TOI-4438 b: a transiting mini-Neptune amenable to atmospheric characterization

Abstract

We report the confirmation and mass determination of a mini-Neptune transiting the M3.5 V star TOI-4438 (G 182-34) every 7.44 days. A transit signal was detected with NASA's TESS space mission in the sectors 40, 52, and 53. In order to validate the planet TOI-4438 b and to determine the system properties, we combined TESS data with high-precision radial velocity measurements from the CARMENES spectrograph, spanning almost one year, and ground-based transit photometry. We found that TOI-4438 b has a radius of Rb = 2.52 +/- 0.13 R_Earth (5% precision), which together with a mass of Mb=5.4 +/- 1.1 M_Earth (20% precision), results in a bulk density of rho = 1.85+0.51-0.44 g cm-3 (28% precision), aligning the discovery with a volatile-rich planet. Our interior structure retrieval with a pure water envelope yields a minimum water mass fraction of 46% (1-sigma). TOI-4438 b is a volatile-rich mini-Neptune with likely H/He mixed with molecules, such as water, CO_2, and CH_4. The primary star has a J-band magnitude of 9.7, and the planet has a high transmission spectroscopy metric (TSM) of 136 +/- 13. Taking into account the relatively warm equilibrium temperature of T_eq = 435 +/- 15 K, and the low activity level of its host star, TOI-4438 b is one of the most promising mini-Neptunes around an M dwarf for transmission spectroscopy studies.

Accepted for publication on Astronomy & Astrophysics

References in corpus (48)

• 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 K2 Mission: Characterization and Early results
• The All-Sky Automated Survey for Supernovae (ASAS-SN) Light Curve Server v1.0
• Habitable Zones Around Main-Sequence Stars: Dependence on Planetary Mass
• VOSA: Virtual Observatory SED Analyzer. An application to the Collinder 69 open cluster
• Growth Model Interpretation of Planet Size Distribution
• Extreme Water Loss and Abiotic O${}_2$ Buildup On Planets Throughout the Habitable Zones of M Dwarfs
• Spectrum radial velocity analyser (SERVAL). High-precision radial velocities and two alternative spectral indicators
• Tidal dissipation in stars and giant planets
• PyTransit: Fast and Easy Exoplanet Transit Modelling in Python
• The CARMENES search for exoplanets around M dwarfs: Different roads to radii and masses of the target stars
• Magnetism, rotation, and nonthermal emission in cool stars -- Average magnetic field measurements in 292 M dwarfs
• The CARMENES search for exoplanets around M dwarfs -- Planet occurrence rates from a subsample of 71 stars
• The CARMENES search for exoplanets around M dwarfs. Activity indicators at visible and near-infrared wavelengths
• Sulphur dioxide in the mid-infrared transmission spectrum of WASP-39b
• The CARMENES search for exoplanets around M dwarfs. Guaranteed time observations Data Release 1 (2016-2020)
• Deflating Super-Puffs: Impact of Photochemical Hazes on the Observed Mass-Radius Relationship of Low Mass Planets
• The Empirical Limits of Gyrochronology
• Monitoring the radio emission of Proxima Centauri
• Two temperate super-Earths transiting a nearby late-type M dwarf
• Characterisation of the hydrospheres of TRAPPIST-1 planets
• Why do warm Neptunes present nonzero eccentricity?
• GJ 273: On the formation, dynamical evolution and habitability of a planetary system hosted by an M dwarf at 3.75 parsec
• Haze Evolution in Temperate Exoplanet Atmospheres Through Surface Energy Measurements
• Rocklines as Cradles for Refractory Solids in the Protosolar Nebula
• A multiple planet system of super-Earths orbiting the brightest red dwarf star GJ887
• TOI-674b: an oasis in the desert of exo-Neptunes transiting a nearby M dwarf
• The HD 260655 system: Two rocky worlds transiting a bright M dwarf at 10 pc
• A super-Earth and a mini-Neptune near the 2:1 MMR straddling the radius valley around the nearby mid-M dwarf TOI-2096
• The CARMENES search for exoplanets around M dwarfs, Wolf 1069 b: Earth-mass planet in the habitable zone of a nearby, very low-mass star
• HD 191939 revisited: New and refined planet mass determinations, and a new planet in the habitable zone
• Formation of aqua planets with water of nebular origin: Effects of water enrichment on the structure and mass of captured atmospheres of terrestrial planets
• Phase curve pollution of exoplanet transmission spectra
• TOI-1468: A system of two transiting planets, a super-Earth and a mini-Neptune, on opposite sides of the radius valley
• A transiting, temperate mini-Neptune orbiting the M dwarf TOI-1759 unveiled by TESS
• Bioverse: The Habitable Zone Inner Edge Discontinuity as an Imprint of Runaway Greenhouse Climates on Exoplanet Demographics
• The CARMENES search for exoplanets around M dwarfs. Telluric absorption corrected high S/N optical and near-infrared template spectra of 382 M dwarf stars
• Two super-Earths at the edge of the habitable zone of the nearby M dwarf TOI-2095
• TOI-969: a late-K dwarf with a hot mini-Neptune in the desert and an eccentric cold Jupiter
• An enhanced slope in the transmission spectrum of the hot Jupiter WASP-104b
• GJ 806 (TOI-4481): A bright nearby multi-planetary system with a transiting hot, low-density super-Earth
• Precise mass determination for the keystone sub-Neptune planet transiting the mid-type M dwarf G 9-40
• TOI-1801 b: A temperate mini-Neptune around a young M0.5 dwarf
• Interior-atmosphere modelling to assess the observability of rocky planets with JWST
• Two sub-Neptunes around the M dwarf TOI-1470
• Correcting the effect of stellar spots on ARIEL transmission spectra II.The limb darkening effect

Cited by in corpus (6)

• Hundreds of TESS exoplanets might be larger than we thought
• Validation of up to seven TESS planet candidates through multi-colour transit photometry using MuSCAT2 data
• The CARMENES search for exoplanets around M dwarfs. The impact of rotation and magnetic fields on the radial velocity jitter in cool stars
• 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