2021A&A...646A..43W

We propose a classification of exoplanet atmospheres based on their H, C, O, and N element abundances below about 600 K. Chemical equilibrium models were run for all combinations of H, C, O, and N abundances, and three types of solutions were found, which are robust against variations of temperature, pressure, and nitrogen abundance. Type A atmospheres contain H₂O, CH₄, NH₃, and either H₂ or N₂, but only traces of CO₂ and O₂. Type B atmospheres contain O₂, H₂O, CO₂, and N₂, but only traces of CH₄, NH₃, and H₂. Type C atmospheres contain H₂O, CO₂, CH₄, and N₂, but only traces of NH₃, H₂, and O₂. Other molecules are only present in ppb or ppm concentrations in chemical equilibrium, depending on temperature. Type C atmospheres are not found in the Solar System, where atmospheres are generally cold enough for water to condense, but exoplanets may well host such atmospheres. Our models show that graphite (soot) clouds can occur in type C atmospheres in addition to water clouds, which can occur in all types of atmospheres. Full-equilibrium condensation models show that the outgassing from warm rock can naturally provide type C atmospheres. We conclude that type C atmospheres, if they exist, would lead to false positive detections of biosignatures in exoplanets when considering the coexistence of CH₄ and CO₂, and suggest other, more robust non-equilibrium markers.