Circumbinary planets - why they are so likely to transit

TL;DR

Circumbinary planets significantly increase transit probability over single-star systems, making transits more likely and sometimes guaranteed, especially around eclipsing binaries, thus enabling exploration of outer exoplanetary regions.

Contribution

The paper analytically derives the transitability probability for circumbinary planets, showing it exceeds single-star probabilities and can be near certainty under certain conditions.

Findings

Transitability probability is always higher than for single stars.

Circumbinary planets around eclipsing binaries can have near 100% transit probability.

Transit probability is weakly dependent on the planet's orbital period.

Abstract

Transits on single stars are rare. The probability rarely exceeds a few per cent. Furthermore, this probability rapidly approaches zero at increasing orbital period. Therefore transit surveys have been predominantly limited to the inner parts of exoplanetary systems. Here we demonstrate how circumbinary planets allow us to beat these unfavourable odds. By incorporating the geometry and the three-body dynamics of circumbinary systems, we analytically derive the probability of transitability, a configuration where the binary and planet orbits overlap on the sky. We later show that this is equivalent to the transit probability, but at an unspecified point in time. This probability, at its minimum, is always higher than for single star cases. In addition, it is an increasing function with mutual inclination. By applying our analytical development to eclipsing binaries, we deduce that transits are highly probable, and in some case guaranteed. For example, a circumbinary planet revolving at 1 AU around a 0.3 AU eclipsing binary is certain to eventually transit - a 100% probability - if its mutual inclination is greater than 0.6 deg. We show that the transit probability is generally only a weak function of the planet's orbital period; circumbinary planets may be used as practical tools for probing the outer regions of exoplanetary systems to search for and detect warm to cold transiting planets.