Scientists believe that Earth may have briefly lost its protection from the Sun two million years ago and was left to endure in the extreme environment of interstellar space as the Solar System passed through a dense cloud of gas and dust between the stars.
At that time, early human ancestors shared our planet with prehistoric animals such as mastodons and saber-toothed tigers. It’s also when Earth was in the middle of an ice age that only ended about 12,000 years ago. Ice ages are caused by a number of factors, including the tilt and rotation of our planet, the level of carbon dioxide in its atmosphere, and the shifting plate tectonics and volcanic eruptions on its surface. But given the timing of when scientists think Earth plunged through interstellar space, this research suggests that radical changes in our planet’s climate, such as the onset and end of ice ages, could also be influenced by the position of our solar system in our home galaxy.
More specifically, the team behind the new findings suggest that the Solar System may have encountered a dense patch of interstellar gas and dust as it passed through the Milky Way two million years ago. And that spot may have been thick enough to interfere with a stream of charged particles called the “solar wind” that flows from the Sun and hits Earth, which can cause temperatures to drop.
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“This paper is the first to quantitatively show that there has been a collision between the Sun and something outside the Solar System that would have affected Earth’s climate,” said lead study author Merav Opher, a space physicist and heliosphere expert at Boston University. in the statement.
Solar system, back in time
Our entire solar system is enveloped in a “giant bubble” of protective plasma that originates from the sun, which is known as the “heliosphere”. This protective shield is created when solar winds push on the interstellar medium, which refers to the material that drifts through the spaces between stars in the Milky Way. The heliosphere is constantly renewed by the constant flow of charged particles from the Sun that flow around Pluto.
The heliosphere protects the Earth’s surface from radiation and galactic rays that could potentially affect the DNA of living things. This shielding is so fundamental that many scientists believe it was integral to the origin and development of life on Earth.
The team thinks it’s possible that a cold cloud of interstellar material may have once held back the solar wind in such a way that the heliosphere was compressed. This may have briefly (cosmically speaking) removed Earth and the other planets of the solar system from the protection offered by the heliosphere.
“The stars are moving, and this paper now shows not only that they are moving, but that they are experiencing drastic changes,” Opher added.
To determine what effect on Earth such a bombardment of dense interstellar dust would have on the heliosphere, Opher set the clock using sophisticated computer models. This allowed her and the team to visualize the position of the Sun two million years ago, as well as determine where the heliosphere and the rest of the Solar System were at that time.
They also tracked the progression of a string of dense, cold gas called the “local cold cloud ribbon system” back in time as it swept through the Milky Way. This revealed that one dense cloud at the end of the Local Ribbon of Cold Clouds system, called the “Local Cold Cloud Feature”, may have collided with the heliosphere.
Earth would thus be exposed to the interstellar medium, including the heavy and radioactive elements that inhabit it, the remnants of massive stars that perish in supernova explosions. The heliosphere usually blocks these particles—without this shielding, these radioactive elements could rain down on Earth. This could explain the increase in iron 60 and plutonium 244 isotopes found in Antarctic snow and ice cores and on the Moon that correspond to the period two million years ago. The timing of the collision of the heliosphere with the Cold Cloud Local Feature also corresponds to a cooling period on Earth two million years ago.
Opher theorizes that pressure from the Cold Cloud Local Feature may have confined the heliosphere to periods of only a few hundred years or as long as a million years. That would all depend on how big the cloud actually was, Opher said. She added that the heliosphere would grow back to surround the planets after the influence of this dense cloud dissipated.
However, at this point it is difficult to determine exactly what impact this cold interstellar cloud would have had on Earth, including whether it could have actually triggered an ice age. The team will now probe even further back in time to see when the Solar System intersected with dense interstellar clouds, and to see if these are also consistent with ice ages.
She and her team are now also investigating the effect of so much hydrogen and radioactive material crossing Earth’s atmosphere.
“This cloud was indeed in our past, and if we crossed something that massive, we were exposed to the interstellar medium,” Opher said. “This is just the beginning.”
The team’s research was published Monday (June 10) in Nature Astronomy.