NIRPS and TESS reveal a peculiar system around the M dwarf TOI-756: A transiting sub-Neptune and a cold eccentric giant

Article Article scientifique Article NIRPS and TESS reveal a peculiar system around the M dwarf TOI-756: A transiting sub-Neptune and a cold eccentric giant eng 2025-10-15 2025-10 author Parc Léna Romane Elisa http://orcid.org/0000-0002-7382-1913 402592 Département d'astronomie (ASTRO) author Bouchy François http://orcid.org/0000-0002-7613-393X 133687 Département d'astronomie (ASTRO) author Grieves Nolan 930970 author Dumusque Xavier 583253 Département d'astronomie (ASTRO) author Ehrenreich David 789146 Département d'astronomie (ASTRO) author Figueira Pedro Ricardo http://orcid.org/0000-0001-8504-283X 637397 author Lovis Christophe http://orcid.org/0000-0001-7120-5837 134919 Département d'astronomie (ASTRO) author Mignon Lucile http://orcid.org/0000-0002-5407-3905 984034 author Pepe Francesco Alfonso http://orcid.org/0000-0002-9815-773X 124203 Département d'astronomie (ASTRO) author Segransan Damien http://orcid.org/0000-0003-2355-8034 134301 Département d'astronomie (ASTRO) author Udry Stéphane http://orcid.org/0000-0001-7576-6236 23409 Département d'astronomie (ASTRO) author Al Moulla Khaled Sleiman http://orcid.org/0000-0002-3212-5778 945221 Khaled Al Moulla author Blind Nicolas http://orcid.org/0000-0003-4232-1999 846350 Département d'astronomie (ASTRO) author Bourrier Vincent http://orcid.org/0000-0002-9148-034X 828250 Département d'astronomie (ASTRO) author Bovay Sébastien 822116 Département d'astronomie (ASTRO) author Carteret Yann Nicolas Philippe 876207 Département d'astronomie (ASTRO) author Chazelas Bruno 668368 Département d'astronomie (ASTRO) author Cointepas Marion http://orcid.org/0009-0001-6168-2178 934336 Département d'astronomie (ASTRO) author Conod Uriel Gwendal 646674 author Da Costa Lopes Da Silva Ana Rita http://orcid.org/0000-0003-2245-9579 418591 author Delisle Jean-Baptiste http://orcid.org/0000-0001-5844-9888 829407 Département d'astronomie (ASTRO) author Faria Joao http://orcid.org/0000-0002-6728-244X 416418 Département d'astronomie (ASTRO) author Frensch Yolanda Gerardina Cécile http://orcid.org/0000-0003-4009-0330 975671 author Genolet Ludovic 714890 Département d'astronomie (ASTRO) author Hobson Melissa 417098 Département d'astronomie (ASTRO) author Hoeijmakers Jens 885290 author Kung Benjamin 849069 author Leath Henry 978041 author De Medeiros Martins Allan 887082 author Messamah Lina Yasmine 429318 Département d'astronomie (ASTRO) author Mounzer Dany 0000-0002-8070-2058 807263 author Nielsen Louise Dyregaard 868873 author Fernandez Otegi Jon 936144 author Pelletier Stefan 417231 Département d'astronomie (ASTRO) author Schnell Robin 653488 Département d'astronomie (ASTRO) author Segovia Milla Alex Géraldo 829065 author Seidel Julia Victoria 873327 author Sordet Michaël 820881 Département d'astronomie (ASTRO) author Sosnowska Danuta 28991 Département d'astronomie (ASTRO) author Srivastava Avidaan 988422 author Vaulato Valentina 0000-0001-7329-3471 402456 Département d'astronomie (ASTRO) author Wildi François 24311 Département d'astronomie (ASTRO) 25 A138 10.1051/0004-6361/202555684 0004-6361 https://www.aanda.org/10.1051/0004-6361/202555684 Context . The Near InfraRed Planet Searcher (NIRPS) joined HARPS on the 3.6-m ESO telescope at La Silla Observatory in April 2023, dedicating part of its Guaranteed Time Observations (GTO) program to the radial velocity follow-up of TESS planet candidates to confirm and characterize transiting planets around M dwarfs. Aims . We present the “Sub-Neptunes” subprogram of the NIRPS-GTO, aimed at investigating the composition and formation of sub-Neptunes orbiting M dwarfs. We report the first results of this program with the characterization of the TOI-756 system, which consists of TOI-756 b, a transiting sub-Neptune candidate detected by TESS, as well as TOI-756 c, an additional