An unusually low-density super-Earth transiting the bright early-type M-dwarf GJ 1018 (TOI-244)

Abstract

Small planets located at the lower mode of the bimodal radius distribution are generally assumed to be composed of iron and silicates in a proportion similar to that of the Earth. However, recent discoveries are revealing a new group of low-density planets that are inconsistent with that description. We intend to confirm and characterize the TESS planet candidate TOI-244.01, which orbits the bright ($K$ = 7.97 mag), nearby ($d$ = 22 pc), and early-type (M2.5 V) M-dwarf star GJ 1018 with an orbital period of 7.4 days. We used Markov Chain Monte Carlo methods to model 57 precise radial velocity measurements acquired by the ESPRESSO spectrograph together with TESS photometry and complementary HARPS data. We find TOI-244 b to be a super-Earth with a radius of $R_{\mathrm{p}}$ = 1.52 $\pm$ 0.12 ${\mathrm{R}}_{\oplus }$ and a mass of $M_{\mathrm{p}}$ = 2.68 $\pm$ 0.30 ${\mathrm{M}}_{\oplus }$. These values correspond to a density of $\rho$ = 4.2 $\pm$ 1.1 $\mathrm{g}\cdot \mathrm{c}{\mathrm{m}}^{-3}$, which is below what would be expected for an Earth-like composition. We find that atmospheric loss processes may have been efficient to remove a potential primordial hydrogen envelope, but high mean molecular weight volatiles such as water could have been retained. Our internal structure modeling suggests that TOI-244 b has a $479_{-96}^{+128}$ km thick hydrosphere over a 1.17 $\pm$ 0.09 ${\mathrm{R}}_{\oplus }$ solid structure composed of a Fe-rich core and a silicate-dominated mantle compatible with that of the Earth. On a population level, we find two tentative trends in the density-metallicity and density-insolation parameter space for the low-density super-Earths, which may hint at their composition. With a 8$\%$ precision in radius and 12$\%$ precision in mass, TOI-244 b is among the most precisely characterized super-Earths, which, together with the likely presence of an extended hydrosphere, makes it a key target for atmospheric observations.

Accepted for publication in A&A

References in corpus (54)

