Geraldía-González, Samuel, Orell-Miquel, Jaume, Pallé, Enric B., Murgas, Felipe, Lacedelli, Gaia, Béjar, Víctor Javier Sánchez, Caballero, José A., Duque-Arribas, C., Lillo-Box, Jorge, & Montes, D. (2025). Discovery of a transiting hot water-world candidate orbiting Ross 176 with TESS and CARMENES. Astronomy and Astrophysics, 700, A216. https://doi.org/10.1051/0004-6361/202553719
The Transiting Exoplanet Survey Satellite (TESS) has discovered many new planet candidates that require confirmation and further study with ground-based observations. One such case is Ross 176, a late K-type star that hosts a promising planet candidate that may be a water world. The star has a radius of 0.569 ± 0.020 times that of the Sun and a mass of 0.577 ± 0.024 times the Sun’s mass. To better determine the planet’s mass, we used spectroscopic data from CARMENES, an instrument that has already been key in confirming planets detected by TESS. Because the host star shows strong activity that affects the radial velocity data, we applied Gaussian Processes (GP) to improve the analysis. We also used GP on the TESS light curves to reduce correlated noise in the dataset. Stellar activity indicators revealed a strong signal tied to the star’s rotation period of about 32 days, which was also visible in the TESS data.
Ross 176 b, the planet candidate, is an inner hot transiting planet with a low-eccentricity orbit (e = 0.25 ± 0.04), a period of about 5 days, and an equilibrium temperature of ~682 K. It has a radius of 1.84 ± 0.08 Earth radii (with 4% precision), a mass of 4.57−0.93+0.89 Earth masses (with 20% precision), and a mean density of 4.03−0.81+0.49 g/cm³. These properties suggest Ross 176 b could be consistent with a water-world scenario. Importantly, it is a strong candidate for atmospheric characterization, which could reveal more about the existence, formation, and composition of water worlds.
This discovery adds to the number of known planets orbiting K-type stars, a sample that is especially useful for studying the distribution of small-radius planets around these stars. It also highlights the effectiveness of combining space-based (TESS) and ground-based (CARMENES) observations for confirming new planets.
GLS periodogram of serval activity indicators. From top to bottom: Hα λ6562.81A, NaI D1 and D2 λλ5889.9, 5895.92Ǻ, CaII IRT λλλ 8498.02,8542.09,8662.14 Ǻ, CRX, and dLW periodograms. In all panels, the period of the planet candidate at 5.01 d is marked with a vertical dashed green line, and the star-related signals at 8, 16, and 32 days are marked in orange, red, and magenta, respectively. The 10%, 1%, and 0.1% FAPs are marked as horizontal dashed, dash-dotted, and dotted horizontal lines, respectively. The peaks near one day correspond to aliases of the stellar rotation signal, introduced by the daily sampling of ground-based observations.