Evidence for the volatile-rich composition of a 1.5-$R_{\oplus }$ planet

Abstract

The population of planets smaller than approximately $1.7R_{\oplus }$ is widely interpreted as consisting of rocky worlds, generally referred to as super-Earths. This picture is largely corroborated by radial-velocity (RV) mass measurements for close-in super-Earths but lacks constraints at lower insolations. Here we present the results of a detailed study of the Kepler-138 system using 13 Hubble and Spitzer transit observations of the warm-temperate $1.51\pm 0.04R_{\oplus }$ planet Kepler-138 d ($T_{\mathrm{eq},{\mathrm{A}}_{\mathrm{B}}=0.3}$~350 K) combined with new Keck/HIRES RV measurements of its host star. We find evidence for a volatile-rich "water world" nature of Kepler-138 d, with a large fraction of its mass contained in a thick volatile layer. This finding is independently supported by transit timing variations, RV observations ($M_d=2.1_{-0.7}^{+0.6}{M}_{\oplus }$), as well as the flat optical/IR transmission spectrum. Quantitatively, we infer a composition of $11_{-4}^{+3}$\% volatiles by mass or ~51% by volume, with a 2000 km deep water mantle and atmosphere on top of a core with an Earth-like silicates/iron ratio. Any hypothetical hydrogen layer consistent with the observations ($<0.003M_{\oplus }$) would have swiftly been lost on a ~10 Myr timescale. The bulk composition of Kepler-138 d therefore resembles those of the icy moons rather than the terrestrial planets in the solar system. We conclude that not all super-Earth-sized planets are rocky worlds, but that volatile-rich water worlds exist in an overlapping size regime, especially at lower insolations. Finally, our photodynamical analysis also reveals that Kepler-138 c ($R_c=1.51\pm 0.04R_{\oplus }$, $M_c=2.3_{-0.5}^{+0.6}{M}_{\oplus }$) is a slightly warmer twin of Kepler-138 d, i.e., another water world in the same system, and we infer the presence of Kepler-138 e, a likely non-transiting planet at the inner edge of the habitable zone.

Published in Nature Astronomy. 4 main figures, 10 extended data figures, 13 supplementary figures. 4 tables

References in corpus (45)

