Deciphering Sub-Neptune Atmospheres: New Insights from Geochemical Models of TOI-270 d

Abstract

The nature of sub-Neptunes is one of the hottest topics in exoplanetary science. Temperate sub-Neptunes are of special interest because some could be habitable. Here, we consider whether these planets might instead be rocky worlds with thick, hot atmospheres. Can recent JWST observations of TOI-270 d be understood in terms of such a model? We perform thermochemical equilibrium calculations to infer conditions of quenching of C-H-O-N species. Our results indicate apparent CO${}_2$-CH${}_4$ equilibrium between ~900 and ~1100 K. The CO abundance should be quenched higher in the atmosphere where the equilibrium CO/CO${}_2$ ratio is lower, potentially explaining a lack of CO. N${}_2$ is predicted to dominate the nitrogen budget. We confirm that the atmosphere of TOI-270 d is strongly enriched in both C and O${}_g$${}_a$${}_s$ relative to protosolar H, whereas N is likely to be less enriched or even depleted. We attempt to reproduce these enrichments by modeling the atmosphere as nebular gas that extracted heavy elements from accreted solids. This type of model can explain the C/H and O${}_g$${}_a$${}_s$/H ratios, but despite supersolar C/N ratios provided by solids, the NH${}_3$ abundance will probably be too high unless there is a nitrogen sink in addition to N${}_2$. A magma ocean may be implied, and indeed the oxygen fugacity of the deep atmosphere seems sufficiently low to support the sequestration of reduced N in silicate melt. The evaluation presented here demonstrates that exoplanetary geochemistry now approaches a level of sophistication comparable to that achieved within our own solar system.

arXiv commentAccepted by ApJ, 49 pages, 11 figures, 5 tables

References in corpus (64)