non-transiting planet discovered by NIRPS and HARPS. Methods . We analyzed TESS and ground-based photometry, high-resolution imaging, and high-precision radial velocities (RVs) from NIRPS and HARPS to characterize the two newly discovered planets orbiting TOI-756, as well as to derive the fundamental properties of the host star. A dedicated approach was employed for the NIRPS RV extraction to mitigate telluric contamination, particularly when the star’s systemic velocity was shown to overlap with the barycentric Earth radial velocity. Results . TOI-756 is a M1V-type star with an effective temperature of T eff ~ 3657 K and a super-solar metallicity ([Fe/H]) of 0.20±0.03 dex. TOI-756 b is a 1.24-day period sub-Neptune with a radius of 2.81 ± 0.10 R ⊕ and a mass of 9.8 −1.6 +1.8 M ⊕ . TOI-756 c is a cold eccentric (e c = 0.45 ± 0.01) giant planet orbiting with a period of 149.6 days around its star with a minimum mass of 4.05 ± 0.11 M Jup . Additionally, a linear trend of 146 m s −1 yr −1 is visible in the radial velocities, hinting at a third component, possibly in the planetary or brown dwarf regime. Conclusions . We present the discovery and characterization of the transiting sub-Neptune TOI-756 b and the non-transiting eccentric cold giant TOI-756 c. This system is unique in the exoplanet landscape, standing as the first confirmed example of such a planetary architecture around an M dwarf. With a density of 2.42 ± 0.49 g cm −3 , the inner planet, TOI-756 b, is a volatile-rich sub-Neptune. Assuming a pure H/He envelope, we inferred an atmospheric mass fraction of 0.023 and a core mass fraction of 0.27, which is well constrained by stellar refractory abundances derived from NIRPS spectra. It falls within the still poorly explored radius cliff and at the lower boundary of the Neptune desert, making it a prime target for a future atmospheric characterization with JWST to improve our understanding of this population. Dewey 520 Fonds de recherche du Québec – Nature et technologies 349961 Canada Foundation for Innovation Université de Montréal Université Laval Ministère de l'Économie, de l'Innovation et de l'Énergie Natural Sciences and Engineering Research Council of Canada 589653 Canada Research Chair Program Arthur B. McDonald Fellowship Trottier Institute for Research on Exoplanets Centre de recherche en astrophysique du Québec NSERC Vanier Canada Graduate Scholarships E. Margaret Burbidge Prize Postdoctoral Fellowship Conselho Nacional de Desenvolvimento Científico e Tecnológico 305804/2022-7 Coordenação de Aperfeiçoamento de Pessoal de Nível Superior Ministerio de Ciencia, Innovación y Universidades PID2020-117493GB-I00 Ramón y Cajal Fellowship RyC2022-035854-I eSSENCE eSSENCE@LU 9:3 Swedish Research Council 2023-05307 The Crafoord Foundation The Royal Physiographic Society of Lund Light Bridges S.L Hewlett Packard Enterprise Canada Economic Development Universidade Federal do Ceará Board of Observational and Instrumental Astronomy (NAOS), Universidade Federal do Rio Grande do Norte Agence Nationale de la Recherche ANR-18-CE31-0019 Avenir program - Origin of life project Horizon 2020 research and innovation programme Fundação para a Ciência e a Tecnologia UIDB/04434/2020 NASA s3449 La Caixa Foundation LCF/BQ/DI23/11990071 Instituto de Astrofísica de Canarias Swiss National Science Foundation NCCR PlanetS: Origin, evolution and characterisation of planets (phase II) 182901 CH Programmes European Commission FInding ExoeaRths: tackling the ChallengEs of stellar activity 101052347 FIERCE EU HE 26 Département d'astronomie 12 false Astronomy & astrophysics 702 e9cff660-3961-441b-ad97-78d6e4a24aa5 Article (Published version) Public Article_scientifique_from_crossref.pdf 12149713 application/pdf CC BY-4.0 b288cf38c2fdbc9d675e1660956865c5afab1dc0c777b7945d58defbf90184bb openAccess Faculté des sciences / Département d'astronomie

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