  Exoplanets that receive stellar irradiance of approximately 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
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 super-solar
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 Hubble, and indicate that
future observations in 0.5-5.0 um 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.
