Revealing Planet Formation With Planets in Binary Star Systems: Close Binaries May Suppress sub-Neptunes

Small planets, with radii less than 4 Earth radii, are divided into rocky super-Earths and gaseous sub-Neptunes separated by a radius gap, but the mechanisms that differentiate these two planet populations remain unclear. Binary stars are the only systems with an observable record of the planet formation timescale and mass reservoir for main sequence systems, so the demographics of planets in binaries may provide insight into the planet formation process. To explore the radius distribution of planets in binary star systems, we observed 185 close binary systems hosting 260 transiting planets detected by the Kepler mission and recharacterized the planets after accounting for the presence of the secondary star. We found that the population of planets in close binaries (separation < 100 au) is significantly different from the planet population in wide binaries (separation > 300 au) or single stars. In contrast to single stars, close binaries have a unimodal radius distribution with a peak near the expected super-Earth peak of R ~ 1.3 Earth radii. We conclude that we are observing the direct impact of a reduced timescale and mass reservoir for planet formation reducing the sub-Neptune formation efficiency. Our results demonstrate the power of binary stars as a laboratory for exploring planet formation and offer intriguing insights into the impact of varying initial conditions on mature planet populations.