Planetary Desert around Compact Binaries: Dynamical Instability Triggered by Resonance-Induced Eccentricity Excitation

TL;DR

This study explains the absence of close-in transiting planets around short-period binary stars by showing how resonance-driven eccentricity growth leads to system-wide instabilities and planet ejections.

Contribution

It demonstrates that resonance-induced eccentricity excitation in multi-planet circumbinary systems causes instabilities that clear out inner planetary regions, explaining the observed desert.

Findings

Resonance locking causes extreme eccentricity growth in outer planets.

Planet-planet interactions trigger scatterings and ejections.

Instability is amplified from single-planet to system-wide chaos.

Abstract

Compact binaries with orbital periods shorter than about 7 days show an absence of transiting planets, a feature known as the ``circumbinary planet desert". The physical mechanism behind this desert remains unclear. We investigate its origin by simulating the long-term dynamics of multi-planet circumbinary systems with evolving inner binaries. Our simulations are based on the single-averaged secular equations that average only over the binary orbital period and fully incorporate planet-planet interactions. When an eccentric binary decays via tides, an outer planet can be captured into resonance advection in eccentricity, a state in which its apsidal precession locks with that of the binary, driving extreme eccentricity growth. While such growth can occur in a binary-single planet system, the parameter space is limited and may not necessarily induce instability. In a multi-planet system, however, the excited orbit inevitably crosses those of its neighbors, which triggers violent planet-planet scatterings and produces collisions or ejections. Crucially, these mutual gravitational interactions amplify the ``localized" instability of a single planet into a system-wide chain reaction, drastically reshaping the orbital architecture and potentially clearing out the inner regions of planetary systems. Our results suggest that the resonance-induced instability provides a natural explanation for the observed circumbinary planet desert.