Characterisation of TOI-406 as showcase of the THIRSTEE program: A 2-planet system straddling the M-dwarf density gap

Abstract

The exoplanet sub-Neptune population currently poses a conundrum, as to whether small-size planets are volatile-rich cores without an atmosphere, or rocky cores surrounded by a H-He envelope. To test the different hypotheses from an observational point of view, a large sample of small-size planets with precise mass and radius measurements is the first step. On top of that, much more information will likely be needed, including atmospheric characterisation and a demographic perspective on their bulk properties. We present here the concept and strategy of the THIRSTEE project, which aims to shed light on the composition of the sub-Neptune population across stellar types by increasing their number and improving the accuracy of bulk density measurements, as well as investigating their atmospheres and performing statistical, demographic analysis. We report the first results of the program, characterising a new two-planet system around the M-dwarf TOI-406. We analyse TESS and ground-based photometry, together with ESPRESSO and NIRPS/HARPS RVs to derive the orbital parameters and investigate the internal composition of the 2 planets orbiting TOI-406, which have radii and masses of $R_c=1.32\pm 0.12R_{\oplus }$, $M_c=2.08_{-0.22}^{+0.23}{M}_{\oplus }$ and $R_b=2.08_{-0.15}^{+0.16}{R}_{\oplus }$, $M_b=6.57_{-0.90}^{+1.00}{M}_{\oplus }$, and periods of $3.3$ and $13.2$ days, respectively. Planet c is consistent with an Earth-like composition, while planet b is compatible with multiple internal composition models, including volatile-rich planets without H/He atmospheres. The 2 planets are located in 2 distinct regions in the mass-density diagram, supporting the existence of a density gap among small exoplanets around M dwarfs. With an T${}_{\mathrm{eq}}$ of only 368 K, TOI-406 b stands up as a particularly interesting target for atmospheric characterisation with JWST in the low-temperature regime.

22 pages, 19 figures. Accepted for publication in A&A. Please note: Planet lettering changed with respect to submitted version

References in corpus (85)

