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

Abstract

We report the discovery of a hot ($T_{\mathrm{eq}}$ $\approx$ 1055 K) planet in the small planet radius valley transiting the Sun-like star TOI-733, 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 $P_{\mathrm{orb}}$ = $4.884765_{-2.4e-5}^{+1.9e-5}$ days and a radius of $R_{\mathrm{p}}$ = $1.992_{-0.090}^{+0.085}$ $R_{\oplus }$. Multi-dimensional Gaussian process modelling of the radial velocity measurements from HARPS and activity indicators, gives a semi-amplitude of $K$ = $2.23\pm 0.26$ m s${}^{-1}$, translating into a planet mass of $M_{\mathrm{p}}$ = $5.72_{-0.68}^{+0.70}$ $M_{\oplus }$. These parameters imply that the planet is of moderate density ($\ddot{I}{}_{\mathrm{p}}$ = $3.98_{-0.66}^{+0.77}$ 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 calculate 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 - one of only a few such planets around G-type stars that are well-characterised.

Accepted for publication in A&A