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Abstract Reconstructions of the paleoclimate indicate that ancient climatic fluctuations on Earth are often correlated with variations in its orbital elements. However, the chaos inherent in the solar system’s orbital evolution prevents numerical simulations from confidently predicting Earth’s past orbital evolution beyond 50–100 Myr. Gravitational interactions among the Sun’s planets and asteroids are believed to set this limiting time horizon, but most prior works approximate the solar system as an isolated system and neglect our surrounding Galaxy. Here we present simulations that include the Sun’s nearby stellar population, and we find that close-passing field stars alter our entire planetary system’s orbital evolution via their gravitational perturbations on the giant planets. This shortens the timespan over which Earth’s orbital evolution can be definitively known by a further ∼10%. In particular, in simulations that include an exceptionally close passage of the Sun-like star HD 7977 2.8 Myr ago, new sequences of Earth’s orbital evolution become possible in epochs before ∼50 Myr ago, which includes the Paleocene–Eocene Thermal Maximum. Thus, simulations predicting Earth’s past orbital evolution before ∼50 Myr ago must consider the additional uncertainty from passing stars, which can open new regimes of past orbital evolution not seen in previous modeling efforts. 

ABSTRACT Given the inexorable increase in the Sun’s luminosity, Earth will exit the habitable zone in ∼1 Gyr. There is a negligible chance that Earth’s orbit will change during that time through internal Solar System dynamics. However, there is a ∼ 1 per cent chance per Gyr that a star will pass within 100 au of the Sun. Here, we use N-body simulations to evaluate the possible evolutionary pathways of the planets under the perturbation from a close stellar passage. We find a ∼ 92 per cent chance that all eight planets will survive on orbits similar to their current ones if a star passes within 100 au of the Sun. Yet a passing star may disrupt the Solar System, by directly perturbing the planets’ orbits or by triggering a dynamical instability. Mercury is the most fragile, with a destruction rate (usually via collision with the Sun) higher than that of the four giant planets combined. The most probable destructive pathways for Earth are to undergo a giant impact (with the Moon or Venus) or to collide with the Sun. Each planet may find itself on a very different orbit than its present-day one, in some cases with high eccentricities or inclinations. There is a small chance that Earth could end up on a more distant (colder) orbit, through re-shuffling of the system’s orbital architecture, ejection into interstellar space (or into the Oort cloud), or capture by the passing star. We quantify plausible outcomes for the post-flyby Solar System.