Orbital instability of close-in exomoons in non-coplanar systems

TL;DR

This study reveals that close-in exomoons in non-coplanar systems can become dynamically unstable if the host planet's oblateness is ignored, due to secular resonances affecting their orbital precession.

Contribution

It demonstrates the importance of including planetary oblateness in simulations of exomoon dynamics in non-coplanar systems, revealing new stability criteria.

Findings

Moons around spherical planets can become highly inclined and eccentric.

Oblate planets stabilize moons by shifting the Laplace plane.

Nodal precession resonances cause instability in non-oblate cases.

Abstract

This work shows the dynamical instability that can happen to close-in satellites when planet oblateness is not accounted for in non-coplanar multiplanet systems. Simulations include two secularly interacting Jupiter-mass planets mutually inclined by 10 degrees, with the host planet either oblate or spherical. With a spherical host planet, moons within a critical planetocentric distance experience high inclinations and in some cases high eccentricities, while more distant moons orbit stably with low inclinations and eccentricities, as expected. These counter-intuitive dynamical phenomena disappear with an oblate host planet, in which case the moons' Laplace plane transitions from the host planet's equatorial plane to the host planet's precessing orbital plane as their semi-major axes increase, and all moons are dynamically stable with very mild changes in orbits. Direct perturbation from the perturbing planet has been investigated and ruled out as an explanation for the behavior of the innermost satellites, therefore leaving the central star's perturbation as the cause. Instability occurs while the nodal precession of the satellite and the central star (as seen from the host planet's frame) approaches the 1:1 secular resonance. In non-coplanar systems, around a non-oblate planet, the nodal precession of the moon becomes slow and comparable to that of the planet, giving rise to resonant configurations. The above effect needs to be taken into account in setting up numerical simulations.