Cold, dense clouds in the interstellar medium of our Milky Way Galaxy are around four-five orders of magnitude denser than their diffuse counterparts.
A team of astronomers from Boston University, Harvard University and Johns Hopkins University has now found evidence that between two and three million years ago, our Solar System encountered one of these dense clouds, which was so dense that it could have interfered with the solar wind. Most stars generate winds and move through the interstellar medium that surrounds them. This motion creates a cocoon that protects planets from the interstellar medium. The Sun's cocoon is the heliosphere. It's made from a constant flow of charged particles, called solar wind, that stretch well past Pluto, wrapping the planets in what astronomers call the Local Bubble. It protects us from radiation and galactic rays that could alter DNA, and scientists believe it's part of the reason life evolved on Earth as it did. According to the new study, the cold interstellar cloud compressed the heliosphere in such a way that it briefly placed Earth and the other planets in the Solar System outside of the heliosphere's influence. "Our 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 Boston University's Professor Merav Opher. "Stars move, and now this paper is showing not only that they move, but they encounter drastic changes." To study this phenomenon, Professor Opher and colleagues essentially looked back in time, using sophisticated computer models to visualize where the sun was positioned two million years…