On The Lack of Circumbinary Planets Orbiting Isolated Binary Stars

TL;DR

This paper proposes a stellar-tidal evolution mechanism explaining the observed scarcity of circumbinary planets around isolated binary stars, showing that orbital expansion destabilizes and often ejects close-in planets.

Contribution

It introduces a new model linking stellar-tidal evolution to the destabilization and removal of circumbinary planets, supported by simulations and application to Kepler-47.

Findings

Most plausible initial conditions lead to expansion of the instability region.

In some cases, the stability semi-major axis doubles from its initial value.

At least one planet is ejected in 87% of multi-planet systems due to instability.

Abstract

We outline a mechanism that explains the observed lack of circumbinary planets (CBPs) via coupled stellar-tidal evolution of isolated binary stars. Tidal forces between low-mass, short-period binary stars on the pre-main sequence slow the stellar rotations, transferring rotational angular momentum to the orbit as the stars approach the tidally locked state. This transfer increases the binary orbital period, expanding the region of dynamical instability around the binary, and destabilizing CBPs that tend to preferentially orbit just beyond the initial dynamical stability limit. After the stars tidally lock, we find that angular momentum loss due to magnetic braking can significantly shrink the binary orbit, and hence the region of dynamical stability, over time impacting where surviving CBPs are observed relative to the boundary. We perform simulations over a wide range of parameter space and find that the expansion of the instability region occurs for most plausible initial conditions and that in some cases, the stability semi-major axis doubles from its initial value. We examine the dynamical and observable consequences of a CBP falling within the dynamical instability limit by running N-body simulations of circumbinary planetary systems and find that typically, at least one planet is ejected from the system. We apply our theory to the shortest period {\it Kepler} binary that possesses a CBP, Kepler-47, and find that its existence is consistent with our model. Under conservative assumptions, we find that coupled stellar-tidal evolution of pre-main sequence binary stars removes at least one close-in CBP in $87\%$ of multi-planet circumbinary systems.