The Mass–Radius Relation for Planets in Binary Systems

Abstract Stellar companion within a few hundred astronomical unit alter the structure and shorten the lifetime of protoplanetary disks, influencing planetary formation and evolution. Such systems host fewer close-in planets, and have fewer ~2.3 R ⊕ “sub-Neptunes” relative to ~1.3 R ⊕ “super-Earths” compared to single-star hosts, observations that can be explained by early dissipation of the gas disk. Here we construct the mass–radius diagram of 15 small (<8 R ⊕ ), well-characterized planets on S-type orbits in systems with projected separations <500 au, and show that it is indistinguishable from that of planets around single stars. This suggests that accretion of the H 2 -dominated envelopes of sub-Neptunes could be much faster than gas disk dissipation and limited instead by available solids for cores, or that many sub-Neptunes have envelopes of condensible volatiles such as H 2 O.