• Array Programming with NumPy
• Atmospheric Escape from Hot Jupiters
• How to Constrain Your M Dwarf: measuring effective temperature, bolometric luminosity, mass, and radius
• Extreme Water Loss and Abiotic O${}_2$ Buildup On Planets Throughout the Habitable Zones of M Dwarfs
• Roche lobe effects on the atmospheric loss of "Hot Jupiters"
• Measuring the rotation period distribution of field M-dwarfs with Kepler
• Atmospheric Retrieval for Super-Earths: Uniquely Constraining the Atmospheric Composition with Transmission Spectroscopy
• Spitzer Secondary Eclipses of the Dense, Modestly-irradiated, Giant Exoplanet HAT-P-20b Using Pixel-Level Decorrelation
• The active lives of stars: a complete description of rotation and XUV evolution of F, G, K, and M dwarfs
• The Extreme Ultraviolet and X-Ray Sun in Time: High-Energy Evolutionary Tracks of a Solar-Like Star
• The Atmospheric Signatures of Super-Earths: How to Distinguish Between Hydrogen-Rich and Hydrogen-Poor Atmospheres
• 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
• Ranges of Atmospheric Mass and Composition of Super Earth Exoplanets
• Satellite dynamics on the Laplace surface
• A new equation of state for dense hydrogen-helium mixtures
• Outgassing History and Escape of the Martian Atmosphere and Water Inventory
• When Do Stalled Stars Resume Spinning Down? Advancing Gyrochronology with Ruprecht 147
• Habitable Evaporated Cores: Transforming Mini-Neptunes into Super-Earths in the Habitable Zones of M Dwarfs
• Habitability and Biosignatures of Hycean Worlds
• TTVFast: An efficient and accurate code for transit timing inversion problems
• Flare Statistics for Young Stars from a Convolutional Neural Network Analysis of $\text{TESS}$ Data
• Efficient Bayesian inference for multimodal problems in cosmology
• The interior and atmosphere of the habitable-zone exoplanet K2-18b
• The mass of the Mars-sized exoplanet Kepler-138 b from transit timing
• Atmosphere Origins for Exoplanet Sub-Neptunes
• Mass-radius relationships for irradiated ocean planets
• Spitzer Observations Confirm and Rescue the Habitable-Zone Super-Earth K2-18b for Future Characterization
• Exoplanet secondary atmosphere loss and revival
• Atmosphere Expansion and Mass Loss of Close-Orbit Giant Exoplanets heated by Stellar XUV. II. Effects of Planetary Magnetic Field, Structuring of inner Magnetosphere
• Using long-term transit timing to detect terrestrial planets
• Retention of water in terrestrial magma oceans and carbon-rich early atmospheres
• WASP-107b's density is even lower: a case study for the physics of planetary gas envelope accretion and orbital migration
• Water on Hot Rocky Exoplanets
• Vertically resolved magma ocean-protoatmosphere evolution: H${}_2$, H${}_2$O, CO${}_2$, CH${}_4$, CO, O${}_2$, and N${}_2$ as primary absorbers
• Rocky super-Earths or waterworlds: the interplay of planet migration, pebble accretion and disc evolution
• Deflating Super-Puffs: Impact of Photochemical Hazes on the Observed Mass-Radius Relationship of Low Mass Planets
• Photochemical hazes in sub-Neptunian atmospheres with focus on GJ 1214 b
• Impact of the measured parameters of exoplanets on the inferred internal structure
• Atmospheric Escape From TOI-700 d: Venus versus Earth Analogs
• The dynamics of the TRAPPIST-1 system in the context of its formation
• The HARPS search for southern extra-solar planets XLV. Two Neptune mass planets orbiting HD 13808: a study of stellar activity modelling's impact on planet detection
• The CARMENES search for exoplanets around M dwarfs. Two terrestrial planets orbiting G 264-012 and one terrestrial planet orbiting Gl 393
• Following up the Kepler field: Masses of Targets for transit timing and atmospheric characterization
• Physical Parameters of the Multi-Planet Systems HD 106315 and GJ 9827

Cited by in corpus (20)

• JWST/NIRISS reveals the water-rich "steam world" atmosphere of GJ 9827 d
• Near-Infrared Transmission Spectroscopy of HAT-P-18b with NIRISS: Disentangling Planetary and Stellar Features in the Era of JWST
• Escaping Helium and a Highly Muted Spectrum Suggest a Metal-Enriched Atmosphere on Sub-Neptune GJ3090b from JWST Transit Spectroscopy
• Shallower radius valley around low-mass hosts: Evidence for icy planets, collisions or high-energy radiation scatter
• Water absorption in the transmission spectrum of the water-world candidate GJ9827d
• When, where, and how many planets end up in first-order resonances?
• Bioverse: The Habitable Zone Inner Edge Discontinuity as an Imprint of Runaway Greenhouse Climates on Exoplanet Demographics
• Where are the Water Worlds?: Self-Consistent Models of Water-Rich Exoplanet Atmospheres
• Dynamics and Clouds in Planetary Atmospheres from Telescopic Observations
• A Mini-Neptune Orbiting the Metal-poor K Dwarf BD+29 2654
• Evolution of steam worlds: energetic aspects
• A New Definition of Exoplanet Habitability: Introducing the Photosynthetic Habitable Zone
• The Fate of Oceans on First-Generation Planets Orbiting White Dwarfs
• Is the hot, dense sub-Neptune TOI-824b an exposed Neptune mantle? Spitzer detection of the hot day side and reanalysis of the interior composition
• Possible Evidence for the Presence of Volatiles on the Warm Super-Earth TOI-270 b
• A Gap in the Densities of Small Planets Orbiting M Dwarfs: Rigorous Statistical Confirmation Using the Open-source Code RhoPop
• Exo-Geoscience Perspectives Beyond Habitability
• Tidally Heated Sub-Neptunes, Refined Planetary Compositions, and Confirmation of a Third Planet in the TOI-1266 System
• Evolution and Observable Properties of Rocky Planet Atmospheres
• Variations in the Radius Distribution of Single- and Compact Multiple-transiting Planets