On the composition of young, directly imaged giant planets
- Space Science Institute, 4750 Walnut Street, Suite 205, Boulder, CO 80301 (United States)
- NASA Ames Research Center, Moffett Field, CA 94035 (United States)
- Department of Astronomy and Astrophysics, University of California, Santa Cruz, CA 95064 (United States)
- Lunar and Planetary Laboratory, University of Arizona, Tucson, AZ 85721 (United States)
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/CH{sub 4} and N{sub 2}/NH{sub 3} ratios much greater than, and H{sub 2}O mixing ratios a factor of a few less than, chemical-equilibrium predictions. Photochemistry can also be important on such planets, with CO{sub 2} and HCN being key photochemical products. Carbon dioxide becomes a major constituent when stratospheric temperatures are low and recycling of water via the H{sub 2} + 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.
- OSTI ID:
- 22868665
- Journal Information:
- Astrophysical Journal, Vol. 829, Issue 2; Other Information: Country of input: International Atomic Energy Agency (IAEA); ISSN 0004-637X
- Country of Publication:
- United States
- Language:
- English
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