TOI-3288 b and TOI-4666 b: Two gas giants transiting low-mass stars characterised by NIRPS

¹ Observatoire de Genève, Département d’Astronomie, Université de Genève, Chemin Pegasi 51, 1290 Versoix, Switzerland
² European Southern Observatory (ESO), Av. Alonso de Cordova 3107, Casilla 19001, Santiago de Chile, Chile
³ Institut Trottier de recherche sur les exoplanètes, Département de Physique, Université de Montréal, Montréal, Québec, Canada
⁴ Departamento de Física Teórica e Experimental, Universidade Federal do Rio Grande do Norte, Campus Universitário, Natal, RN 59072-970, Brazil
⁵ Instituto de Astrofísica e Ciências do Espaço, Universidade do Porto, CAUP, Rua das Estrelas, 4150-762 Porto, Portugal
⁶ Observatoire du Mont-Mégantic, Québec, Canada
⁷ Univ. Grenoble Alpes, CNRS, IPAG, 38000 Grenoble, France
⁸ Centro de Astrobiología (CAB), CSIC-INTA, Camino Bajo del Castillo s/n, 28692 Villanueva de la Cañada (Madrid), Spain
⁹ Light Bridges S.L., Observatorio del Teide, Carretera del Observatorio, s/n Guimar, 38500 Tenerife, Canarias, Spain
¹⁰ Instituto de Astrofísica de Canarias (IAC), Calle Vía Láctea s/n, 38205 La Laguna, Tenerife, Spain
¹¹ Departamento de Astrofísica, Universidad de La Laguna (ULL), 38206 La Laguna, Tenerife, Spain
¹² Departamento de Física e Astronomia, Faculdade de Ciências, Universidade do Porto, Rua do Campo Alegre, 4169-007 Porto, Portugal
¹³ Department of Earth, Planetary, and Space Sciences, University of California, Los Angeles, CA 90095, USA
¹⁴ Department of Physics, University of Toronto, Toronto, ON M5S 3H4, Canada
¹⁵ Department of Physics & Astronomy, McMaster University, 1280 Main St W, Hamilton, ON L8S 4L8, Canada
¹⁶ Department of Physics, McGill University, 3600 rue University, Montréal, QC H3A 2T8, Canada
¹⁷ Department of Earth & Planetary Sciences, McGill University, 3450 rue University, Montréal, QC H3A 0E8, Canada
¹⁸ Centre Vie dans 1’Univers, Faculté des sciences de l’Université de Genève, Quai Ernest-Ansermet 30, 1205 Geneva, Switzerland
¹⁹ European Southern Observatory (ESO), Karl-Schwarzschild-Str. 2, 85748 Garching bei München, Germany
²⁰ Space Research and Planetary Sciences, Physics Institute, University of Bern, Gesellschaftsstrasse 6, 3012 Bern, Switzerland
²¹ Consejo Superior de Investigaciones Científicas (CSIC), 28006 Madrid, Spain
²² Bishop’s University, Dept of Physics and Astronomy, Johnson-104E, 2600 College Street, Sherbrooke, QC J1M 1Z7, Canada
²³ Department of Physics, Engineering Physics, and Astronomy, Queen’s University, 99 University Avenue, Kingston, ON K7L 3N6, Canada
²⁴ Department of Physics and Space Science, Royal Military College of Canada, 13 General Crerar Cres., Kingston, ON K7P 2M3, Canada
²⁵ Astrobiology Research Unit, Université de Liège, 19C Allée du 6 Août, 4000 Liège, Belgium
²⁶ Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA 02139, USA
²⁷ Center for astrophysics | Harvard & Smithsonian, 60 Garden Street, Cambridge, MA 02138, USA
²⁸ Caltech/IPAC, Mail Code 100-22, Pasadena, CA 91125, USA
²⁹ Department of Astronomy, Westlake University, Hangzhou 310030, Zhejiang Province, PR China
³⁰ NASA Ames Research Center, Moffett Field, CA 94035, USA
³¹ University Observatory, Faculty of Physics, Ludwig-Maximilians-Universität München, Scheinerstr. 1, 81679 Munich, Germany
³² Department of Astronomy & Astrophysics, University of Chicago, 5640 South Ellis Avenue, Chicago, IL 60637, USA
³³ Department of Physics and Astronomy, Vanderbilt University, VU Station 1807, Nashville, TN 37235, USA
³⁴ SETI Institute, Mountain View, CA 94043, USA NASA Ames Research Center, Moffett Field, CA 94035, USA

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Received: 10 October 2025
Accepted: 20 December 2025

Abstract

Context. Gas giant planets orbiting low-mass stars (T_eff ≲ 4600 K) are uncommon outcomes of planet formation. Increasing the sample of well-characterised giants around early M dwarfs will enable population-level studies of their properties, offering valuable insights into their formation and evolutionary histories.

Aims. We aim to confirm and characterise giant exoplanets transiting M dwarfs identified by the TESS mission. To this end, we have started the Gas giAnts Transiting 1Ow-mass Stars (GATOS) programme within the NIRPS guaranteed time observations (GTO).

Methods. High-resolution spectroscopic data were obtained in the optical and near-infrared (nIR), combining HARPS and NIRPS. We derived radial velocities (RVs) via the cross-correlation function and implemented a novel post-processing procedure to further mitigate telluric contamination in the nIR. The resulting RVs were jointly fit with TESS and ground-based photometry to derive the orbital and physical parameters of the systems.

Results. We present the GATOS programme and its first results. We confirm two gas giants transiting the low-mass stars TOI-3288 A (K9V, T_eff = 3933 ± 48 K) and TOI-4666 (M2.5V, T_eff = 3512 ± 36 K). TOI-3288 A hosts a hot Jupiter with a mass of 2.11 ± 0.08 M_Jup and a radius of 1.00 ± 0.03 R_Jup, with an orbital period of 1.43 days (T_eq = 1059 ± 20 K). TOI-4666 hosts a 0.70 ± 0.06 M_Jup warm Jupiter (T_eq = 713 ± 14 K) with a radius of 1.11 ± 0.04 R_Jup, with an orbital period of 2.91 days. At a population level, we identify a decrease in planetary mass with spectral type, whereby late M dwarfs host less massive giant planets than early M dwarfs. More massive gas giants that deviate from this trend are preferentially hosted by more metal-rich stars. Furthermore, we find an increased binarity fraction among low-mass stars hosting gas giants, which may play a role in enhancing giant planet formation around low-mass stars.

Conclusions. These mass characterisations contribute to the growing catalogue of well-defined giant exoplanets around low-mass stars. The observed population trends agree with theoretical predictions, whereby higher metallicity can compensate for lower disc masses, and wide binary systems may influence planet formation and migration through Kozai–Lidov cycles or disc instabilities.