Tilted circumbinary planetary systems as efficient progenitors of free-floating planets

TL;DR

This paper demonstrates that tilted circumbinary planetary systems can become unstable and eject planets, especially around eccentric binaries, providing an efficient mechanism for producing free-floating planets, including low-mass ones.

Contribution

It reveals that planet instability can occur in widely separated tilted circumbinary systems, expanding the understanding of free-floating planet origins beyond single-star systems.

Findings

Instability occurs even with widely separated planets on tilted orbits.

Most massive planets tend to remain stable while lower mass planets are ejected.

Ejection of low-mass planets is more common around eccentric binaries.

Abstract

The dominant mechanism for generating free-floating planets has so far remained elusive. One suggested mechanism is that planets are ejected from planetary systems due to planet-planet interactions. However, instability around a single star requires a very compactly spaced planetary system. We find that around binary star systems instability can occur even with widely separated planets that are on tilted orbits relative to the binary orbit due to combined effects of planet-binary and planet-planet interactions, especially if the binary is on an eccentric orbit. We investigate the orbital stability of planetary systems with various planet masses and architectures. We find that the stability of the system depends upon the mass of the highest mass planet. The order of the planets in the system does not significantly affect stability but, generally, the most massive planet remains stable and the lower mass planets are ejected. The minimum planet mass required to trigger the instability is about that of Neptune for a circular orbit binary and a super-Earth of about $10$ Earth masses for highly eccentric binaries. Hence, we suggest that planet formation around misaligned binaries can be an efficient formation mechanism for free-floating planets. While most observed free-floating planets are giant planets, we predict that there should be more low-mass free floating planets that are as yet unobserved than higher mass planets.