Top: abundance and temperature–pressure profiles for a 3 M _⊕, 600 K equilibrium temperature planet, with 0.1% soot, orbiting an M-dwarf star. Even under the harsh UV irradiation environment of the host star, high abundances of methane (thick brown line) persist at high altitude and low pressure. A combination of methane photolysis in the upper atmosphere, vertical mixing, and thermochemistry gives rise to significant quantities of higher-order hydrocarbons such as C₂H₂, C₂H₄, and C₂H₆, Subsequent photolysis and polymerization reactions result in the formation of hydrocarbon haze (thick dashed red line). Bottom: the resulting transmission spectrum (orange line) of this planet is shaped considerably by haze with some strong methane features permeating through the haze at longer wavelengths. The teal line shows the transmission spectrum of the same planet but with the haze opacity artificially removed, while the transparent colored line shows these data at a spectral resolution of R = 1000.