On the Composition of Young, Directly Imaged Giant Planets

Abstract

The past decade has seen significant progress on the direct detection and characterization of young, self-luminous giant planets at wide orbital separations from their host stars. Some of these planets show evidence for disequilibrium processes like transport-induced quenching in their atmospheres; photochemistry may also be important, despite the large orbital distances. These disequilibrium chemical processes can alter the expected composition, spectral behavior, thermal structure, and cooling history of the planets, and can potentially confuse determinations of bulk elemental ratios, which provide important insights into planet-formation mechanisms. Using a thermo/photochemical kinetics and transport model, we investigate the extent to which disequilibrium chemistry affects the composition and spectra of directly imaged giant exoplanets. Results for specific "young Jupiters" such as HR 8799 b and 51 Eri b are presented, as are general trends as a function of planetary effective temperature, surface gravity, incident ultraviolet flux, and strength of deep atmospheric convection. We find that quenching is very important on young Jupiters, leading to CO/CH4 and N2/NH3 ratios much greater than, and H2O mixing ratios a factor of a few less than, chemical-equilibrium predictions. Photochemistry can also be important on such planets, with CO2 and HCN being key photochemical products. Carbon dioxide becomes a major constituent when stratospheric temperatures are low and recycling of water via the H2 + OH reaction becomes kinetically stifled. Young Jupiters with effective temperatures <~ 700 K are in a particularly interesting photochemical regime that differs from both transiting hot Jupiters and our own solar-system giant planets.

Accepted in The Astrophysical Journal

References in corpus (21)

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Cited by in corpus (21)

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• Atmospheric Regimes and Trends on Exoplanets and Brown Dwarfs
• Keck/NIRC2 $L$'-Band Imaging of Jovian-Mass Accreting Protoplanets around PDS 70
• A medium-resolution spectrum of the exoplanet HIP 65426 b
• Four-of-a-kind? Comprehensive atmospheric characterisation of the HR 8799 planets with VLTI/GRAVITY
• Retrieving C and O Abundance of HR 8799 c by Combining High- and Low-Resolution Data
• The ESO SupJup Survey I: Chemical and isotopic characterisation of the late L-dwarf DENIS J0255-4700 with CRIRES${}^{+}$
• Jet Streams and Tracer Mixing in the Atmospheres of Brown Dwarfs and Isolated Young Giant Planets
• Chemical mapping of temperate sub-Neptune atmospheres: Constraining the deep-interior H2O/H2 using the atmospheric CO2/CH4
• Black hole-neutron star coalescence: effects of the neutron star spin on jet launching and dynamical ejecta mass
• JWST-TST High Contrast: JWST/NIRCam observations of the young giant planet $\beta$ Pic b
• Automated chemical reaction network generation and its application to exoplanet atmospheres
• The compositions of the HR 8799 planets reflect accretion of both solids and metal-enriched gas
• Multi-modal atmospheric characterization of $\beta$ Pictoris b: Adding high-resolution continuum spectra from GRAVITY
• The Effect of Metallicity on the Non-Equilibrium Abundance of Hydrogen Dominated Exoplanet Atmosphere
• Time-Resolved High Spectral Resolution Observation of 2MASSW J0746425+200032AB
• Experimental Investigation of the Photochemical Production of Hydrocarbons in Warm Gas Giant Exoplanet Atmospheres
• Global Chemistry and Thermal Structure Models for the Hot Jupiter WASP-43b and Predictions for JWST
• Hydrocarbon complexity and photochemical shielding of prebiotic feedstock molecules in exoplanet atmospheres
• Photochemistry and Spectral Characterization of Temperate and Gas-Rich Exoplanets