Orbital Dynamics and Numerical N-body Simulations of Extrasolar Moons and Giant Planets

Abstract

This thesis work focuses on computational orbital dynamics of exomoons and exoplanets. Exomoons are highly sought-after astrobiological targets. Two candidates have been discovered to-date (Bennett et al. 2014, Teachey et al. 2018). We developed the first N-body integrator that can handle exomoon orbits in close planet-planet interactions, for the following three projects. (1) Instability of moons around non-oblate planets associated with slowed nodal precession and resonances with stars.This work reversed the commonsensical notion that spinning giant planets should be oblate. Moons around spherical planets were destabilized by 3:2 and 1:1 resonance overlap or the chaotic zone around 1:1 resonance between the orbital precession of the moons and the star. Normally, the torque from planet oblateness keeps the orbit of close-in moons precess fast ( period ~ 7 yr for Io). Without planet oblateness, Io's precession period is much longer (10,000 yr), which allowed resonance with the star, thus the instability. Therefore, realistic treatment of planet oblateness is critical in moon dynamics. (2) Orbital stability of moons in planet-planet scattering. Planet-planet scattering is the best model to date for explaining the eccentricity distribution of exoplanets. Planets encounter each other closely, and moons are easily destabilized. The orbital evolution of planets also destabilizes moons via Kozai (highly inclined) perturbations and violation of Hill stability. Moons showed rich dynamical outcomes, including ejected free-floating exomoons, moon exchange between planets, moons turning to orbit the star, and moons orbiting ejected free-floating planets. Planets involved in planet-planet scattering develops high inclinations and high obliquities. Relevant instability effects for moons requires the code to address planet spin evolution. Planet-planet scattering is efficient at removing moons (80-90%). (3) Obliquity of extrasolar giant planets in planet-planet scattering. Planet-planet scattering can generate high obliquity giant planets and retrograde obliquity like Uranus. The close interaction during close encounters can't generate high obliquity (Brunini 2006, retracted), but the correspondence between the planets' spin and orbital precession rates can efficiently drive obliquity evolution (Storch et al. 2014). When planets are scattered close to the star, their obliquity evolves.