2024ApJ...972..201O

The Sun moves through the interstellar medium (ISM) at a velocity of ∼19 pc Myr^–1, making the conditions outside the solar system vary with time over millions of years. Today's solar system is protected from interstellar particles by the heliosphere, the bubble formed by the solar wind as the Sun moves through the ISM, which engulfs the planets. There is geological evidence from ⁶⁰Fe that Earth was in direct contact with the ISM 2-3 and 5-7 million years ago (MYA). Recent work argues that the Sun encountered a massive cold cloud 2 MYA as part of the Local Ribbon of Cold Clouds that shrunk the heliosphere and exposed Earth to the ISM. Here, we consider the effects of the passage of the Sun through the edge of the Local Bubble occurring at ${6.8}_{-0.4}^{+0.5}$ MYA assuming that the Sun encountered a cloud with a density of 900 cm^–3. If we consider additional turbulent motion within the cloud due to shocks, the density encountered can be as low as 283 cm^–3. Clouds of this density cover a small but nonzero (≲4.6%) fraction of the surface of the Local Bubble, making an encounter plausible. Using a state-of-the art magnetohydrodynamic model, we show that the heliosphere shrank to a scale smaller than Earth's orbit, thereby exposing Earth to cold dense ISM, consistent with ⁶⁰Fe evidence. The timing of the event matches perturbations observed in the paleoclimate record recovered from deep-sea sediments. The passage through the Local Bubble's surface and contraction of the heliosphere therefore may have impacted the climate and biosphere significantly, suggesting a new driver of major events in Earth's history.