Abstract

Binary stars are ubiquitous; about two-thirds of solar-type stars exist in binaries. Exoplanet occurrence rate is suppressed in binaries, but some multiples do still host planets. Multiplicity reduces the size and lifetime of protoplanetary disks, suggesting that binaries not only suppress planet formation but also could fundamentally alter the properties of planets in multiples. Binaries also cause observational biases in planet parameters, with undetected multiplicity causing transiting planets to appear smaller than they truly are. To understand the impact of the observational and physical effects of binaries on planet populations, we have analyzed the properties of two samples of exoplanets in binaries. First, we studied the observational effects of multiplicity on apparent rocky super-Earth planets, and found that more than half of them were actually gaseous sub-Neptunes after correcting for the presence of the secondary star. Six of the supposedly rocky planets were originally in the habitable zone (HZ), but only two of them remained rocky and in the HZ after our analysis. These results indicate that carefully accounting for undetected multiples in demographic samples is important for calculating accurate occurrence rates. Second, we studied the underlying population of planets in binaries that fall in and around the radius valley, a demographic feature in period-radius space that marks the transition from predominantly rocky to predominantly gaseous planets. We found that the radius valley shifts to smaller planetary radii in binaries, suggesting that the planetary core mass function in binaries may peak at smaller values than in single stars. These results reveal demographic differences between planets in binary and single stars for the first time, suggesting that stellar multiplicity may fundamentally alter the outcome of the planet formation process.