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

Abstract: The case of Ross 176 is a late K-type star that hosts a promising water-world candidate planet. The star has a radius of $R_*$=0.569$\pm$0.020$R_{\odot}$ and a mass of $M_{\star}$ = 0.577 $\pm$ 0.024 $M_{\odot}$. 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 $\sim$ 32 days. This stellar activity signal was also confirmed on the TESS light curves. Ross 176b is an inner hot transiting planet with a low-eccentricity orbit of $e = 0.25 \pm 0.04$, an orbital period of $P \sim 5$ days, and an equilibrium temperature of $T_{eq}\sim 682K$. With a radius of $R_p = 1.84\pm0.08R_{\oplus}$ (4% precision), a mass of $M_p = 4.57^{{+0.89}_{-0.93}} M_{\oplus}$ (20% precision), and a mean density of $$\rho_p = 4.03^{{+0.49}_{-0.81}} g cm<sup>{-3}$$, the composition of Ross 176b might be consistent with a water-world scenario. Moreover, Ross 176b 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.