Three million years ago, a cosmic cloud left Earth exposed to interstellar space

Two to three million years ago, the solar system encountered galactic-scale turbulence and collided with dense interstellar clouds, potentially altering both the Earth's climate and evolution.

Only recently have researchers been able to map the Sun's orbit through the Galaxy, particularly in relation to the relatively dense hydrogen clouds that pass through the interstellar medium, the vast expanse of space between star systems.

the current, Merab Offer A research team from Boston University in Massachusetts has found evidence that one of these clouds, a “local cold cloud ribbon” in Lynx, likely intersects with the Sun's heliosphere.

The heliosphere is a protective cocoon or bubble formed by the solar wind pushing out to the edge of the solar system. Within the heliosphere, the planet is protected from the worst gamma radiation in the galaxy.

The new study proposes that as the solar system passed through the interstellar cloud, the heliosphere retreated from it and moved inward toward the Sun. The researchers think that the heliosphere may have shrunk so much that Earth was outside the protective cocoon provided by the solar wind, perhaps for around 10,000 years.

Merab and his colleagues used the European Space Agency's Gaia satellite to map the location of the dense, cold clouds and the sun's past orbit.

Ofer says the heliosphere's encounter with the cold cloud coincides with deposits of the elements plutonium-244 and radioactive iron-60 in Antarctic ice, deep-sea cores and lunar samples. These elements, which originated from distant supernovae, would have been captured in interstellar clouds and deposited while Earth was outside the heliosphere.

“There are signs of an increase in these elements over the past two years. [million] “The solar cloud record going back 3 million years provides compelling evidence that the Sun did in fact pass through it around 2 million years ago,” Offer says. “The exposure of Earth to a cloud of cold interstellar material and the associated increase in atmospheric hydrogen and radiation almost certainly had a major impact on Earth and its climate.”

Sarah Spitzer The University of Michigan researcher says the paper provides “compelling” evidence that the heliosphere was exposed to a much denser interstellar cloud two to three million years ago. As the solar system passed through that dense, cold cloud, Earth would have been outside the heliosphere and directly exposed to the interstellar environment, she says.

“Understanding this can teach us about the impact interstellar material has had on life on Earth in the past,” Spitzer says, “but it also helps us better understand the impact the heliosphere has on life on Earth today, what would happen if Earth were exposed to interstellar material again in the future, and when that might happen.”

Evan Economo Researchers from Japan's Okinawa Institute of Science and Technology say it's intriguing to consider how encounters in “our nearby space” could have influenced the environment experienced by life on Earth.

“The heliosphere is part of the extended environment experienced by life on the Earth's surface, influencing climate and radiation from space,” he says. “If we had been outside the heliosphere for a period of time, it could have altered the evolutionary trajectory of a wide range of life, including humans. Such connections are highly speculative at this point, but they provide us with new research directions.”