Photochemistry and Spectral Characterization of Temperate and Gas-rich Exoplanets

• Published in: The Astrophysical journal 2021-11, Vol.921 (1), p.27
• Main Author: Hu, Renyu
• Format: Article
• Language: English
• Subjects: Ammonia; Astrophysics; Atmosphere; Atmospheric chemistry; Atmospheric models; Carbon dioxide; Chemical reactions; Exoplanet atmospheres; Extrasolar gaseous planets; Extrasolar ice giants; Extrasolar planets; Habitable zone; Hubble Space Telescope; Hydrocarbons; Hydrogen sulfide; Irradiance; K stars; M stars; Metallicity; Methane; Middle atmosphere; Mini Neptunes; Photochemicals; Photochemistry; Photodissociation; Photodissociation of water; Planetary atmospheres; Solar system; Space telescopes; Spectra; Transmission spectroscopy
• ISSN: 0004-637X; 1538-4357
• DOI: 10.3847/1538-4357/ac1789

Exoplanets that receive stellar irradiance approximately equal to Earth’s or less have been discovered and many are suitable for spectral characterization. Here, we focus on the temperate planets that have massive H 2 -dominated atmospheres, and trace the chemical reactions and transport following the photodissociation of H 2 O, CH 4 , NH 3 , and H 2 S, with K2-18 b, PH2 b, and Kepler-167 e representing temperate/cold planets around M and G/K stars. We find that NH 3 is likely depleted by photodissociation to the cloud deck on planets around G/K stars but remains intact in the middle atmosphere of planets around M stars. A common phenomenon on temperate planets is that the photodissociation of NH 3 in the presence of CH 4 results in HCN as the main photochemical product. The photodissociation of CH 4 together with H 2 O leads to CO and CO 2 , and the synthesis of hydrocarbon is suppressed. Temperate planets with a supersolar atmospheric metallicity and appreciable internal heat may have additional CO and CO 2 from the interior and less NH 3 , and thus less HCN. Our models of K2-18 b can explain the transmission spectrum measured by the Hubble Space Telescope, and indicate that future observations in 0.5–5.0 μ m wavelength range would provide the sensitivity to detect the equilibrium gases CH 4 , H 2 O, and NH 3 , the photochemical gas HCN, as well as CO 2 in some cases. Temperate and H 2 -rich exoplanets are thus laboratories of atmospheric chemistry that operate in regimes not found in the solar system, and spectral characterization of these planets in transit or reflected starlight promises to greatly expand the types of molecules detected in exoplanet atmospheres.