• Array Programming with NumPy
• The Astropy Project: Sustaining and Growing a Community-oriented Open-source Project and the Latest Major Release (v5.0) of the Core Package
• 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
• Bayes in the sky: Bayesian inference and model selection in cosmology
• The All-Sky Automated Survey for Supernovae (ASAS-SN) Light Curve Server v1.0
• Mass-Radius Relationships for Solid Exoplanets
• VOSA: Virtual Observatory SED Analyzer. An application to the Collinder 69 open cluster
• Growth Model Interpretation of Planet Size Distribution
• Planets and Stellar Activity: Hide and Seek in the CoRoT-7 system
• Roche lobe effects on the atmospheric loss of "Hot Jupiters"
• Planetary population synthesis coupled with atmospheric escape: a statistical view of evaporation
• Can we constrain interior structure of rocky exoplanets from mass and radius measurements?
• Density, not radius, separates rocky and water-rich small planets orbiting M dwarf stars
• Transit Least Squares: Optimized transit detection algorithm to search for periodic transits of small planets
• Inferring probabilistic stellar rotation periods using Gaussian processes
• A generalized bayesian inference method for constraining the interiors of super Earths and sub-Neptunes
• 197 Candidates and 104 Validated Planets in K2's First Five Fields
• The CARMENES search for exoplanets around M dwarfs: Different roads to radii and masses of the target stars
• The L 98-59 System: Three Transiting, Terrestrial-Sized Planets Orbiting a Nearby M-dwarf
• The mass of the Mars-sized exoplanet Kepler-138 b from transit timing
• The Precision of Mass Measurements Required for Robust Atmospheric Characterization of Transiting Exoplanets
• A giant impact as the likely origin of different twins in the Kepler-107 exoplanet system
• Revisiting Proxima with ESPRESSO
• Aeronomical constraints to the minimum mass and maximum radius of hot low-mass planets
• Warm terrestrial planet with half the mass of Venus transiting a nearby star
• Metallicity of M dwarfs IV. A high-precision [Fe/H] and Teff technique from high-resolution optical spectra for M dwarfs
• Constraining planet structure and composition from stellar chemistry: trends in different stellar populations
• Transit detection of the long-period volatile-rich super-Earth ${\nu }^2$ Lupi d with $CHEOPS$
• Empirical limb-darkening coefficients & transit parameters of known exoplanets from TESS
• Constraints on Super-Earths Interiors from Stellar Abundances
• HADES RV Programme with HARPS-N at TNG XII. The abundance signature of M dwarf stars with planets
• GJ 1252b: A Hot Terrestrial Super-Earth With No Atmosphere
• A discontinuity in the $T_{\mathrm{eff}}$-radius relation of M-dwarfs
• A precise architecture characterization of the $\pi$ Men planetary system
• Characterisation of the hydrospheres of TRAPPIST-1 planets
• Chemical Diversity of Super-Earths As a Consequence of Formation
• Two temperate Earth-mass planets orbiting the nearby star GJ1002
• GJ 1252 b: A 1.2 $R_{\oplus }$ planet transiting an M3-dwarf at 20.4 pc
• Rocklines as Cradles for Refractory Solids in the Protosolar Nebula
• Investigating the architecture and internal structure of the TOI-561 system planets with CHEOPS, HARPS-N and TESS
• Possible Internal Structures and Compositions of Proxima Centauri b
• HD 23472: A multi-planetary system with three super-Earths and two potential super-Mercuries
• Characterization of the K2-38 planetary system. Unraveling one of the densest planets known to date
• A sub-Neptune and a non-transiting Neptune-mass companion unveiled by ESPRESSO around the bright late-F dwarf HD 5278 (TOI-130)
• Masses for the seven planets in K2-32 and K2-233. Four diverse planets in resonant chain and the first young rocky worlds
• 37 New Validated Planets in Overlapping K2 Campaigns
• The Gold Standard: Accurate Stellar and Planetary Parameters for Eight Kepler M dwarf Systems Enabled by Parallaxes
• The KOBE experiment: K-dwarfs Orbited By habitable Exoplanets. Project goals, target selection and stellar characterization
• Estimating $T_{\mathrm{eff}}$, radius and luminosity of M-dwarfs using high resolution optical and NIR spectral features
• Interior-atmosphere modelling to assess the observability of rocky planets with JWST
• Three small transiting planets around the M dwarf host star LP 358-499
• The Curie line in protoplanetary disks and the formation of Mercury-like planets

Cited by in corpus (19)

• Super-Earths and Earth-like Exoplanets
• TOI-5005 b: A super-Neptune in the savanna near the ridge
• A sub-Earth-mass planet orbiting Barnard's star
• Two super-Earths at the edge of the habitable zone of the nearby M dwarf TOI-2095
• ODUSSEAS: Upgraded version with new reference scale and parameter determinations for 82 planet-host M dwarf stars in SWEET-Cat
• Three super-Earths and a possible water world from TESS and ESPRESSO
• Hundreds of TESS exoplanets might be larger than we thought
• A systematic bias in template-based RV extraction algorithms
• ATREIDES I. Embarking on a trek across the exo-Neptunian landscape with the TOI-421 system
• Searching for Hot Water World Candidates with CHEOPS: Refining the radii and analysing the internal structures and atmospheric lifetimes of TOI-238 b and TOI-1685 b
• KOBE-1: The first planetary system from the KOBE survey. Two planets likely residing in the sub-Neptune mass regime around a late K-dwarf
• Two neighbours of the ultra-short-period Earth-sized planet K2-157 b in the warm Neptunian savanna
• Tidally Heated Sub-Neptunes, Refined Planetary Compositions, and Confirmation of a Third Planet in the TOI-1266 System
• The Magnetically Induced Radial Velocity Variation of Gliese 341 and an Upper Limit to the Mass of Its Transiting Earth-sized Planet
• TOI-2322: two transiting rocky planets close to the stellar rotation period and its first harmonic
• No Robust Statistical Evidence for a Population of Water Worlds in a 2025 Sample of Planets Orbiting M Stars
• Stellar-activity analysis of the nearby M dwarf GJ 526. Multi-dimensional Gaussian-process modelling of RV, FWHM and S-index
• 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
• GJ 3929 b as the First Complete Rocky Worlds DDT Data Set