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A carbon-to-oxygen ratio (C/O) of around unity is believed to act as anatural separator of water- and methane-dominated spectra when characterizingexoplanet atmospheres. In this paper we quantify the C/O ratios at which thisseparation occurs by calculating a large self-consistent grid of cloud-freeatmospheric models in chemical equilibrium, using the latest version ofpetitCODE. Our study covers a broad range of parameter space: 400K$<$T$_{eff}<$2600 K, 2.0$<$log(g)$<$5.0, -1.0$<$[Fe/H]$<$2.0,0.25$<$C/O$<$1.25, and stellar types from M to F. We make the synthetictransmission and emission spectra, as well as the temperature structurespublicly available. We find that the transition C/O ratio depends on manyparameters such as effective temperature, surface gravity, metallicity andspectral type of the host star, and could have values less, equal, or higherthan unity. By mapping all the transition C/O ratios we propose a "four-class"classification scheme for irradiated planets in this temperature range. We finda parameter space where methane always remains the cause of dominant spectralfeatures. CH$_4$ detection in this region, or the lack of it, provides adiagnostic tool to identify the prevalence of cloud formation andnon-equilibrium chemistry. As another diagnostic tool, we construct syntheticSpitzer IRAC color-diagrams showing two distinguishable populations of planets.Since most of the exoplanet atmospheres appear cloudy when studied intransmission, we regard this study as a starting point of how such aC/O-sensitive observation-based classification scheme should be constructed.This preparatory work will have to be refined by future cloudy andnon-equilibrium modeling, to further investigate the existence and exactlocation of the classes, as well as the color-diagram analysis.

From cold to hot irradiated gaseous exoplanets: Toward an observation-based classification scheme