Evidence for unresolved exoplanet-hosting binaries in Gaia DR2

TL;DR

This paper investigates unresolved stellar companions in known transiting exoplanet systems using Gaia DR2 data, revealing that many host stars are likely unresolved binaries which bias planetary measurements and our understanding of exoplanet demographics.

Contribution

It introduces a method to identify unresolved binaries in exoplanet host stars using Gaia DR2 astrometric indicators, highlighting the prevalence of such binaries.

Findings

Many exoplanet host stars show signs of unresolved binarity.

Unresolved binaries significantly bias planetary radius and mass estimates.

Systematic underestimation of exoplanet parameters due to unresolved companions.

Abstract

This note describes an effort to detect additional stellar sources in known transiting exoplanet (TEP) systems, which are unresolved or barely resolved in the Gaia Data Release 2 (DR2) catalogue. The presence of multiple unresolved stars in photometric and spectroscopic observations of a transiting planetary system biases measurements of the planet's radius, mass, and atmospheric conditions. In addition to the effect on individual planetary systems, the presence of unresolved stars across the sample of known exoplanets biases our overall understanding of planetary systems, due to the systematic underestimation of both masses and radii. This work uses the Astrometric Goodness of Fit in the Along-Scan direction (GOF_AL) and the Astrometric Excess Noise as indicators of poorly-resolved binaries. Many known close binaries in the exoplanet host star sample have highly significant GOF_AL and Astrometric Excess Noise values, such as WASP-20AB with Astrometric Excess Noise significant at $4720\sigma$ and GOF_AL=124.