• The false positive rate of Kepler and the occurrence of planets
• The effects of snowlines on C/O in planetary atmospheres
• The origins and concentrations of water, carbon, nitrogen and noble gases on Earth
• A Framework for Prioritizing the TESS Planetary Candidates Most Amenable to Atmospheric Characterization
• Disequilibrium Carbon, Oxygen, and Nitrogen Chemistry in the Atmospheres of HD 189733b and HD 209458b
• Mass-Radius Relation for Rocky Planets based on PREM
• A Precise Water Abundance Measurement for the Hot Jupiter WASP-43b
• The imprint of exoplanet formation history on observable present-day spectra of hot Jupiters
• Early Release Science of the exoplanet WASP-39b with JWST NIRSpec PRISM
• A Framework for Characterizing the Atmospheres of Low-Mass Low-Density Transiting Planets
• Density, not radius, separates rocky and water-rich small planets orbiting M dwarf stars
• Compositional diversity in the atmospheres of hot Neptunes, with application to GJ 436b
• Hydrogen Greenhouse Planets Beyond the Habitable Zone
• Towards Chemical Constraints on Hot Jupiter Migration
• Three Possible Origins for the Gas Layer on GJ 1214b
• Mass-Metallicity Trends in Transiting Exoplanets from Atmospheric Abundances of H${}_2$O, Na, and K
• Methane, Carbon Monoxide, and Ammonia in Brown Dwarfs and Self-Luminous Giant Planets
• Ranges of Atmospheric Mass and Composition of Super Earth Exoplanets
• VULCAN: an Open-Source, Validated Chemical Kinetics Python Code for Exoplanetary Atmospheres
• Elemental and molecular abundances in comet 67P/Churyumov-Gerasimenko
• Chemistry of atmospheres formed during accretion of the Earth and other terrestrial planets
• Quenching of Carbon Monoxide and Methane in the Atmospheres of Cool Brown Dwarfs and Hot Jupiters
• Habitability and Biosignatures of Hycean Worlds
• Jupiter's composition: sign of a (relatively) late formation in a chemically evolved protosolar disk
• The nature and origins of sub-Neptune size planets
• Reorientation of Sputnik Planitia implies a Subsurface Ocean on Pluto
• Oxygen fugacities of extrasolar rocks: Evidence for an Earth-like geochemistry of exoplanets
• Beyond Equilibrium Temperature: How the Atmosphere/Interior Connection Affects the Onset of Methane, Ammonia, and Clouds in Warm Transiting Giant Planets
• Atmosphere Origins for Exoplanet Sub-Neptunes
• Chemical Kinetics on Extrasolar Planets
• A super-Earth and two sub-Neptunes transiting the nearby and quiet M dwarf TOI-270
• Earth shaped by primordial H${}_2$ atmospheres
• Water on Hot Rocky Exoplanets
• Vertical Tracer Mixing in Hot Jupiter Atmospheres
• Masses and compositions of three small planets orbiting the nearby M dwarf L231-32 (TOI-270) and the M dwarf radius valley
• Distinguishing oceans of water from magma on mini-Neptune K2-18b
• Probable detection of hydrogen sulphide (H${}_2$S) in Neptune's atmosphere
• Towards Consistent Modeling of Atmospheric Chemistry and Dynamics in Exoplanets: Validation and Generalization of Chemical Relaxation Method
• How to identify exoplanet surfaces using atmospheric trace species in hydrogen-dominated atmospheres
• Majority of water hides deep in the interiors of exoplanets
• Formation and dynamics of water clouds on temperate sub-Neptunes: The example of K2-18b
• Dry or water world? How the water contents of inner sub-Neptunes constrain giant planet formation and the location of the water ice line
• Hubble Space Telescope transmission spectroscopy for the temperate sub-Neptune TOI-270d: a possible hydrogen-rich atmosphere containing water vapour
• Atmospheres as windows into sub-Neptune interiors: coupled chemistry and structure of hydrogen-silane-water envelopes
• Hints of a sulfur-rich atmosphere around the 1.6 R${}_{\oplus }$ Super-Earth L98-59 d from JWST NIRSpec G395H transmission spectroscopy
• The methane distribution and polar brightening on Uranus based on HST/STIS, Keck/NIRC2, and IRTF/SpeX observations through 2015
• The erosion of large primary atmospheres typically leaves behind substantial secondary atmospheres on temperate rocky planets
• Atmospheric retrievals suggest the presence of a secondary atmosphere and possible sulfur species on L 98-59 d from JWST NIRSpec G395H transmission spectroscopy
• A 3D picture of moist-convection inhibition in hydrogen-rich atmospheres: Implications for K2-18 b
• The distribution of volatile elements during rocky planet formation
• Nitrogen as a Tracer of Giant Planet Formation. I.: A Universal Deep Adiabatic Profile and Semi-analytical Predictions of Disequilibrium Ammonia Abundances in Warm Exoplanetary Atmospheres
• The Fundamental Connections Between the Solar System and Exoplanetary Science
• Composition of Terrestrial Exoplanet Atmospheres from Meteorite Outgassing Experiments
• Primordial N2 provides a cosmochemical explanation for the existence of Sputnik Planitia, Pluto
• Helium Enhanced Planets Along the Upper Edge of the Radius Valley
• Chemical mapping of temperate sub-Neptune atmospheres: Constraining the deep-interior H2O/H2 using the atmospheric CO2/CH4
• Moderate D/H Ratios in Methane Ice on Eris and Makemake as Evidence of Hydrothermal or Metamorphic Processes in Their Interiors: Geochemical Analysis
• Super-adiabatic Temperature Gradient at Jupiter's Equatorial Zone and Implications for the Water Abundance
• Oxygen depletion in giant planets with different formation histories
• A mineralogical reason why all exoplanets cannot be equally oxidising
• Transit Timing Variations in the three-planet system: TOI-270
• Planet Mass and Metallicity: The Exoplanets and Solar System Connection
• The deep oxygen abundance in Solar System Giant Planets, with a new derivation for Saturn
• Stability of Hydrides in Sub-Neptune Exoplanets with Thick Hydrogen-Rich Atmospheres

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