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

¹ Instituto de Astrofísica de Canarias (IAC), 38205 La Laguna, Tenerife, Spain
² Departamento de Astrofísica, Universidad de La Laguna (ULL), 38206 La Laguna, Tenerife, Spain
³ Department of Astronomy, University of Texas at Austin, 2515 Speedway, Austin, TX 78712, USA
⁴ Centro de Astrobiología (CSIC-INTA), Camino Bajo del Castillo s/n, Campus ESAC, 28692 Villanueva de la Cañada, Madrid, Spain
⁵ Departamento de Física de la Tierra y Astrofísica & IPARCOS-UCM (Instituto de Física de Partículas y del Cosmos de la UCM), Facultad de Ciencias Físicas, Universidad Complutense de Madrid, 28040 Madrid, Spain
⁶ Instituto de Astrofísica de Andalucía (IAA-CSIC), Glorieta de la Astronomía s/n, 18008 Granada, Spain
⁷ Institut für Astrophysik und Geophysik, Georg-August-Universität, Friedrich-Hund-Platz 1, 37077 Göttingen, Germany
⁸ Hamburger Sternwarte, Gojenbergsweg 112, 21029 Hamburg, Germany
⁹ Komaba Institute for Science, The University of Tokyo, 3-8-1 Komaba, Meguro, Tokyo 153-8902, Japan
¹⁰ Department of Multi-Disciplinary Sciences, Graduate School of Arts and Sciences, The University of Tokyo, 3-8-1 Komaba, Meguro, Tokyo, Japan
¹¹ Department of Astronomy & Astrophysics, University of Chicago, Chicago, IL 60637, USA
¹² Astrobiology Center, 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan
¹³ National Astronomical Observatory of Japan, 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan
¹⁴ Aristotle University of Thessaloniki, University Campus, 54124, Thessaloniki, Greece
¹⁵ Institut de Ciències de l’Espai (ICE, CSIC), Campus UAB, c/de Can Magrans s/n, 08193 Bellaterra, Barcelona, Spain
¹⁶ Institut d’Estudis Espacials de Catalunya (IEEC), 08860 Castellde-fels (Barcelona), Spain
¹⁷ Steward Observatory and Department of Astronomy, The University of Arizona, Tucson, AZ 85721, USA
¹⁸ Department of Physics and Kavli Institute for Astrophysics and Space Research, Massachusetts Institute of Technology, Cambridge, MA 02139, USA
¹⁹ Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA 02139, USA
²⁰ Department of Aeronautics and Astronautics, MIT, 77 Massachusetts Avenue, Cambridge, MA 02139, USA
²¹ Department of Physics and Astronomy, Vanderbilt University, Nashville, TN 37235, USA
²² Landessternwarte, Zentrum für Astronomie der Universtät Heidelberg, Königstuhl 12, 69117 Heidelberg, Germany
²³ Department of Astronomy, Faculty of Physics, Sofia University “St. Kliment Ohridski”, 5 James Bourchier Blvd., BG-1164 Sofia, Bulgaria
²⁴ Vereniging Voor Sterrenkunde, Oude Bleken 12, 2400 Mol, Belgium
²⁵ AstroLAB IRIS, Provinciaal Domein “De Palingbeek”, Verbrande-molenstraat 5, 8902 Zillebeke, Ieper, Belgium
²⁶ Centre for Mathematical Plasma-Astrophysics, Department of Mathematics, KU Leuven, Celestijnenlaan 200B, 3001 Heverlee, Belgium
²⁷ Department of Astrophysical Sciences, Princeton University, Princeton, NJ 08544, USA

^★ Corresponding author: sgeraldia@iac.es

Received: 10 January 2025
Accepted: 25 June 2025

Abstract

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_−0.93^+0.89M_⊕ (20% precision), and a mean density of ρ_p = 4.03_−0.81^+0.49g 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.