Precise mass determination for the keystone sub-Neptune planet transiting the mid-type M dwarf G 9-40

Abstract

Context. Despite being a prominent subset of the exoplanet population discovered in the past three decades, the nature and provenance of sub-Neptune-sized planets are still one of the open questions in exoplanet science. Aims. For planets orbiting bright stars, precisely measuring the orbital and planet parameters of the system is the best approach to distinguish between competing theories regarding their formation and evolution. Methods. We obtained 69 new radial velocity observations of the mid-M dwarf G 9-40 with the CARMENES instrument to measure for the first time the mass of its transiting sub-Neptune planet, G 9-40 b, discovered in data from the K2 mission. Results. Combined with new observations from the TESS mission during Sectors 44, 45, and 46, we are able to measure the radius of the planet to an uncertainty of 3.4% (Rb = 1.900 +- 0.065 Re) and determine its mass with a precision of 16% (Mb = 4.00 +- 0.63 Me). The resulting bulk density of the planet is inconsistent with a terrestrial composition and suggests the presence of either a water-rich core or a significant hydrogen-rich envelope. Conclusions. G 9-40 b is referred to as a keystone planet due to its location in period-radius space within the radius valley. Several theories offer explanations for the origin and properties of this population and this planet is a valuable target for testing the dependence of those models on stellar host mass. By virtue of its brightness and small size of the host, it joins L 98-59 d as one of the two best warm (Teq ~ 400 K) sub-Neptunes for atmospheric characterization with JWST, which will probe cloud formation in sub-Neptune-sized planets and break the degeneracies of internal composition models.

Accepted for publication in A&A

References in corpus (30)

• The generalised Lomb-Scargle periodogram. A new formalism for the floating-mean and Keplerian periodograms
• Bayes in the sky: Bayesian inference and model selection in cosmology
• Growth Model Interpretation of Planet Size Distribution
• Spectrum radial velocity analyser (SERVAL). High-precision radial velocities and two alternative spectral indicators
• Precision Stellar Characterization of FGKM Stars using an Empirical Spectral Library
• Spitzer Secondary Eclipses of the Dense, Modestly-irradiated, Giant Exoplanet HAT-P-20b Using Pixel-Level Decorrelation
• Make Super-Earths, Not Jupiters: Accreting Nebular Gas onto Solid Cores at 0.1 AU and Beyond
• Revisited Mass-Radius relations for exoplanets below 120 Earth masses
• A Sub-Neptune Exoplanet with a Low-Metallicity Methane-Depleted Atmosphere and Mie-Scattering Clouds
• The CARMENES search for exoplanets around M dwarfs: Different roads to radii and masses of the target stars
• CARMENES input catalogue of M dwarfs. I. Low-resolution spectroscopy with CAFOS
• The Gaia-Kepler Stellar Properties Catalog. II. Planet Radius Demographics as a Function of Stellar Mass and Age
• The California-Kepler Survey. X. The Radius Gap as a Function of Stellar Mass, Metallicity, and Age
• The Precision of Mass Measurements Required for Robust Atmospheric Characterization of Transiting Exoplanets
• Could we identify hot Ocean-Planets with CoRoT, Kepler and Doppler velocimetry?
• Microlensing Results Challenge the Core Accretion Runaway Growth Scenario for Gas Giants
• The CARMENES search for exoplanets around M dwarfs -- Photospheric parameters of target stars from high-resolution spectroscopy. II. Simultaneous multiwavelength range modeling of activity insensitive lines
• Rocky super-Earths or waterworlds: the interplay of planet migration, pebble accretion and disc evolution
• Transit detection of the long-period volatile-rich super-Earth ${\nu }^2$ Lupi d with $CHEOPS$
• A sub-Neptune sized planet transiting the M2.5-dwarf G 9-40: Validation with the Habitable-zone Planet Finder
• How Deep Is the Ocean? Exploring the phase structure of water-rich sub-Neptunes
• MuSCAT2 multicolour validation of TESS candidates: an ultra-short-period substellar object around an M dwarf
• Three Small Super-Earths Transiting the nearby star GJ 9827
• A Heavy Molecular Weight Atmosphere for the Super-Earth π Men c
• Haze Evolution in Temperate Exoplanet Atmospheres Through Surface Energy Measurements
• Evidence for Spin-orbit Alignment in the TRAPPIST-1 System
• The CARMENES search for exoplanets around M dwarfs. Mapping stellar activity indicators across the M dwarf domain
• Planets Across Space and Time (PAST). III. Morphology of the Planetary Radius Valley as a Function of Stellar Age and Metallicity in the Galactic Context Revealed by the LAMOST-Gaia-Kepler Sample
• Phase curve pollution of exoplanet transmission spectra
• A transiting, temperate mini-Neptune orbiting the M dwarf TOI-1759 unveiled by TESS

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