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

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 • , 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 counterintuitive 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.

Innocent Bystanders: Orbital Dynamics of Exomoons During Planet–Planet Scattering

“…Various sources of perturbations also affect the satellites, such as secular perturbations from the perturbing planets (as opposed to the host planet of the satellites) , and stellar perturbations through the change of the host planet's spin and orbits in the instability phase. The orbital parameter space of moons in this work covers close-in regions where planet oblateness plays a major role in moon stability, as an already well-established fact, and the absence of planet oblateness will cause unrealistic orbital instability effect (Hong et al 2015). This work also simulates moons within and beyond the critical semi-major axis (0.04 Hill radii for Jupiter) of the planets where planet spin can affect moon stability (Tremaine et al 2009), and moons up to 0.35 Hill radii where prograde moons can be stable.…”

The effect of multiple heat sources on exomoon habitable zones

“…Beyond the fact that our models are physically plausible only for moderate values (≤ 0.1), tidal circularization will act to decrease eccentricities to zero on time scales that are typically much shorter than 1 Gyr (Porter and Grundy 2011;Heller and Barnes 2013). Hence, even moderate eccentricities can only be expected in real exomoon systems if the star has a significant effect on the moon's orbit (Heller 2012), if other planets can act as orbital perturbers (Gong et al 2013;Payne et al 2013;Hong et al 2015), if other massive moons are present around the same planet, or if the planet-moon system has migrated through orbital resonances with the circumstellar orbit (Namouni 2010;Spalding et al 2016).…”

Spin Dynamics of Extrasolar Giant Planets in Planet-Planet Scattering

“…The numerical scheme of this work builds on Hong et al (2015Hong et al ( , 2018. The code uses Mercury (Chambers 1999) as a base, and incorporates the shape of planets and spin evolution using a non-secular equation of motion.…”