The Fragility of the Terrestrial Planets During a Giant Planet Instability

TL;DR

The study investigates how a giant planet instability could have disrupted the orbits of terrestrial planets, suggesting such an event likely occurred before terrestrial planet formation was complete, challenging previous theories about the Late Heavy Bombardment.

Contribution

This work provides the first direct simulation-based analysis of terrestrial planet orbital excitation during giant planet instability, highlighting the low probability of current orbital configurations arising from such an event.

Findings

At least 85% chance of losing one or more terrestrial planets during instability.

Systems retaining all terrestrial planets often have higher than observed orbital eccentricities and inclinations.

Less than 5% chance of reproducing observed terrestrial planet orbits after instability.

Abstract

Many features of the outer solar system are replicated in numerical simulations if the giant planets undergo an orbital instability that ejects one or more ice giants. During this instability, Jupiter and Saturn's orbits diverge, crossing their 2:1 mean motion resonance (MMR), and this resonance-crossing can excite the terrestrial planet orbits. Using a large ensemble of simulations of this giant planet instability, we directly model the evolution of the terrestrial planet orbits during this process, paying special attention to systems that reproduce the basic features of the outer planets. In systems that retain four giant planets and finish with Jupiter and Saturn beyond their 2:1 MMR, we find at least an 85% probability that at least one terrestrial planet is lost. Moreover, systems that manage to retain all four terrestrial planets often finish with terrestrial planet eccentricities and inclinations larger than the observed ones. There is less than a ~5% chance that the terrestrial planet orbits will have a level of excitation comparable to the observed orbits. If we factor in the probability that the outer planetary orbits are well-replicated, we find a probability of 1% or less that the orbital architectures of the inner and outer planets are simultaneously reproduced in the same system. These small probabilities raise the prospect that the giant planet instability occurred before the terrestrial planets had formed. This scenario implies that the giant planet instability is not the source of the Late Heavy Bombardment and that terrestrial planet formation finished with the giant planets in their modern configuration.