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

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${}_{-1.6}^{+1.8}$ $M_{\oplus }$. 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${\mathrm{m}\mathrm{s}}^{-1}{\mathrm{yr}}^{-1}$ 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.

arXiv comment25 pages, 15 figures, published in A&A, 2025, 702, A138. Full abstract in the article. All data used to produce the results presented in this article are publicly available at the following link: https://dace.unige.ch/openData/?record=10.82180/dace-voj8hff0

References in corpus (128)

• emcee: The MCMC Hammer
• Gaia Data Release 2. Summary of the contents and survey properties
• The Wide-field Infrared Survey Explorer (WISE): Mission Description and Initial On-orbit Performance
• Intrinsic Colors, Temperatures, and Bolometric Corrections of Pre-Main Sequence Stars
• dynesty: A Dynamic Nested Sampling Package for Estimating Bayesian Posteriors and Evidences
• The James Webb Space Telescope
• A new extensive library of PHOENIX stellar atmospheres and synthetic spectra
• Local Kinematics and the Local Standard of Rest
• New evolutionary models for pre-main sequence and main sequence low-mass stars down to the hydrogen-burning limit
• The California-Kepler Survey. III. A Gap in the Radius Distribution of Small Planets
• The PLATO 2.0 Mission
• Las Cumbres Observatory Global Telescope Network
• Planet Occurrence within 0.25 AU of Solar-type Stars from Kepler
• Exploring the Milky Way stellar disk. A detailed elemental abundance study of 714 F and G dwarf stars in the Solar neighbourhood
• batman: BAsic Transit Model cAlculatioN in Python
• Planetary Candidates Observed by Kepler, III: Analysis of the First 16 Months of Data
• Characteristics of planetary candidates observed by Kepler, II: Analysis of the first four months of data
• Kepler Presearch Data Conditioning II - A Bayesian Approach to Systematic Error Correction
• Fast and scalable Gaussian process modeling with applications to astronomical time series
• The Revised TESS Input Catalog and Candidate Target List
• Evolution of the Solar Activity over Time and Effects on Planetary Atmospheres: I. High-energy Irradiances (1-1700 A)
• Prevalence of Earth-size planets orbiting Sun-like stars
• Kepler Presearch Data Conditioning I - Architecture and Algorithms for Error Correction in Kepler Light Curves
• The evaporation valley in the Kepler planets
• How to Constrain Your M Dwarf: measuring effective temperature, bolometric luminosity, mass, and radius
• AstroImageJ: Image Processing and Photometric Extraction for Ultra-Precise Astronomical Light Curves (Expanded Edition)
• AstroImageJ: Image Processing and Photometric Extraction for Ultra-Precise Astronomical Light Curves
• Planet Migration and Binary Companions: the case of HD 80606b
• The HARPS search for southern extra-solar planets XXXI. The M-dwarf sample
• The Occurrence Rate of Small Planets around Small Stars
• VOSA: Virtual Observatory SED Analyzer. An application to the Collinder 69 open cluster
• A Framework for Prioritizing the TESS Planetary Candidates Most Amenable to Atmospheric Characterization
• Growth Model Interpretation of Planet Size Distribution
• ESPRESSO@VLT -- On-sky performance and first results
• RadVel: The Radial Velocity Modeling Toolkit
• The Role of Core Mass in Controlling Evaporation: the Kepler Radius Distribution and the Kepler-36 Density Dichotomy
• The Solar Neighborhood XVII: Parallax Results from the CTIOPI 0.9m Program -- Twenty New Members of the RECONS 10 Parsec Sample
• Towards a deterministic model of planetary formation. III. Mass distribution of short-period planets around stars of various masses
• A Steeper than Linear Disk Mass-Stellar Mass Scaling Relation
• A million binaries from Gaia eDR3: sample selection and validation of Gaia parallax uncertainties
• Core-Accretion Model Predicts Few Jovian-Mass Planets Orbiting Red Dwarfs
• Planetary evaporation by UV & X-ray radiation: basic hydrodynamics
• Mass-Radius Relation for Rocky Planets based on PREM
• Secular Chaos and the Production of Hot Jupiters
• The TESS Objects of Interest Catalog from the TESS Prime Mission
