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 ≈ 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_⊕. These parameters imply that the planet is of moderate density ({ρ _{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.

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