Effects of the Sun’s trajectory through the galaxy on Earth’s climate over the past 10 million years

Abstract: With the advent of the Gaia space mission, there has been a revolution in astronomers’ ability to precisely locate the interstellar structures the Sun may have encountered on its voyage around the galaxy. We now have the spatial resolution to trace the Sun’s trajectory back through its interstellar environment up to 60 million years in the past (4000 light-years in distance). This timescale is commensurate with the timescale over which we can reconstruct the paleoclimate of Earth from deep ocean foraminiferas. 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 with 19pc/Myr and has traversed many different structures in the interstellar medium that affect the heliosphere, at times collapsing it to sub-au scales. These periods of collapse introduce climate and radiation changes in Earth environment.
The frequency of such encounters could be as often as every couple of Myrs, 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 Myr ago (Mya), 6-7 Mya and 2-3Mya, for which the driving mechanisms are subject to ongoing debate. Such cooling events might be triggered by heliosphere collapse. I will discuss our recent work that show that in two occasions 3 and 7 million years ago (Mya), the Sun encountered a massive cloud that shrank the heliosphere —the solar cocoon protecting our solar system— exposing Earth to its interstellar environment, in agreement with geological evidence from 60Fe and 244Pu isotopes and discuss the implications for Earth’s climate and biodiversity.