Exoplanets in binary star systems: on the switch from prograde to retrograde orbits

Abstract

The eccentric Kozai–Lidov mechanism, based on the secular theory, has been proposed as a mechanism that plays an important role in producing orbits that switch from prograde to retrograde. In the present work we study the secular dynamics of a triple system composed of a Sun-like central star and a Jupiter-like planet, which are under the gravitational influence of another perturbing star (brown dwarf). The perturbation potential is developed in closed form up to the fifth order in a small parameter ($\alpha =a_{1}/a_{2}$), where $a_{1}$ is the semimajor axis of the extrasolar planet and $a_{2}$ is the semimajor axis of the perturbing star. To eliminate the short-period terms of the perturbation potential, the double-average method is applied. In this work we do not eliminate the nodes, a standard method in the literature, before deriving the equations of motion. The main goal is to study the effects of the higher-order terms of the expansion of the perturbing force due to the third body in the orbital evolution of the planet. In particular, we investigate the inclination and the shape (eccentricity) of these orbits. We show the importance of the higher-order terms in changing the inversion times of the flip, i.e., the times where the inclination of the inner planet flips from prograde to retrograde trajectories. We also show the dependence of the first flip with respect to the semimajor axis and eccentricity of the orbit of the planet. The general conclusion is that the analytical model increases its accuracy with the inclusion of higher-order terms. We also performed full numerical integrations using the Bulirsch–Stoer method available in the Mercury package for comparison with the analytical model. The results obtained with the equations developed in this work are in accordance with direct numerical simulations.