Hot Jupiters from Secular Planet--Planet Interactions

TL;DR

This paper demonstrates that secular gravitational interactions in hierarchical planetary systems, combined with tidal friction, can produce hot Jupiters in retrograde orbits, explaining observed orbital configurations.

Contribution

It extends secular perturbation theory to planetary systems, showing how retrograde hot Jupiters can form through long-term interactions and tidal effects.

Findings

Retrograde hot Jupiters can form via secular interactions and tidal friction.

Planetary perturbers can cause the inner orbit's angular momentum to change sign.

High eccentricity phases enable tidal circularization into retrograde orbits.

Abstract

About 25 per cent of `hot Jupiters' (extrasolar Jovian-mass planets with close-in orbits) are actually orbiting counter to the spin direction of the star. Perturbations from a distant binary star companion can produce high inclinations, but cannot explain orbits that are retrograde with respect to the total angular momentum of the system. Such orbits in a stellar context can be produced through secular (that is, long term) perturbations in hierarchical triple-star systems. Here we report a similar analysis of planetary bodies, including both octupole-order effects and tidal friction, and find that we can produce hot Jupiters in orbits that are retrograde with respect to the total angular momentum. With distant stellar mass perturbers, such an outcome is not possible. With planetary perturbers, the inner orbit's angular momentum component parallel to the total angular momentum need not be constant. In fact, as we show here, it can even change sign, leading to a retrograde orbit. A brief excursion to very high eccentricity during the chaotic evolution of the inner orbit allows planet-star tidal interactions to rapidly circularize that orbit, decoupling the planets and forming a retrograde hot Jupiter.