A triple origin for the lack of tight coplanar circumbinary planets around short-period binaries

TL;DR

This paper investigates why transiting circumbinary planets are absent around short-period binaries, showing that secular Kozai-Lidov cycles and tidal friction prevent their formation or survival, especially in coplanar, tight orbits.

Contribution

The study introduces a comprehensive analysis combining secular evolution and population synthesis to explain the scarcity of circumbinary planets around short-period binaries, highlighting the role of planetary mass and orbit.

Findings

Massive planets can prevent short-period binary formation by quenching KLCTF.

Secular evolution often leads to planetary ejection or wide, eccentric, inclined orbits.

Circumbinary planets around short-period binaries are unlikely to be tight and coplanar.

Abstract

Transiting circumbinary planets are more easily detected around short-period than long-period binaries, but none have yet been observed by {\it Kepler} orbiting binaries with periods shorter than seven days. In triple systems, secular Kozai-Lidov cycles and tidal friction (KLCTF) have been shown to reduce the inner orbital period from $\sim 10^4$ to a few days. Indeed, the majority of short-period binaries are observed to possess a third stellar companion. Using secular evolution analysis and population synthesis, we show that KLCTF makes it unlikely for circumbinary transiting planets to exist around short-period binaries. We find the following outcomes. (1) Sufficiently massive planets in tight and/or coplanar orbits around the inner binary can quench the KL evolution because they induce precession in the inner binary. The KLCTF process does not take place, preventing the formation of a short-period binary. (2) Secular evolution is not quenched and it drives the planetary orbit into a high eccentricity, giving rise to an unstable configuration, in which the planet is most likely ejected from the system. (3) Secular evolution is not quenched but the planet survives the KLCTF evolution. Its orbit is likely to be much wider than the currently observed inner binary orbit, and is likely to be eccentric and inclined with respect to the inner binary. These outcomes lead to two main conclusions: (1) it is unlikely to find a massive planet on a tight and coplanar orbit around a short-period binary, and (2) the properties of circumbinary planets in short-period binaries are constrained by secular evolution.