• The HARPS-TERRA project I. Description of the algorithms, performance and new measurements on a few remarkable stars observed by HARPS
• How to Constrain Your M dwarf II: the mass-luminosity-metallicity relation from 0.075 to 0.70$M_{\odot }$
• Can we constrain interior structure of rocky exoplanets from mass and radius measurements?
• Juliet: a versatile modelling tool for transiting and non-transiting exoplanetary systems
• Kepler Data Validation I -- Architecture, Diagnostic Tests, and Data Products for Vetting Transiting Planet Candidates
• A new list of thorium and argon spectral lines in the visible
• Understanding The Effects Of Stellar Multiplicity On The Derived Planet Radii From Transit Surveys: Implications for Kepler, K2, and TESS
• Origin of water in the inner Solar System: Planetesimals scattered inward during Jupiter and Saturn's rapid gas accretion
• Limb darkening and exoplanets: testing stellar model atmospheres and identifying biases in transit parameters
• Planetary nebulae seen with TESS: Discovery of new binary central star candidates from Cycle 1
• Kepler Data Validation II -- Transit Model Fitting and Multiple-Planet Search
• Revisited Mass-Radius relations for exoplanets below 120 Earth masses
• SOAR TESS Survey. I: Sculpting of TESS planetary systems by stellar companions
• Radius and Structure models for the First Super-Earth Planet
• An Increase in the Mass of Planetary Systems around Lower-Mass Stars
• Magnetic Cycles and Rotation Periods of Late Type Stars from photometric time series
• The New Generation Planetary Population Synthesis (NGPPS). IV. Planetary systems around low-mass stars
• The CARMENES search for exoplanets around M dwarfs: Different roads to radii and masses of the target stars
• A Resolved Census of Millimeter Emission from Taurus Multiple Star Systems
• Rotation periods of late-type dwarf stars from time-series high-resolution spectroscopy of chromospheric indicators
• They are Small Worlds After All: Revised Properties of Kepler M Dwarf Stars and their Planets
• Ten years of speckle interferometry at SOAR
• SPIRou: nIR velocimetry & spectropolarimetry at the CFHT
• TAPAS, a web-based service of atmospheric transmission computation for astronomy
• Evolution of the radius valley around low mass stars from $Kepler$ and $K2$
• The Solar Neighborhood XXXV: Distances to 1404 M Dwarf Systems Within 25 pc in the Southern Sky
• How formation time-scales affect the period dependence of the transition between rocky super-Earths and gaseous sub-Neptunes and implications for ${\eta }_{\oplus }$
• Primordial Circumstellar Disks in Binary Systems: Evidence for Reduced Lifetimes
• The 10 parsec sample in the Gaia era
• Impact of Binary Stars on Planet Statistics -- I. Planet Occurrence Rates, Trends with Stellar Mass, and Wide Companions to Hot Jupiter Hosts
• Metallicity of M dwarfs III. Planet-metallicity and planet-stellar mass correlations of the HARPS GTO M dwarf sample
• A remnant planetary core in the hot-Neptune desert
• Mass-radius relationships for irradiated ocean planets
• The Precision of Mass Measurements Required for Robust Atmospheric Characterization of Transiting Exoplanets
• Line-by-line velocity measurements, an outlier-resistant method for precision velocimetry
• SWEET-Cat 2.0: The Cat just got SWEETer; Higher quality spectra and precise parallaxes from GAIA eDR3
• Occurrence and core-envelope structure of 1--4x Earth-size planets around Sun-like stars
• Superabundance of Exoplanet Sub-Neptunes Explained by Fugacity Crisis
• The New Generation Planetary Population Synthesis (NGPPS). III. Warm super-Earths and cold Jupiters: A weak occurrence correlation, but with a strong architecture-composition link
• Chemical Fingerprints of Formation in Rocky Super-Earths' Data
• Formation and composition of planets around very low mass stars
• Deserts and pile-ups in the distribution of exoplanets due to photoevaporative disc clearing
• APERO: A PipelinE to Reduce Observations -- Demonstration with SPIRou
• The occurrence rate of giant planets orbiting low-mass stars with TESS
• Constraints on Super-Earths Interiors from Stellar Abundances
