. However, their event rate is not high enough to account for a sufficiently nearby supernova that would yield the needed enhancement in cosmic-rays 10-million years ago. Collimated jets from Gamma-Ray Bursts would have produced a cosmic-ray pulse that is too short to account for the extended period of enhancement in beryllium-10 production.
The full-width at half-maximum of the beryllium-10 temporal peak is 1.4-million years. At the speed of the Solar system through the local standard-of-rest of the Milky-Way galaxy, 12 kilometers per second, the passage duration would imply a dense cloud with a diameter of order 50 light years. At the required density enhancement, the corresponding mass of the molecular gas would have been about a few hundred thousand solar masses. These are the characteristic size and mass of a giant molecular cloud. Such a cloud should have been located at a distance of about 400 light years, for the Solar system to cross it 10 million years ago.
In the original paper that I published last year with Merav Opher and Josh Peek, we considered the possibility of a more recent passage of the Solar system, merely 2–3 million years ago, through a smaller and closer cloud located within the so-called Local Ribbon of Cold Clouds. But like any other learning experience, science is done by iterations. Perhaps the actual cloud through which the Sun passed was bigger and farther away.
Molecular clouds are a common occurrence in the Milky-Way galaxy, representing the short-lived nurseries of young stars. Currently, the closest molecular cloud to the Sun is the Taurus Molecular Cloud, at a distance of 430 light years, similar to the distance inferred above. However, the typical lifetime of molecular clouds is of order 10 million years, as they are transient, barely gravitationally-bound structures that are easily dispersed by differential motions and the explosive feedback from stellar winds and supernovae. It is possible that the cloud through which the Sun plowed 10 million years ago is not around anymore. In that case, the remaining testimony for the passage is imprinted through the enhanced beryllium-10 abundance in the deep ocean floor.
When I visited my childhood home on the day of my mother’s funeral, I realized that it does not resemble the house I lived in as a young child. The street was much noisier than I remembered and the rooms were renovated beyond recognition by the new owner. It became apparent to me that the only way for me to go back to my childhood home is through photos and memories, because it was an entity that existed in both space and time and not just a physical entity in space. In a similar fashion, the only way for us to reconstruct the 10-million years old Galactic history of our home planet, the Earth, might be by studying the scars imprinted on its isotope abundances. After all, humans arrived on the scene just after this event and recorded human history is only 8,600 years old.
This raises the interesting question: is it possible that the emergence of humanity was triggered by the passage of the Solar system through a giant molecular cloud?
ABOUT THE AUTHOR
Avi Loeb is the head of the Galileo Project, founding director of Harvard University’s — Black Hole Initiative, director of the Institute for Theory and Computation at the Harvard-Smithsonian Center for Astrophysics, and the former chair of the astronomy department at Harvard University (2011–2020). He is a former member of the President’s Council of Advisors on Science and Technology and a former chair of the Board on Physics and Astronomy of the National Academies. He is the bestselling author of “Extraterrestrial: The First Sign of Intelligent Life Beyond Earth” and a co-author of the textbook “Life in the Cosmos”, both published in 2021. The paperback edition of his new book, titled “Interstellar”, was published in August 2024.
