TOI-733 b: A planet in the small-planet radius valley orbiting a Sun-like star^★,★★

¹ Department of Space, Earth and Environment, Chalmers University of Technology, Onsala Space Observatory, 439 92 Onsala, Sweden
e-mail: iskra.georgieva@chalmers.se
² Dipartimento di Fisica, Universita degli Studi di Torino, via Pietro Giuria 1, 10125, Torino, Italy
³ Thüringer Landessternwarte Tautenburg, Sternwarte 5, 07778 Tautenburg, Germany
⁴ Aix-Marseille Université, Institut Origines, CNRS, CNES, LAM, Marseille, France
⁵ Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
⁶ Department of Astronomy & Astrophysics, University of California, Santa Cruz, CA 95064, USA
⁷ Institute of Planetary Research, German Aerospace Center (DLR), Rutherfordstrasse 2, 12489 Berlin, Germany
⁸ Center for Astrophysics I Harvard & Smithsonian, 60 Garden Street, Cambridge, MA 02138, USA
⁹ NASA Ames Research Center, Moffett Field, CA 94035, USA
¹⁰ Division of Geological and Planetary Sciences, 1200 E California Blvd, Pasadena CA 91125, USA
¹¹ Department of Astronomy, California Institute of Technology, Pasadena CA 91125, USA
¹² Leiden Observatory, University of Leiden, PO Box 9513, 2300 RA, Leiden, The Netherlands
¹³ Lund Observatory, Division of Astrophysics, Department of Physics Lund University, Box 43, 221 00 Lund, Sweden
¹⁴ Department of Space, Earth and Environment, Chalmers University of Technology, Chalmersplatsen 4, 412 96 Gothenburg, Sweden
¹⁵ Sub-department of Astrophysics, Department of Physics, University of Oxford, Oxford OX1 3RH, UK
¹⁶ McDonald Observatory and Center for Planetary Systems Habitability, The University of Texas, Austin, TX, USA
¹⁷ Instituto de Astrofísica de Canarias, C. Via Lactea s/n, 38205 La Laguna, Tenerife, Spain
¹⁸ Universidad de La Laguna, Dept. de Astrofísica, 38206 La Laguna, Tenerife, Spain
¹⁹ Astrobiology Center, 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan
²⁰ Department of Astronomical Science, The Graduate University for Advanced Studies (SOKENDAI), 2-21-1 Osawa, Mitaka, Tokyo, Japan
²¹ National Astronomical Observatory of Japan, 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan
²² Department of Astronomy & Astrophysics, University of Chicago, Chicago, IL 60637, USA
²³ Mullard Space Science Laboratory, University College London, Holmbury St Mary, Dorking, Surrey RH5 6NT, UK
²⁴ Astronomy Department and Van Vleck Observatory, Wesleyan University, Middletown, CT 06459, USA
²⁵ SETI Institute, Mountain View, CA 94043, USA
²⁶ Department of Astrophysical Sciences, Princeton University, Princeton, NJ 08544, USA
²⁷ George Mason University, 4400 University Drive, Fairfax, VA 22030, USA
²⁸ Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
²⁹ NASA Exoplanet Science Institute, Caltech/IPAC, Mail Code 100-22, 1200 E. California Blvd., Pasadena, CA 91125, USA
³⁰ MIT Kavli Institute for Astrophysics and Space Research & MIT Physics Department, 70 Vassar St, Cambridge, MA 02139, USA
³¹ Royal Astronomical Society, Burlington House, Piccadilly, London W1J 0BQ, UK
³² Department of Earth, Atmospheric, and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA 02139, USA
³³ Department of Physics and Kavli Institute for Astrophysics and Space Research, Massachusetts Institute of Technology, Cambridge, MA 02139, USA
³⁴ Department of Aeronautics and Astronautics, Massachusetts Institute of Technology, Cambridge, MA 02139, USA
³⁵ Perth Exoplanet Survey Telescope, Perth, Western Australia, Australia
³⁶ Department of Astronomy, The University of Wisconsin–Madison, Madison, WI 53706, USA
³⁷ Department of Astronomy, The University of Texas at Austin, Austin, TX 78712, USA

Received: 20 January 2023
Accepted: 12 April 2023

Abstract

We report the discovery of a hot (T_eq ≈ 1055 K) planet in the small-planet radius valley that transits the Sun-like star TOI-733. It was discovered as part of the KESPRINT follow-up program of TESS planets carried out with the HARPS spectrograph. TESS photometry from sectors 9 and 36 yields an orbital period of $P_{\text{orb}}={4.884765}_{-2.4e-5}^{+1.9e-5}$ days and a radius of $R_{\text{p}}={1.992}_{-0.090}^{+0.085}$ R_⊕. Multi-dimensional Gaussian process modelling of the radial velocity measurements from HARPS and activity indicators gives a semi-amplitude of K = 2.23 ± 0.26 m s⁻¹, translating into a planet mass of $M_{\text{p}}={5.72}_{-0.68}^{+0.70}$ M_⊕. These parameters imply that the planet is of moderate density (${\rho }_{\text{p}}={3.98}_{-0.66}^{+0.77}$ g cm⁻³) and place it in the transition region between rocky and volatile-rich planets with H/He-dominated envelopes on the mass-radius diagram. Combining these with stellar parameters and abundances, we calculated planet interior and atmosphere models, which in turn suggest that TOI-733 b has a volatile-enriched, most likely secondary outer envelope, and may represent a highly irradiated ocean world. This is one of only a few such planets around G-type stars that are well characterised.