A Resonant Beginning for the Solar System Terrestrial Planets

TL;DR

This paper investigates whether the early terrestrial planets formed in a resonance chain can evolve into the current configuration, using simulations that show the giant planet instability can trigger impacts and produce present-day orbital characteristics.

Contribution

It demonstrates through N-body simulations that the giant planet instability can destabilize a primordial terrestrial resonance chain, leading to impacts and current orbital configurations.

Findings

Giant planet instability triggers moon-forming impacts in 20-50% of systems.

Post-instability, planets' eccentricities and inclinations match current observations.

The current Mars-Venus period ratio can be a relic of the original resonance chain.

Abstract

In the past two decades, transit surveys have revealed a class of planets with thick atmospheres -- sub-Neptunes -- that must have completed their accretion in protoplanet disks. When planets form in the gaseous disk, the gravitational interaction with the disk gas drives their migration and results in the trapping of neighboring planets in mean motion resonances, though these resonances can later be broken when the damping effects of disk gas or planetesimals wane. It is widely accepted that the outer Solar System gas giant planets originally formed in a resonant chain, which was later disrupted by dynamical instabilities. Here, we explore whether the early formation of the terrestrial planets in a resonance chain (including Theia) can evolve to the present configuration. Using N-body simulations, we demonstrate that the giant planet instability would also have destabilized the terrestrial resonance chain, triggering moon-forming giant impacts in 20--50\% of our simulated systems, dependent on the initial resonance architecture. After the instability, the eccentricity and inclination of the simulated planets match their present-day values. Under the proposed scenario, the current period ratio of 3.05 between Mars and Venus -- devoid of any special significance in traditional late formation models -- naturally arises as a relic of the former resonance chain.