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

We report the discovery of a hot (T-eq approximate to 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-orb = 4.884765(-2.4e-5)(+1.9e-5) days and a radius of R-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-p = 5.72(-0.68)(+0.70) M-circle dot. These parameters imply that the planet is of moderate density (rho(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.