Orbital stability of two circumbinary planets around misaligned eccentric binaries

Abstract

ABSTRACT With n-body simulations, we investigate the stability of tilted circumbinary planetary systems consisting of two non-zero mass planets. The planets are initially in circular orbits that are coplanar to each other, as would be expected if they form in a flat but tilted circumbinary gas disc and decouple from the disc within a time difference that is much less than the disc nodal precession period. We constrain the parameters of stable multiple planet circumbinary systems. Both planet–planet and planet–binary interactions can cause complex planet tilt oscillations that can destabilize the orbits of one or both planets. The system is considerably more unstable than the effects of these individual interactions would suggest, due to the interplay between these two interactions. The stability of the system is sensitive to the binary eccentricity, the orbital tilt, and the semimajor axes of the two circumbinary planets. With an inner planet semimajor axis of $5\, a_{\rm b}$, where ab is the semimajor axis of the binary, the system is generally stable if the outer planet is located at $\gtrsim 8\, a_{\rm b}$, beyond the 2:1 mean motion resonance with the inner planet. For larger inner planet semimajor axis, the system is less stable because the von-Zeipel–Kozai–Lidov mechanism plays a significant role, particularly for low binary-eccentricity cases. For the unstable cases, the most likely outcome is that one planet is ejected and the other remains bound on a highly eccentric orbit. Therefore, we suggest that this instability is an efficient mechanism for producing free-floating planets.