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 $R_{\ast }$=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 $R_p=1.84\pm 0.08R_{\oplus }$ (4% precision), a mass of $M_p=4.57_{-0.93}^{+0.89}{M}_{\oplus }$ (20% precision), and a mean density of ${\rho }_p=4.03_{-0.81}^{+0.49}gc{m}^{-3}$, 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.

Accepted for publication in A&A. 16 pages. 13 figures

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