• The Gaia mission
• Gaia Data Release 3: Summary of the content and survey properties
• The Transiting Exoplanet Survey Satellite
• The generalised Lomb-Scargle periodogram. A new formalism for the floating-mean and Keplerian periodograms
• The All-Sky Automated Survey for Supernovae (ASAS-SN) Light Curve Server v1.0
• Most 1.6 Earth-Radius Planets are not Rocky
• EXOFAST: A fast exoplanetary fitting suite in IDL
• VOSA: Virtual Observatory SED Analyzer. An application to the Collinder 69 open cluster
• Growth Model Interpretation of Planet Size Distribution
• ESPRESSO@VLT -- On-sky performance and first results
• Spectrum radial velocity analyser (SERVAL). High-precision radial velocities and two alternative spectral indicators
• A new list of thorium and argon spectral lines in the visible
• Density, not radius, separates rocky and water-rich small planets orbiting M dwarf stars
• Make Super-Earths, Not Jupiters: Accreting Nebular Gas onto Solid Cores at 0.1 AU and Beyond
• Transit Least Squares: Optimized transit detection algorithm to search for periodic transits of small planets
• Revisited Mass-Radius relations for exoplanets below 120 Earth masses
• PyTransit: Fast and Easy Exoplanet Transit Modelling in Python
• 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
• A Guide to Realistic Uncertainties on Fundamental Properties of Solar-Type Exoplanet Host Stars
• The CARMENES search for exoplanets around M dwarfs: Different roads to radii and masses of the target stars
• The Gaia-Kepler Stellar Properties Catalog. II. Planet Radius Demographics as a Function of Stellar Mass and Age
• SPIRou: nIR velocimetry & spectropolarimetry at the CFHT
• Stellar chromospheric activity of 1,674 FGK stars from the AMBRE-HARPS sample I. A catalogue of homogeneous chromospheric activity
• The California-Kepler Survey. X. The Radius Gap as a Function of Stellar Mass, Metallicity, and Age
• Irradiated ocean planets bridge super-Earth and sub-Neptune populations
• Mass-radius relationships for irradiated ocean planets
• The Precision of Mass Measurements Required for Robust Atmospheric Characterization of Transiting Exoplanets
• Cold Jupiters and improved masses in 38 Kepler and K2 small planet systems from 3661 HARPS-N radial velocities. No excess of cold Jupiters in small planet systems
• A secondary atmosphere on the rocky exoplanet 55 Cancri e
• The CARMENES search for exoplanets around M dwarfs. Stellar atmospheric parameters of target stars with SteParSyn
• Line-by-line velocity measurements, an outlier-resistant method for precision velocimetry
• Conclusive evidence for a population of water-worlds around M-dwarfs remains elusive
• Occurrence and core-envelope structure of 1--4x Earth-size planets around Sun-like stars
• Superabundance of Exoplanet Sub-Neptunes Explained by Fugacity Crisis
• From planetesimals to planets: volatile molecules
• APERO: A PipelinE to Reduce Observations -- Demonstration with SPIRou
• JWST/NIRISS reveals the water-rich "steam world" atmosphere of GJ 9827 d
• Rocky super-Earths or waterworlds: the interplay of planet migration, pebble accretion and disc evolution
• Radial Velocity Data Analysis with Compressed Sensing Techniques
• The Exoplanet Radius Valley from Gas-driven Planet Migration and Breaking of Resonant Chains
• A new perspective on interiors of ice-rich planets: Ice-rock mixture instead of ice on top of rock
• The HARPS search for southern extra-solar planets XLII. A system of Earth-mass planets around the nearby M dwarf YZ Ceti
• Unveiling shrouded oceans on temperate sub-Neptunes via transit signatures of solubility equilibria vs. gas thermochemistry
• Majority of water hides deep in the interiors of exoplanets
• From super-Earths to sub-Neptunes: Observational constraints and connections to theoretical models
• Creating the Radius Gap without Mass Loss
• Clouds and Clarity: Revisiting Atmospheric Feature Trends in Neptune-size Exoplanets
• A fading radius valley towards M-dwarfs, a persistent density valley across stellar types
• SteParSyn: A Bayesian code to infer stellar atmospheric parameters using spectral synthesis
• The TESS-Keck Survey XX: 15 New TESS Planets and a Uniform RV Analysis of all Survey Targets
• TOI-1452 b: SPIRou and TESS reveal a super-Earth in a temperate orbit transiting an M4 dwarf
• Pebbles versus planetesimals: the outcomes of population synthesis models
• Haze Evolution in Temperate Exoplanet Atmospheres Through Surface Energy Measurements
• A planetary system with two transiting mini-Neptunes near the radius valley transition around the bright M dwarf TOI-776
• Hubble Space Telescope transmission spectroscopy for the temperate sub-Neptune TOI-270d: a possible hydrogen-rich atmosphere containing water vapour
• TOI-1235 b: a keystone super-Earth for testing radius valley emergence models around early M dwarfs
• TOI-431/HIP 26013: a super-Earth and a sub-Neptune transiting a bright, early K dwarf, with a third RV planet
• K2-66b and K2-106b: Two extremely hot sub-Neptune-size planets with high densities
• Investigating the architecture and internal structure of the TOI-561 system planets with CHEOPS, HARPS-N and TESS
• The Magellan-TESS Survey I: Survey Description and Mid-Survey Results
• TOI-561 b: A Low Density Ultra-Short Period "Rocky" Planet around a Metal-Poor Star
• The TESS-Keck Survey: Science Goals and Target Selection
• Super-Earths and Earth-like Exoplanets
• The Reversibility Error Method (REM): a new, dynamical fast indicator for planetary dynamics
• The K2-3 system revisited: testing photoevaporation and core-powered mass loss with three small planets spanning the radius valley
• The CARMENES search for exoplanets around M dwarfs: Benchmarking the impact of activity in high-precision radial velocity measurements
• Detection Limits of Low-mass, Long-period Exoplanets Using Gaussian Processes Applied to HARPS-N Solar RVs
• The PEPSI Exoplanet Transit Survey (PETS) I: Investigating the presence of a silicate atmosphere on the super-Earth 55 Cnc e
• The K2-OjOS Project: New and revisited planets and candidates in K2 campaigns 5, 16, & 18
• TOI-836: A super-Earth and mini-Neptune transiting a nearby K-dwarf
• Long Period Jovian Tilts the Orbits of Two sub-Neptunes Relative to Stellar Spin Axis in Kepler-129
• A HARPS-N mass for the elusive Kepler-37d: a case study in disentangling stellar activity and planetary signals
• TOI-1695 b: A Water World Orbiting an Early M Dwarf in the Planet Radius Valley
• Revisiting K2-233 spectroscopic time-series with multidimensional Gaussian Processes
• Detection and characterization of an ultra-dense sub-Neptune planet orbiting the Sun-like star HD 119130
• TESS Habitable Zone Star Catalog
• The TESS-Keck Survey. XV. Precise Properties of 108 TESS Planets and Their Host Stars
• Three planets transiting the evolved star EPIC 249893012: a hot 8.8-M${}_{\oplus }$ super-Earth and two warm 14.7 and 10.2-M${}_{\oplus }$ sub-Neptunes
• Unified simulations of planetary formation and atmospheric evolution II: Rapid disk clearing by photoevaporation yields low-mass super-Earth atmospheres
• Precise mass determination for the keystone sub-Neptune planet transiting the mid-type M dwarf G 9-40
• The origins of nearly coplanar, non-resonant systems of close-in super-Earths
• Another super-dense sub-Neptune in K2-182 b and refined mass measurements for K2-199 b and c
• CHEOPS and TESS view of the ultra-short period super-Earth TOI-561 b
• A Gap in the Densities of Small Planets Orbiting M Dwarfs: Rigorous Statistical Confirmation Using the Open-source Code RhoPop

Cited by in corpus (8)

• NIRPS joining HARPS at ESO 3.6 m. On-sky performance and science objectives
• Two neighbours of the ultra-short-period Earth-sized planet K2-157 b in the warm Neptunian savanna
• A transiting rocky super-Earth and a non-transiting sub-Neptune orbiting the M dwarf TOI-771
• No Robust Statistical Evidence for a Population of Water Worlds in a 2025 Sample of Planets Orbiting M Stars
• Discovery of a transiting hot water-world candidate orbiting Ross 176 with TESS and CARMENES
• Evolution and Observable Properties of Rocky Planet Atmospheres
• Sibling Sub-Neptunes Around Sibling M Dwarfs: TOI-521 and TOI-912
• GJ 3929 b as the First Complete Rocky Worlds DDT Data Set