Confronting compositional confusion through the characterisation of the sub-Neptune orbiting HD 77946

Abstract

We report on the detailed characterization of the HD 77946 planetary system. HD 77946 is an F5 ($M_{\ast }$ = 1.17 M${}_{\odot }$, $R_{\ast }$ = 1.31 R${}_{\odot }$) star, which hosts a transiting planet recently discovered by NASA's Transiting Exoplanet Survey Satellite (TESS), classified as TOI-1778 b. Using TESS photometry, high-resolution spectroscopic data from HARPS-N, and photometry from CHEOPS, we measure the radius and mass from the transit and RV observations, and find that the planet, HD 77946 b, orbits with period $P_{\mathrm{b}}$ = $6.527282_{-0.000020}^{+0.000015}$ d, has a mass of $M_{\mathrm{b}}=8.38\pm 1.32$M${}_{\oplus }$, and a radius of $R_{\mathrm{b}}=2.705_{-0.081}^{+0.086}$R${}_{\oplus }$. From the combination of mass and radius measurements, and the stellar chemical composition, the planet properties suggest that HD 77946 b is a sub-Neptune with a $\sim$1\% H/He atmosphere. However, a degeneracy still exists between water-world and silicate/iron-hydrogen models, and even though interior structure modelling of this planet favours a sub-Neptune with a H/He layer that makes up a significant fraction of its radius, a water-world composition cannot be ruled out, as with $T_{\mathrm{eq}}=1248_{-38}^{+40}$K, water may be in a supercritical state. The characterisation of HD 77946 b, adding to the small sample of well-characterised sub-Neptunes, is an important step forwards on our journey to understanding planetary formation and evolution pathways. Furthermore, HD 77946 b has one of the highest transmission spectroscopic metrics for small planets orbiting hot stars, thus transmission spectroscopy of this key planet could prove vital for constraining the compositional confusion that currently surrounds small exoplanets.

19 pages, 13 figures, 5 tables. Published in MNRAS

References in corpus (63)

• The chemical composition of the Sun
• The Wide-field Infrared Survey Explorer (WISE): Mission Description and Initial On-orbit Performance
• Gaia Data Release 3: Summary of the content and survey properties
• dynesty: A Dynamic Nested Sampling Package for Estimating Bayesian Posteriors and Evidences
• The Dartmouth Stellar Evolution Database
• The California-Kepler Survey. III. A Gap in the Radius Distribution of Small Planets
• Improved Age Estimation for Solar-Type Dwarfs Using Activity-Rotation Diagnostics
• Bayes in the sky: Bayesian inference and model selection in cosmology
• Exploring the Milky Way stellar disk. A detailed elemental abundance study of 714 F and G dwarf stars in the Solar neighbourhood
• Gaia Early Data Release 3: Parallax bias versus magnitude, colour, and position
• Elemental Abundance Survey of The Galactic Thick Disk
• The TESS Input Catalog and Candidate Target List
• A Framework for Prioritizing the TESS Planetary Candidates Most Amenable to Atmospheric Characterization
• Growth Model Interpretation of Planet Size Distribution
• Planets and Stellar Activity: Hide and Seek in the CoRoT-7 system
• An asteroseismic view of the radius valley: stripped cores, not born rocky
• Core-powered mass loss and the radius distribution of small exoplanets
• A Gaussian process framework for modelling stellar activity signals in radial velocity data
• Sculpting the Valley in the Radius Distribution of Small Exoplanets as a by-product of Planet Formation: The Core-Powered Mass-Loss Mechanism
• Can we constrain interior structure of rocky exoplanets from mass and radius measurements?
• A Framework for Quantifying the Degeneracies of Exoplanet Interior Compositions
• Three regimes of extrasolar planets inferred from host star metallicities
• Density, not radius, separates rocky and water-rich small planets orbiting M dwarf stars
• The CHEOPS mission
• Ocean Planet or Thick Atmosphere: On the Mass-Radius Relationship for Solid Exoplanets with Massive Atmospheres
• Gaia Early Data Release 3: Summary of the contents and survey properties
• A generalized bayesian inference method for constraining the interiors of super Earths and sub-Neptunes
• Six transiting planets and a chain of Laplace resonances in TOI-178
• AQUA: A Collection of H${}_2$O Equations of State for Planetary Models
• Three years of Sun-as-a-star radial-velocity observations on the approach to solar minimum
• The Mount Wilson Observatory S-index of the Sun
• The GAPS programme with HARPS-N at TNG XI. Pr~0211 in M~44: the first multi-planet system in an open cluster
• The California-Kepler Survey. X. The Radius Gap as a Function of Stellar Mass, Metallicity, and Age
• Expected performances of the Characterising Exoplanet Satellite (CHEOPS) III. Data reduction pipeline: architecture and simulated performances
• A candidate short-period sub-Earth orbiting Proxima Centauri
• A new ab initio equation of state of hcp-Fe and its implication on the interior structure and mass-radius relations of rocky super-Earths
• Warm terrestrial planet with half the mass of Venus transiting a nearby star
• Conclusive evidence for a population of water-worlds around M-dwarfs remains elusive
• Correcting the spectroscopic surface gravity using transits and asteroseismology. No significant effect on temperatures or metallicities with ARES+MOOG in LTE
• Separating planetary reflex Doppler shifts from stellar variability in the wavelength domain
• From planetesimals to planets: volatile molecules
• ARES+MOOG - a practical overview of an EW method to derive stellar parameters
• Characterisation of the radial velocity signal induced by rotation in late-type dwarfs
• Analysis of Early Science observations with the CHaracterising ExOPlanets Satellite (CHEOPS) using pycheops
• A deep radius valley revealed by Kepler short cadence observations
• Irradiation-driven escape of primordial planetary atmospheres II. Evaporation efficiency of sub-Neptunes through hot Jupiters
• A pair of Sub-Neptunes transiting the bright K-dwarf TOI-1064 characterised with CHEOPS
• Evolution of the Exoplanet Size Distribution: Forming Large Super-Earths Over Billions of Years
• The CARMENES search for exoplanets around M dwarfs. Three temperate to warm super-Earths
• Coupling thermal evolution of planets and hydrodynamic atmospheric escape in MESA
• Keck/NIRSPEC studies of He I in the atmospheres of two inflated hot gas giants orbiting K dwarfs: WASP-52b and WASP-177b
• K2-111: an old system with two planets in near-resonance
• Uncovering the true periods of the young sub-Neptunes orbiting TOI-2076
• Investigating the architecture and internal structure of the TOI-561 system planets with CHEOPS, HARPS-N and TESS
• New Perspectives on the Exoplanet Radius Gap from a Mathematica Tool and Visualized Water Equation of State
• The mass-radius relation of intermediate-mass planets outlined by hydrodynamic escape and thermal evolution
• Sub-m s${}^{-1}$ upper limits from a deep HARPS-N radial-velocity search for planets orbiting HD 166620 and HD 144579
• A HARPS-N mass for the elusive Kepler-37d: a case study in disentangling stellar activity and planetary signals
• The HD 93963 A transiting system: A 1.04d super-Earth and a 3.65 d sub-Neptune discovered by TESS and CHEOPS
• Non-detection of Helium in the Hot Jupiter WASP-48b
• Evolution of the Velocity Ellipsoids in the Thin Disk of the Galaxy and the Radial Migration of Stars
• K2-79b and K2-222b: Mass measurements of two small exoplanets with periods beyond 10 days that overlap with periodic magnetic activity signals
• Identification of the Top TESS Objects of Interest for Atmospheric Characterization of Transiting Exoplanets with JWST