Discovery of a transiting hot water-world candidate orbiting Ross 176 with TESS and CARMENES

The Transiting Exoplanet Survey Satellite (TESS) discovered several new planet candidates that need to be confirmed and characterized with ground-based observations. This is the case of Ross 176, a late K-type star that hosts a promising water-world candidate planet. The star has a radius of R_★ = 0.569 ± 0.020 R_☉ and a mass of M_★ = 0.577 ± 0.024 M_☉. We constrained the planetary mass using spectroscopic data from CARMENES, an instrument that has already played a major role in confirming the planetary nature of the transit signal detected by TESS. We used Gaussian Processes (GP) to improve the analysis because the host star has a relatively strong activity that affects the radial velocity dataset. In addition, we applied a GP to the TESS light curves to reduce the correlated noise in the detrended dataset. The stellar activity indicators show a strong signal that is related to the stellar rotation period of ∼32 days. This stellar activity signal was also confirmed on the TESS light curves. Ross 176 b is an inner hot transiting planet with a low-eccentricity orbit of e = 0.25 ± 0.04, an orbital period of P ∼ 5 days, and an equilibrium temperature of T_eq ∼ 682 K. With a radius of R_p = 1.84 ± 0.08 R_⊕ (4% precision), a mass of M_p = 4.57⁺₋⁰_0.⁸⁹₉₃ M_⊕ (20% precision), and a mean density of ρ_p = 4.03⁺₋⁰_0.⁴⁹₈₁g cm⁻³, the composition of Ross 176 b might be consistent with a water-world scenario. Moreover, Ross 176 b is a promising target for atmospheric characterization, which might lead to more information on the existence, formation and composition of water worlds. This detection increases the sample of planets orbiting K-type stars. This sample is valuable for investigating the valley of planets with small radii around this type of star. This study also shows that the dual detection of space- and ground-based telescopes is efficient for confirm new planets.