A Criterion for the Onset of Chaos in Compact, Eccentric Multiplanet Systems

TL;DR

This paper develops a semi-analytic criterion for chaos onset in compact, eccentric multiplanet systems, linking resonance overlap and secular evolution, and provides an improved stability criterion with an open-source tool.

Contribution

It introduces a new semi-analytic chaos criterion based on resonance overlap and secular effects, and refines the AMD stability threshold for tightly packed planetary systems.

Findings

Chaos onset is determined by resonance overlap at maximum MMR widths.

Secular evolution modulates MMR widths, affecting chaos boundaries.

The improved AMD criterion accounts for interplanetary spacing, lowering stability thresholds.

Abstract

We derive a semi-analytic criterion for the presence of chaos in compact, eccentric multiplanet systems. Beyond a minimum semimajor-axis separation, below which the dynamics are chaotic at all eccentricities, we show that (i) the onset of chaos is determined by the overlap of two-body mean motion resonances (MMRs), like it is in two-planet systems; (ii) secular evolution causes the MMR widths to expand and contract adiabatically, so that the chaotic boundary is established where MMRs overlap at their greatest width. For closely spaced two-planet systems, a near-symmetry strongly suppresses this secular modulation, explaining why the chaotic boundaries for two-planet systems are qualitatively different from cases with more than two planets. We use these results to derive an improved angular-momentum-deficit (AMD) stability criterion, i.e., the critical system AMD below which stability should be guaranteed. This introduces an additional factor to the expression from Laskar and Petit (2017) that is exponential in the interplanetary separations, which corrects the AMD threshold toward lower eccentricities by a factor of several for tightly packed configurations. We make routines for evaluating the chaotic boundary available to the community through the open-source SPOCK package.