Around two million years ago, Earth was a very different place, with our early human ancestors living alongside saber-toothed tigers, mastodons, and enormous rodents. And, depending on where they were, they may have been cold: Earth had fallen into a deep freeze, with multiple ice ages coming and going until about 12,000 years ago.

Around two million years ago, Earth was a very different place, with our early human ancestors living alongside saber-toothed tigers, mastodons, and enormous rodents. And, depending on where they were, they may have been cold: Earth had fallen into a deep freeze, with multiple ice ages coming and going until about 12,000 years ago.

This 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.

Normally, the heliosphere filters out most of these radioactive particles. But without protection, they can easily reach Earth. According to the paper, this aligns with geological evidence that shows increased 60Fe (iron 60) and 244Pu (plutonium 244) isotopes in the ocean, on the moon, Antarctic snow, and ice cores from the same time period. The timing also matches with temperature records that indicate a cooling period.