• High-precision stellar limb-darkening in exoplanetary transits
• Metallicity-Dependent Signatures in the Kepler Planets
• The sub-Neptune desert and its dependence on stellar type: Controlled by lifetime X-ray irradiation
• The HARPS search for southern extra-solar planets XLII. A system of Earth-mass planets around the nearby M dwarf YZ Ceti
• Majority of water hides deep in the interiors of exoplanets
• From super-Earths to sub-Neptunes: Observational constraints and connections to theoretical models
• The atmosphere and architecture of WASP-189 b probed by its CHEOPS phase curve
• A pair of Sub-Neptunes transiting the bright K-dwarf TOI-1064 characterised with CHEOPS
• A fading radius valley towards M-dwarfs, a persistent density valley across stellar types
• Bias and robustness of eccentricity estimates from radial velocity data
• The Kepler Giant Planet Search. I: A Decade of Kepler Planet-host Radial Velocities from W. M. Keck Observatory
• Dry or water world? How the water contents of inner sub-Neptunes constrain giant planet formation and the location of the water ice line
• Gaia Search for stellar Companions of TESS Objects of Interest
• ODUSSEAS: A machine learning tool to derive effective temperature and metallicity for M dwarf stars
• Mapping the exo-Neptunian landscape. A ridge between the desert and savanna
• Mid-to-Late M Dwarfs Lack Jupiter Analogs
• Stellar Companions to TESS Objects of Interest: A Test of Planet-Companion Alignment
• A Possible Alignment Between the Orbits of Planetary Systems and their Visual Binary Companions
• Giants are bullies: how their growth influences systems of inner sub-Neptunes and super-Earths
• Outer-planet scattering can gently tilt an inner planetary system
• Automatic model-based telluric correction for the ESPRESSO data reduction software. Model description and application to radial velocity computation
• TOI-269 b: An eccentric sub-Neptune transiting a M2 dwarf revisited with ExTrA
• Small Planets Around Cool Dwarfs: Enhanced Formation Efficiency of Super-Earths around M dwarfs
• Shallower radius valley around low-mass hosts: Evidence for icy planets, collisions or high-energy radiation scatter
• How does the mass and activity history of the host star affect the population of low-mass planets?
• EPIC247098361b: a transiting warm Saturn on an eccentric $P=11.2$ days orbit around a $V=9.9$ star
• NIRPS joining HARPS at ESO 3.6 m. On-sky performance and science objectives
• Revising Properties of Planet-Host Binary Systems. III. There is No Observed Radius Gap For Kepler Planets in Binary Star Systems
• TOI-969: a late-K dwarf with a hot mini-Neptune in the desert and an eccentric cold Jupiter
• Observation uncertainty effects on the precision of interior planetary parameters
• ODUSSEAS: Upgraded version with new reference scale and parameter determinations for 82 planet-host M dwarf stars in SWEET-Cat
• TOI-4010: A System of Three Large Short-Period Planets With a Massive Long-Period Companion
• Revising Properties of Planet-Host Binary Systems. IV. The Radius Distribution of Small Planets in Binary Star Systems is Dependent on Stellar Separation
• Estimating $T_{\mathrm{eff}}$, radius and luminosity of M-dwarfs using high resolution optical and NIR spectral features
• Detection and Characterisation of Giant Planets with Gaia Astrometry
• Four Small Planets Buried in K2 Systems: What Can We Learn for TESS?
• Evolution of steam worlds: energetic aspects
• Diving into the planetary system of Proxima with NIRPS -- Breaking the metre per second barrier in the infrared

Cited by in corpus (5)

• TOI-3288 b and TOI-4666 b: two gas giants transiting low-mass stars characterised by NIRPS
• TOI-4552 b: A new ultra-short period rocky world revealed by NIRPS and TESS
• Confirmation of the hot super-Neptune TOI-672 b with NIRPS and HARPS and Insights into the Neptunian desert around M dwarfs
• Densities of small planets around the M dwarfs TOI-4336 A and TOI-4342 with ESPRESSO: Three sub-Neptunes, one super-Earth, and a Neptune-mass candidate
• NIRPS tightens the mass estimate of GJ 3090 b and detects a planet near the stellar rotation period