Earth’s weather changes based on where it is in its annual cycle around the Sun – but is it possible the climate can also be affected by the solar system’s position in its orbit around the center of the Milky Way? A new study suggests an ice age period about two million years ago may have been triggered by a kind of interstellar winter.
Much like Earth’s magnetic field protects the surface from dangerous radiation from the Sun, the Sun actually protects Earth and the other planets from interstellar cosmic rays. The solar wind creates a kind of bubble called the heliosphere, which extends 130 Astronomical Units (AU) from the Sun – for reference, Earth is 1 AU away, and even Pluto is less than 40 AU out.
But a new study suggests that a few million years ago, the heliosphere shrank to a fraction of its current size. During that time the Sun’s influence extended out to just 0.22 AU, or about halfway to Mercury – and Earth’s exposure to the interstellar medium might have plunged the planet into a deep freeze.
What could overpower the Sun so dramatically? Cold clouds of gas drifting around the galaxy have been calculated to be at least 10,000 times denser than the usual interstellar medium, and if the solar system passed through one that pressure could theoretically shrink the heliosphere. So for the new study, scientists at Harvard and Boston University investigated the possibility.
Just like everything in the solar system orbits the Sun, the contents of the galaxy revolve around the supermassive black hole at the center. It takes about 230 million years for the solar system to do a full lap – so this time one galactic year ago, dinosaurs were just starting to take off.
By tracing the path of the Sun over the last few million years, the team found that we may have passed through a structure known as the Local Ribbon of Cold Clouds (LRCC) around two to three million years ago. In particular, the Local Lynx of Cold Cloud, near the end of the ribbon, could have collided with the solar system.
The scientists simulated this scenario, and found the cloud could have blocked the heliosphere for anywhere from a few centuries to a million years, leaving Earth fully exposed to particles in the cloud during that time.
And sure enough, the researchers say this time frame lines up with geological evidence of increased levels of iron 60 and plutonium 244 isotopes, found in samples from the ocean, Antarctic snow, ice cores and even on the Moon. Temperature records also indicate that this coincides with the beginning of the Quaternary glaciation period, a long-term ice age that began around 2.5 million years ago.
“Only rarely does our cosmic neighborhood beyond the solar system affect life on Earth,” said Avi Loeb, co-author of the study. “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. Our results open a new window into the relationship between the evolution of life on Earth and our cosmic neighborhood.”
Other studies have linked Earth cycles with events happening on a galactic scale. One for instance suggests that continents form faster when the solar system enters or exits the spiral arms of the Milky Way.
While it’s hard to say for certain that a cosmic cloud caused an ice age, the evidence is intriguing. Even if it did, it was likely just one of many factors that can influence climate, such as volcanic eruptions, plate tectonics, solar cycles, and changes in the shape of the Earth's orbit. Next, the team plans to track the Sun’s path back even farther.
The research was published in the journal Nature Astronomy.
Source: Boston University
