Passing Stars and the Orbital Evolution of Our Planets

For centuries, scientists have debated how our planets’ orbits evolve over long timescales: whether this evolution is chaotic, how orbital evolution impacts Earth’s climate, and if the planets’ orbits are stable forever (e.g. Newton, 1730; Milankovitch, 1941). During the last 4 decades, computer N-body simulations have enabled major advances across all of these topics. It is now known that our planets’ orbital evolution is chaotic and impossible to predict beyond ~60-Myr timescales, that there is a ~1% chance that Mercury will be lost before the Sun leaves the main sequence, and that long-term variations in Earth’s sedimentation records (and, by extension, climate) correlate with the Earth’s past orbital evolution predicted in backwards-run simulations (Lourens et al., 2005; Laskar & Gastineau, 2009; Laskar et al., 2011).

However, previous simulations have nearly always approximated the solar system as completely isolated, neglecting any influences from the Sun’s local galactic environment. We recently relaxed this assumption and discovered that passing field stars are a significant accelerator of the chaos governing Earth’s orbital evolution, and they further limit how precisely Earth’s past orbital history can be known (Kaib & Raymond 2024). For many other aspects of our solar system’s orbital evolution, though, the dynamical consequences of passing stars still remain unknown but could also be very important. We will use well-established N-body simulation techniques to fully characterize the dynamical influence of nearby passing stars, a potentially critical ingredient missing from most past studies of our planets’ orbital evolution. This will be accomplished with the following three tasks:

1. We will run 5-Gyr simulations of the planets’ orbital evolution while they are perturbed by passing field stars to understand how the inclusion of stars alters the solar system’s stability and the chaotic diffusion of planetary orbits.

2. We will perform many stellar passage experiments revealing the relationship between specific passage parameters and the changes expected to each planet’s orbit.

3. We will exploit newly discovered anisotropies in the distribution of long-period comet orbits (Kaib, 2022) to better constrain the strength of the Sun’s past encounter with HD 7977, the only known recent stellar encounter with the potential to alter simulations’ predictions of the past 50-60 Myrs of Earth’s orbital evolution (Bailer-Jones, 2022; Kaib & Raymond, 2024).

To fully model the planets’ long-term dynamical evolution, the effects of stellar encounters must be understood, and this proposed work will do this. We expect to greatly improve our understanding of the solar system’s stability, the range of dynamical states through which the Earth and other planets will pass or have passed, and the reliability with which simulations of Earth’s past orbital evolution can aid in interpreting our geological record.