TOI-733 b: A planet in the small-planet radius valley orbiting a Sun-like star

Abstract

We report the discovery of a hot (Teq ≈ 1055 K) planet in the small-planet radius valley that transits the Sun-like star TOI-733. It was discovered as part of the KESPRINT follow-up program of TESS planets carried out with the HARPS spectrograph. TESS photometry from sectors 9 and 36 yields an orbital period of ${P_{{\rm{orb}}}} = 4.884765_{ - 2.4e - 5}^{ + 1.9e - 5}$ days and a radius of ${R_{\rm{p}}} = 1.992_{ - 0.090}^{ + 0.085}$ R⊕. Multi-dimensional Gaussian process modelling of the radial velocity measurements from HARPS and activity indicators gives a semi-amplitude of K = 2.23 ± 0.26 m s−1, translating into a planet mass of ${M_{\rm{p}}} = 5.72_{ - 0.68}^{ + 0.70}$ M⊕. These parameters imply that the planet is of moderate density (${\rho _{\rm{p}}} = 3.98_{ - 0.66}^{ + 0.77}$ g cm−3) and place it in the transition region between rocky and volatile-rich planets with H/He-dominated envelopes on the mass-radius diagram. Combining these with stellar parameters and abundances, we calculated planet interior and atmosphere models, which in turn suggest that TOI-733 b has a volatile-enriched, most likely secondary outer envelope, and may represent a highly irradiated ocean world. This is one of only a few such planets around G-type stars that are well characterised.