Mind the Companion: Demographics of Transiting S-type Exoplanets

Exoplanet demographic studies rely on large and homogeneous catalogues, yet stellar multiplicity remains incompletely characterised in many planet samples. We update the PlanetS catalogue of transiting exoplanets by systematically identifying gravitationally bound stellar companions using Gaia data release 3 (DR3), constructing a matched control sample of single hosts to mitigate selection and observational biases. Using this curated dataset of 860 transiting exoplanets including 133 S-type planets, we perform a comparative demographic analysis as a function of host multiplicity, stellar mass, and binary separation. We find a binary fraction of 19.4% relative to the control sample, consistent with previous estimates but derived from a larger and more homogeneous dataset. In the giant planet regime, less affected by observational biases, planets in binaries tend to be more massive than their single-star counterparts and orbit closer to their host stars, making their radii more inflated. In particular, 53% of giant planets orbiting M-dwarfs reside in binary systems with separations < 1000 AU, a 2.6 sigma excess compared to FGK-type hosts, suggesting that stellar multiplicity plays a key role in the formation or survival of giant planets around low-mass stars.