Orbital Architectures of Planet-Hosting Binaries II. Low Mutual Inclinations Between Planetary and Stellar Orbits

TL;DR

This study investigates the orbital alignments in binary star systems hosting Kepler planets, revealing that planetary and stellar orbits tend to be closely aligned with low mutual inclinations, which impacts theories of planet formation.

Contribution

The paper provides the first statistical analysis showing that binary star and planetary orbits are generally aligned with low mutual inclinations in systems with Kepler planets.

Findings

Binary orbits are not randomly oriented, with a significance of 4.7σ.

Most systems have mutual inclinations between 0-30 degrees.

Results support models of planet formation in aligned binary systems.

Abstract

Planet formation is often considered in the context of one circumstellar disk around one star. Yet stellar binary systems are ubiquitous, and thus a substantial fraction of all potential planets must form and evolve in more complex, dynamical environments. We present the results of a five-year astrometric monitoring campaign studying 45 binary star systems that host Kepler planet candidates. The planet-forming environments in these systems would have literally been shaped by the binary orbits that persist to the present day. Crucially, the mutual inclinations of star-planet orbits can only be addressed by a statistical sample. We describe in detail our sample selection and Keck/NIRC2 laser guide star adaptive optics observations collected from 2012 to 2017. We measure orbital arcs, with a typical accuracy of ~0.1 mas/yr, that test whether the binary orbits tend to be aligned with the edge-on transiting planet orbits. We rule out randomly-distributed binary orbits at 4.7$\sigma$, and we show that low mutual inclinations are required to explain the observed orbital arcs. If the stellar orbits have a field binary-like eccentricity distribution, then the best match to our observed orbital arcs is a distribution of mutual inclinations ranging from 0-30 degrees. We discuss the implications of such widespread planet-binary alignment in the theoretical context of planet formation and circumstellar disk evolution.