TOI-283 b: A transiting mini-Neptune in a 17.6-day orbit discovered with TESS and ESPRESSO

¹ Instituto de Astrofísica de Canarias (IAC), E-38205 La Laguna, Tenerife, Spain
² Departamento de Astrofísica, Universidad de La Laguna (ULL), 38206 La Laguna, Tenerife, Spain
³ Observatoire Astronomique de l’Université de Genève, Chemin Pegasi 51b, 1290 Versoix, Switzerland
⁴ Lund Observatory, Division of Astrophysics, Department of Physics, Lund University, Box 118, 22100 Lund, Sweden
⁵ Instituto de Astrofísica de Andalucía (IAA-CSIC), Glorieta de la Astronomía s/n, 18008 Granada, Spain
⁶ Instituto de Astrofísica e Ciências do Espaço, Universidade do Porto, CAUP, Rua das Estrelas, 4150-762 Porto, Portugal
⁷ Departamento de Física e Astronomia, Faculdade de Ciências, Universidade do Porto, Rua do Campo Alegre, 4169-007 Porto, Portugal
⁸ Centro de Astrobiología, CSIC-INTA, Camino Bajo del Castillo s/n, 28692 Villanueva de la Cañada, Madrid, Spain
⁹ Physics Institute of University of Bern, Gesellschafts strasse 6, 3012 Bern, Switzerland
¹⁰ SOAR Telescope/NSF NOIRLab, Casilla 603, La Serena, Chile
¹¹ SETI Institute, Mountain View, CA 94043 USA/NASA Ames Research Center, Moffett Field, CA 94035, USA
¹² NASA Exoplanet Science Institute, Caltech/IPAC, Mail Code 100-22, 1200 E. California Blvd., Pasadena, CA 91125, USA
¹³ Center for Astrophysics | Harvard & Smithsonian, 60 Garden Street, Cambridge, MA 02138, USA
¹⁴ George Mason University, 4400 University Drive, Fairfax, VA 22030, USA
¹⁵ INAF – Osservatorio Astronomico di Trieste, Via Tiepolo 11, 34143 Trieste, Italy
¹⁶ NASA Ames Research Center, Moffett Field, CA 94035, USA
¹⁷ Gemini Observatory/NSF’s NOIRLab, 670 A’ohoku Place, Hilo, HI 96720, USA
¹⁸ Department of Physics and Astronomy, The University of North Carolina at Chapel Hill, Chapel Hill, NC 27599-3255, USA
¹⁹ Bay Area Environmental Research Institute, Moffett Field, CA 94035, USA
²⁰ European Southern Observatory, Av. Alonso de Cordova, 3107, Vitacura, Santiago de Chile, Chile
²¹ Centro de Astrofísica da Universidade do Porto, Rua das Estrelas, 4150-762 Porto, Portugal
²² INAF – Osservatorio Astronomico di Palermo, Piazza del Parlamento 1, 90134 Palermo, Italy
²³ Instituto de Astrofísica e Ciências do Espaço, Faculdade de Ciências da Universidade de Lisboa, Campo Grande, 1749-016, Lisboa, Portugal
²⁴ Consejo Superior de Investigaciones Científicas (CSIC), 28006 Madrid, Spain
²⁵ Department of Physics and Kavli Institute for Astrophysics and Space Research, Massachusetts Institute of Technology, Cambridge, MA 02139, USA
²⁶ Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA 02139, USA
²⁷ Department of Aeronautics and Astronautics, MIT, 77 Massachusetts Avenue, Cambridge, MA 02139, USA
²⁸ INAF – Osservatorio Astrofisico di Torino, Strada Osservatorio, 20 10025 Pino Torinese (TO), Italy
²⁹ Department of Astrophysical Sciences, Princeton University, Princeton, NJ 08544, USA
³⁰ Department of Physics, Engineering and Astronomy, Stephen F. Austin State University, 1936 North St, Nacogdoches, TX 75962, USA

^★ Corresponding author: fmurgas@iac.es

Received: 30 January 2025
Accepted: 14 September 2025

Abstract

Context. Super-Earths and mini-Neptunes are missing from our Solar System, yet they appear to be the most abundant planetary types in our Galaxy. A detailed characterization of key planets within this population is important for understanding the formation mechanisms of rocky and gas giant planets and the diversity of planetary interior structures.

Aims. In 2019, NASA’s TESS satellite found a transiting planet candidate in a 17.6-day orbit around the star TOI-283. We started radial velocity (RV) follow-up observations with ESPRESSO to obtain a mass measurement. Mass and radius are measurements critical for planetary classification and internal composition modeling.

Methods. We used ESPRESSO spectra to derive the stellar parameters of the planet candidate host star TOI-283. We then performed a joint analysis of the photometric and RV data of this star, using Gaussian processes to model the systematic noise present in both datasets.

Results. We find that the host is a bright K-type star (d = 82.4 pc, T_eff = 5213 ± 70 K, V = 10.4 mag) with a mass and radius of M_⋆ = 0.80 ± 0.01 M_⊙ and R_⋆ = 0.85 ± 0.03 R_⊙. The planet has an orbital period of P = 17.617 days, a size of R_p = 2.34 ± 0.09 R_⊕, and a mass of M_p = 6.54 ± 2.04 M_⊕. With an equilibrium temperature of ~600 K and a bulk density of ρ_p = 2.81 ± 0.93 g cm⁻³, this planet is positioned in the mass-radius diagram where planetary models predict H₂O- and H/He-rich envelopes. The ESPRESSO RV data also reveal a long-term trend that is probably related to the star’s activity cycle. Further RV observations are required to confirm whether this signal originates from stellar activity or another planetary body in the system.