2021ApJ...921...27H

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 H₂-dominated atmospheres, and trace the chemical reactions and transport following the photodissociation of H₂O, CH₄, NH₃, and H₂S, with K2-18 b, PH2 b, and Kepler-167 e representing temperate/cold planets around M and G/K stars. We find that NH₃ 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 NH₃ in the presence of CH₄ results in HCN as the main photochemical product. The photodissociation of CH₄ together with H₂O leads to CO and CO₂, and the synthesis of hydrocarbon is suppressed. Temperate planets with a supersolar atmospheric metallicity and appreciable internal heat may have additional CO and CO₂ from the interior and less NH₃, 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 µm wavelength range would provide the sensitivity to detect the equilibrium gases CH₄, H₂O, and NH₃, the photochemical gas HCN, as well as CO₂ in some cases. Temperate and H₂-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.