Scientists explore Earth and skies to find clues about our planet’s climate history. Powerful and prolonged volcanic eruptions can change climate over long periods of time, and sunlight can change Earth’s climate over millions of years. What about interstellar hydrogen clouds? Could these regions of gas and dust change Earth’s climate if a planet crashes into them?

Interstellar clouds are not all the same. Some are scattered, some are much more intense. new research Nature Astronomy It suggests that our solar system may have passed through one of the dense clouds two to three million years ago. This effect can affect cloud formation and climate by changing the chemical composition of the Earth’s atmosphere.

The study is called “Possible direct impact of the cold, dense interstellar medium on Earth 2-3 million years ago.” The lead author is Merav Ofer of Harvard University’s Radcliffe Institute for Advanced Study and Boston University’s Department of Astronomy.

“Our results open a new window into the relationship between the evolution of life on Earth and our cosmic neighborhood.” – Avi Loeb, co-author, Institute for Theory and Computation, Harvard University

The Sun passes through a large gap in the interstellar medium (ISM) called the local bubble. Inside the LB, sunlight forms a cocoon called the heliosphere. It protects the solar system from cosmic radiation.

There is more to LB than the Sun. It also includes other stars and the Local Interstellar Cloud (LIC). The Sun has passed through LIC and will leave there after a few thousand years. LIC is not very dense.

However, over the last few million years, as the Sun passed through the local bubble, it encountered much denser clouds than the LIC. Researchers examined the effects of these collisions on the Sun’s ability to form a cocoon for the Solar System and what impact this had on Earth.

“Stars are moving, and now this paper shows that they are not only moving, but also undergoing dramatic changes.” – Merav Ofer, Professor of Astronomy, BU Faculty of Arts and Sciences

“Here we show that there are cold compact clouds in the ISM that the Sun has passed through in the last few million years, which could seriously affect the heliosphere. We investigate a scenario in which the Solar System passed through a cold gas cloud several million years ago,” Ofer and colleagues write.

Most of what the Sun passes through is thin ISM. The Sun moves continuously within the thin ISM without any impact.

“There are many of these clouds around the sun, but their density is too low to compress the heliosphere over any distance.

However, the denser clouds in the ISM are dense enough to significantly impact the protective heliosphere.

“The ISM near the Solar System also contains a few rare dense cold clouds called local cold cloud bands,” they write.

One of the clouds in this band is called the Leo Cold Cloud (LLCC). It is one of the largest clouds in the band and has been studied extensively by astronomers. They know its density and temperature. Researchers haven’t paid much attention to other clouds in the strip, but they expect them to be similar. The authors of this article say there is about a 1.3% chance that the Sun will pass through the tail of the LLCC.

“We call this part the Local Lynx of Cold Clouds (LxCC). “LxCCs account for almost half of the total LRCC mass and are larger than the better-studied LLCCs,” they write.

There have been questions about the nature of these clouds in the past.

“Note that these clouds are anomalous and unexplained structures in the ISM, and their origins and physics are poorly understood,” the authors write. Their study is based on the assumption that they have not changed significantly in the 2 million years since the supposed encounter.

“We assumed that these clouds have not changed significantly over the last 2 million years, but future studies may provide more information about their evolution.”

Researchers used simulations to study the effects of the dense cloud on the heliosphere and thus our planet. They say the cloud’s hydrogen density pushes the Sun away, making the heliosphere less compressed than Earth’s orbit around the Sun. The meeting was confirmed by the presence of the radioisotope 60Fe on Earth. 60Fe occurs mainly in supernovae and has a half-life of 2.6 million years.

Previous studies had linked 60Fe to a supernova explosion, where it was buried in dust and then sent to Earth. It is also available on the Moon. 244Pu was also delivered at the same time by a supernova eruption. Although there are many uncertainties, researchers say the accumulation of 60Fe on Earth is consistent with the hypothetical passage of our solar system through a dense cloud that compresses the protective heliosphere and allows isotopes to reach Earth.

“Our proposed scenario is consistent with geological evidence from 60Fe and 244Pu isotopes indicating that the Earth was in direct contact with the ISM during this period,” they write.

But if the supernova was sending out radioisotopes, it would have to be pretty close, and other evidence rules out a supernova source.

“A nearby supernova explosion contradicts the recent model for the formation of a local bubble,” the authors explain. “The scenario does not require 60Fe and 244Pu to be absorbed by dust particles specifically delivering them to Earth, as in the scenario of nearby supernova explosions.”

The question at the heart of this question is how does this affect the Earth?

An in-depth examination of these results is beyond the scope of this study. The team commented on some possibilities but also cautioned that little research has been done on the subject.

“Very few studies have quantitatively investigated the climatic consequences of such collisions in the context of collisions with dense giant molecular clouds. Some suggest that such a high density would deplete ozone in the central part of the atmosphere (50-100 km) and eventually cool the Earth,” they write.

It’s a leap, but some research suggests that this cooling may have contributed to the spread of our species.

“The hypothesis is that the emergence of our species, Homo sapiens, was shaped by the need to adapt to climate change. With the compression of the heliosphere, the Earth was exposed to the direct impact of the ISM,” they write.

In the conclusion, they remind that the probability of this meeting taking place is low. But it’s not zero.

“Stars are moving, and this paper shows not only that they are moving, but that they are undergoing dramatic changes,” said Ofer, a professor of astronomy in BU’s College of Arts and Sciences and a member of the university’s Center for Space Physics.