NIRPS and TESS reveal a peculiar system around the M dwarf TOI-756: A transiting sub-Neptune and a cold eccentric giant
Authors
Léna Parc
François Bouchy
Neil J. Cook
Nolan Grieves
Étienne Artigau
Alexandrine L'Heureux
René Doyon
Yuri S. Messias
Frédérique Baron
Susana C. C. Barros
Björn Benneke
Xavier Bonfils
Marta Bryan
Bruno L. Canto Martins
Ryan Cloutier
Nicolas B. Cowan
Daniel Brito de Freitas
Jose Renan De Medeiros
Xavier Delfosse
Elisa Delgado-Mena
Xavier Dumusque
David Ehrenreich
Pedro Figueira
Jonay I. González Hernández
David Lafrenière
Izan de Castro Leão
Christophe Lovis
Lison Malo
Claudio Melo
Lucile Mignon
Christoph Mordasini
Francesco Pepe
Rafael Rebolo
Jason Rowe
Nuno C. Santos
Damien Ségransan
Alejandro Suárez Mascareño
Stéphane Udry
Diana Valencia
Gregg Wade
Manuel Abreu
José L. A. Aguiar
Khaled Al Moulla
Guillaume Allain
Romain Allart
Jose Manuel Almenara
Tomy Arial
Hugues Auger
Luc Bazinet
Nicolas Blind
David Bohlender
Isabelle Boisse
Anne Boucher
Vincent Bourrier
Sébastien Bovay
Pedro Branco
Christopher Broeg
Denis Brousseau
Alexandre Cabral
Charles Cadieux
Andres Carmona
Yann Carteret
Zalpha Challita
David Charbonneau
Bruno Chazelas
Catherine A. Clark
João Coelho
Marion Cointepas
Karen A. Collins
Kevin I. Collins
Uriel Conod
Eduardo Cristo
Ana Rita Costa Silva
Antoine Darveau-Bernier
Laurie Dauplaise
Jean-Baptiste Delisle
Roseane de Lima Gomes
João Faria
Dasaev O. Fontinele
Thierry Forveille
Yolanda G. C. Frensch
Jonathan Gagné
Frédéric Genest
Ludovic Genolet
João Gomes da Silva
Félix Gracia Témich
Nicole Gromek
Olivier Hernandez
Melissa J. Hobson
Jens H. Hoeijmakers
Norbert Hubin
Marziye Jafariyazani
Farbod Jahandar
Ray Jayawardhana
Hans-Ulrich Käufl
Dan Kerley
Johann Kolb
Vigneshwaran Krishnamurthy
Benjamin Kung
Pierrot Lamontagne
Pierre Larue
Henry Leath
Olivia Lim
Gaspare Lo Curto
Allan M. Martins
Elisabeth C. Matthews
Jaymie Matthews
Jean-Sébastien Mayer
Stan Metchev
Lina Messamah
Leslie Moranta
Dany Mounzer
Nicola Nari
Louise D. Nielsen
Ares Osborn
Mathieu Ouellet
Jon Otegi
Luca Pasquini
Vera M. Passegger
Stefan Pelletier
Céline Peroux
Caroline Piaulet-Ghorayeb
Mykhaylo Plotnykov
Emanuela Pompei
Anne-Sophie Poulin-Girard
José Luis Rasilla
Vladimir Reshetov
Jonathan Saint-Antoine
Mirsad Sarajlic
Ivo Saviane
Robin Schnell
Alex Segovia
Julia Seidel
Armin Silber
Peter Sinclair
Michael Sordet
Danuta Sosnowska
Avidaan Srivastava
Atanas K. Stefanov
Márcio A. Teixeira
Simon Thibault
Philippe Vallée
Thomas Vandal
Valentina Vaulato
Joost P. Wardenier
Bachar Wehbe
Drew Weisserman
Ivan Wevers
François Wildi
Vincent Yariv
Gérard Zins
Abstract
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. 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. TOI-756 b is a 1.24-day period sub-Neptune with a radius of 2.81 $\pm$ 0.10 $R_\oplus$ and a mass of 9.8$^{+1.8}_{-1.6}$ $M_\oplus$. TOI-756 c is a cold eccentric (e$_c$ = 0.45 $\pm$ 0.01) giant planet orbiting with a period of 149.6 days around its star with a minimum mass of 4.05 $\pm$ 0.11 $M_\mathrm{jup}$. Additionally, a linear trend of 146$~\mathrm{m\,s}^{-1}\,\mathrm{yr}^{-1}$ is visible in the radial velocities, hinting at a third component, possibly in the planetary or brown dwarf regime. 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 $\pm$ 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.
