Understanding the Heliospheric Shield: Laying the Groundwork to Predict Habitable Astrospheres

This article reviews today's open issues, frontiers of research, and advances in our understanding of the heliosphere throughout the Sun's last 10 million years. Today's heliosphere, the cocoon formed by the solar wind as it moves through the Galaxy, engulfs all the planets extending in the nose direction to ∼120 astronomical units (AU). ▪ The Sun moves at 19 pc/Myr and has traversed many different structures in the interstellar medium that affect the heliosphere, at times collapsing it to sub-astronomical-unit scales. These periods of collapse introduce climate and radiation changes in Earth's environment. ▪ The frequency of such encounters could be as often as every couple of million years, making it a major external disturbance to the development of life on Earth. ▪ Stepwise shifts in past global climate, seen in deep-sea sediment cores, indicate intervals of more rapid cooling at 13–14 Mya, 6–7 Mya, and 2–3 Mya, for which the driving mechanisms are subject to ongoing debate. Such cooling events might be triggered by heliosphere collapse.