The First Direct Occurrence Rate Estimates for Kepler Exoplanets in Small-separation Binary Star Systems: Planet Occurrence Is Suppressed in Binary Stars

Abstract Exoplanet occurrence rates facilitate comparisons between observations of planets and theoretical models of planet formation. Despite their deductive power, exoplanet occurrence rates for half the stars in the sky are missing because occurrence rate studies systematically exclude binary star systems. We assembled a large sample of high-likelihood binaries from the Kepler mission to calculate occurrence rates for circumstellar (S-type) planets in small-separation binary star systems (≲100 au) for the first time. For a sample of high-likelihood small-separation binaries, we found binaries to host 58% fewer planets per system than single stars to 11.4 σ significance within 1–4 R ⊕ and 1–50 days, and 50% fewer planets compared to single stars when integrating over the full parameter space of 1–10 R ⊕ and 1–100 days to 3.8 σ significance. We found no evidence for a radius valley or radius cliff, instead detecting a smooth decline in planet occurrence with increasing planetary radius. The difference between the single-star planet radius distribution and the binary-star planet radius distribution is 4.3 σ significant from a Kolmogorov–Smirnov test. These results suggest significantly different planet formation and survival outcomes in binaries compared to single stars, and support other studies that have measured a deficit of observed planets in binary star systems.