Two Million Years Ago, an Encounter between the Sun and Something Outside the Solar System Altered All Life on Earth

New research from Boston University and the Harvard Radcliffe Institute suggests that the Sun passed through a huge hydrogen cloud two million years ago, directly affecting its protective heliosphere and may have permanently altered all life on Earth. Evidence for this event and its effects on Earth’s climate also shows that the Sun’s location in the galaxy can have a direct effect on the plants and animals living on Earth.

“This paper is the first to quantitatively show there was an encounter between the sun and something outside of the solar system that would have affected Earth’s climate,” said astrophysicist Merav Opher, an astronomy professor at Boston University and fellow at Harvard Radcliffe Institute in the press release announcing the team’s study.

Although climate change is currently a politically charged topic, plants and animals living on Earth have experienced repeated and dramatic swings in their environment for millions of years. Historically, events like volcanic eruptions, shifting plate tectonics, shifts in atmospheric carbon dioxide levels, and even the tilt and rotation of the Earth have all likely contributed to those extreme climatological shifts.

As an astrophysicist who specializes in studying the Sun’s heliosphere, which is a halo of stellar wind made up of highly charged particles that encompasses and protects the rest of the solar system from harmful interstellar radiation and galactic rays, Opher speculated that if the Sun were to come into contact with one of the large hydrogen bubbles floating around our galaxy, it also might have a direct and dramatic effect on the planet’s climate. Now, she believes her team’s study supports that theory. Adding yet another factor to the ever-evolving picture of Earth’s climate.

“Stars move,” said Opher, “and now this paper is showing not only that they move, but they encounter drastic changes.”

Astrophysics ‘Time Travel’ Reveals Sun’s Position Two Million Years Ago

To test the team’s theory, Opher, whose previous work suggesting that the heliosphere is shaped like a puffy croissant purportedly “shook the space physics community,” decided to use computer models to track the Sun’s motion through the galaxy. By turning the astronomical clock dial back in time, Opher came across an event that occurred around two million years ago.

In their study published in the journal Nature Astronomy, Opher and colleagues explain how a string of large, dense interstellar clouds made up of extremely cold hydrogen atoms called the Local Ribbon of Cold Clouds system was rather precariously positioned. When checking closer, the simulators ‘astrophysical way-back machine’ showed that one of the clouds, the Local Lynx of Cold Cloud, may have collided with the heliosphere due to its position near the end of one particular ribbon of the Local Ribbon of Cold Clouds.

two million years ago
For a brief period of time two million years ago, Earth may have been plunged out of the sun’s protective plasma shield, called the heliosphere, which is depicted here as the dark gray bubble over the backdrop of interstellar space. According to new research, this could have exposed Earth to high levels of radiation and influenced the climate (Credit: Opher et al., Nature Astronomy).

According to Opher, if such a collision did occur, the team’s simulations indicate it could have altered the heliosphere enough to temporarily deprive Earth of its critical protections. This means that all life on Earth would not only have been dosed with incredibly powerful levels of interstellar radiation and cosmic rays (which can alter DNA) but also been exposed to dangerous amounts of iron and plutonium atoms left over from exploded stars that exist in measurable concentrations within the interstellar medium.

“This cloud was indeed in our past, and if we crossed something that massive, we were exposed to the interstellar medium,” Opher says.

Supporting this idea is geological evidence showing unusually high concentrations of 60Fe (iron 60) and 244Pu (plutonium 244) isotopes in the ocean, on the moon, buried in Antarctic snow, and collected from ice cores from the same time period two million years ago. While no direct research has been done to determine what the collective effects of an extended removal of the heliosphere’s protection and all of this exposure might be, the authors of the paper outlining this possibility believe it may have had a direct effect on everything living on the Earth.

“Only rarely does our cosmic neighborhood beyond the solar system affect life on Earth,” says Avi Loeb, director of Harvard University’s Institute for Theory and Computation and coauthor on the paper. “It is exciting to discover that our passage through dense clouds a few million years ago could have exposed the Earth to a much larger flux of cosmic rays and hydrogen atoms.” Loeb is also a regular contributor to The Debrief.

Next Steps and Other Potential Cosmic Collisions

While there is no direct way to travel back in time to see how the Sun colliding with an interstellar cloud of cold hydrogen atoms may have affected our ancient human ancestors or the plants and animals living alongside them, one theory suggests a large enough event could have triggered one of the many ice ages the Earth has encountered in its lifetime. In fact, given the over four billion years since our Solar System’s creation, Opher believes it is likely the Sun has encountered many similar events in the past, all of which could have affected life in a number of different and unpredictable ways.

“And it will likely stumble across more in another million years or so,” the press release announcing the study explains.

Next, Opher and her team at the university’s NASA-funded SHIELD (Solar Wind with Hydrogen Ion Exchange and Large-scale Dynamics) DRIVE Science Center will begin exploring how such a massive collision between the sun and an interstellar cloud of hydrogen atoms might have affected Earth’s atmosphere, and climate, but also it’s radiation.

After that, the researchers plan to trace the sun’s position back to seven million years ago. If their assumptions are correct, data collected by the European Space Agency’s Gaia mission, which is building the largest ever three-dimensional map of the galaxy and also tracing how fast stars move, should not only help them pinpoint the Sun’s location within the galaxy but also pinpoint where the ribbons of the cold cloud system were as well. If the two coincide with any known climatic events in Earth’s past, that could add even more evidence to the team’s theory.

“This is only the beginning,” Opher says.

Moving forward, the astrophysicist says she hopes that this paper will open the door to much more exploration of how the solar system was influenced by outside forces in the deep past and how these forces have, in turn, shaped life on our planet. Loeb agrees.

“Our results open a new window into the relationship between the evolution of life on Earth and our cosmic neighborhood,” he said.

Christopher Plain is a Science Fiction and Fantasy novelist and Head Science Writer at The Debrief. Follow and connect with him on X, learn about his books at, or email him directly at