Long-term instability of the inner Solar System: numerical experiments

TL;DR

This study uses numerical experiments to analyze the long-term stability of the inner Solar System, revealing that truncations of the Hamiltonian at degree 4 show remarkable stability over 5 billion years, with instability mainly arising from degree 6 terms.

Contribution

The paper demonstrates that the inner Solar System's stability over billions of years can be explained by Hamiltonian truncation analysis, highlighting the role of higher-degree terms in destabilization.

Findings

Degree 4 Hamiltonian truncation shows stability over 5 Gyr.

Instability mainly caused by degree 6 terms.

Long-term stability analogous to Fermi-Pasta-Ulam-Tsingou problem.

Abstract

Apart from being chaotic, the inner planets in the Solar System constitute an open system, as they are forced by the regular long-term motion of the outer ones. No integrals of motion can bound a priori the stochastic wanderings in their high-dimensional phase space. Still, the probability of a dynamical instability is remarkably low over the next 5 billion years, a timescale thousand times longer than the Lyapunov time. The dynamical half-life of Mercury has indeed been estimated recently at 40 billion years. By means of the computer algebra system TRIP, we consider a set of dynamical models resulting from truncation of the forced secular dynamics recently proposed for the inner planets at different degrees in eccentricities and inclinations. Through ensembles of $10^3$ to $10^5$ numerical integrations spanning 5 to 100 Gyr, we find that the Hamiltonian truncated at degree 4 practically does not allow any instability over 5 Gyr. The destabilisation is mainly due to terms of degree 6. This surprising result suggests an analogy to the Fermi-Pasta-Ulam-Tsingou problem, in which tangency to Toda Hamiltonian explains the very long timescale of thermalisation, which Fermi unsuccessfully looked for.