Astrophysicists calculate the likelihood that Earth was exposed to cold harsh interstellar clouds 2 million years ago

Around two million years ago, Earth experienced significant changes, marked by the presence of early human ancestors and a diverse range of prehistoric creatures. During this period, the planet underwent several ice ages, which lasted until around 12,000 years ago. Scientists have attributed ice ages to various causes, including Earth's tilt and rotation, tectonic plate movements, volcanic eruptions, and levels of carbon dioxide in the atmosphere. However, a new study published in Nature Astronomy suggests that there may be another factor influencing Earth's history: the location of our sun in the galaxy.

Lead author Merav Opher, an astrophysicist and astronomy professor at Boston University, proposes that approximately two million years ago, our solar system encountered an unusually dense interstellar cloud. This cloud was so thick that it potentially interfered with the sun's solar wind, causing a significant impact on Earth. Opher's findings challenge the conventional understanding of Earth's environment, suggesting that the sun's position in space could shape our planet's history to a greater extent than previously thought.

Our solar system is enveloped by a protective plasma shield known as the heliosphere. This shield consists of charged particles emitted by the sun, called solar wind. It extends beyond the orbit of Pluto, creating a "giant bubble" that shields Earth and other planets from harmful radiation and cosmic rays. According to the study, the dense interstellar cloud compressed the heliosphere in a way that briefly placed Earth and other planets outside of its protective influence.

Opher's groundbreaking investigations into the heliosphere have revolutionized our understanding of its structure and interaction with the interstellar medium. She argues that the heliosphere resembles a puffy croissant, challenging previously held beliefs in the space physics community.

In her latest research endeavors, Opher delves into the potential effects of the heliosphere's shape and the sun's movement through space on Earth's atmospheric chemistry. The study involves reconstructing the positions of the sun and heliosphere from two million years ago using advanced computer models. Additionally, researchers track the path of the Local Ribbon of Cold Clouds, a series of substantial and extremely frigid hydrogen clouds situated beyond the heliosphere.

Computer simulations indicate that one of these clouds, specifically the Local Lynx of Cold Cloud, plausibly collided with the heliosphere. Had this impact occurred, Earth would have been exposed to the interstellar medium—a mixture of gas, dust, and remnants of exploded stars, including elements such as iron and plutonium. Normally, the heliosphere filters out the majority of these radioactive particles. However, without the heliosphere's protection, these particles could have reached Earth, which aligns with geological evidence from that time period, suggesting an increased presence of iron-60 and plutonium-244 isotopes in various locations.

Lead co-author Avi Loeb, director of Harvard University's Institute for Theory and Computation, emphasizes that encounters between our cosmic neighborhood and the solar system have rarely impacted life on Earth. Thus, the team's discovery of our cosmic passage through dense clouds millions of years ago brings forth a new understanding of the relationship between Earth's evolution and our galactic environment.

Depending on the size of the Local Lynx of Cold Cloud, it could have obstructed the heliosphere for a few hundred years to a million years. Ultimately, as our planet distanced itself from the cloud, the heliosphere resumed its protective influence over Earth and the other planets. While the precise effects of these cold clouds on Earth remain uncertain, it is probable that the sun had previously encountered other cold clouds throughout the billions of years since its formation, and will likely encounter more in the future.

Moving forward, Opher and collaborators plan to investigate the sun's situation seven million years ago and beyond, utilizing data from the European Space Agency's Gaia mission. This project aims to construct a comprehensive 3D map of the galaxy, offering insights into the movement of stars and facilitating precise determinations of the sun's historical position as well as the cold cloud system's location.

Opher considers this study just the beginning of a larger exploration into the influence of external forces on our solar system's past and their effects on our planet. She hopes that through further investigation, we can gain a deeper understanding of how outside factors have shaped Earth's history and influenced life as we know it.