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We present a comprehensive study of the abundance of carbon dioxide inexoplanetary atmospheres in hot, hydrogen-dominated atmospheres. We constructnovel analytical models of systems in chemical equilibrium that include carbonmonoxide, carbon dioxide, water, methane and acetylene and relate theequilibrium constants of the chemical reactions to temperature and pressure viathe tabulated Gibbs free energies. We prove that such chemical systems may bedescribed by a quintic equation for the mixing ratio of methane. By examiningthe abundances of these molecules across a broad range of temperatures(spanning equilibrium temperatures from 600 to 2500 K), pressures (viatemperature-pressure profiles that explore albedo and opacity variations) andcarbon-to-oxygen ratios, we conclude that carbon dioxide is subdominantcompared to carbon monoxide and water. Atmospheric mixing does not alter thisconclusion if carbon dioxide is subdominant everywhere in the atmosphere.Carbon dioxide and carbon monoxide may attain comparable abundances if themetallicity is greatly enhanced, but this property is negated by temperaturesabove 1000 K. For hydrogen-dominated atmospheres, our generic result has theimplication that retrieval studies may wish to set the subdominance of carbondioxide as a prior of the calculation and not let its abundance completely roamfree as a fitting parameter, because it directly affects the inferred value ofthe carbon-to-oxygen ratio and may produce unphysical conclusions. We discussthe relevance of these implications for the hot Jupiter WASP-12b and suggestthat some of the previous results are chemically impossible. The relativeabundance of carbon dioxide to acetylene is potentially a sensitive diagnosticof the carbon-to-oxygen ratio.

Carbon Dioxide in Exoplanetary Atmospheres: Rarely Dominant Compared to Carbon Monoxide and Water in Hot, Hydrogen-dominated Atmospheres