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

TL;DR

This study models exoplanet atmospheres across a broad parameter space to establish a classification scheme based on C/O ratios, spectral features, and observational diagnostics, considering cloud and chemistry effects.

Contribution

It introduces a new four-class observational classification scheme for irradiated exoplanets based on C/O ratios and spectral diagnostics, incorporating extensive atmospheric modeling.

Findings

Transition C/O ratios depend on multiple parameters.

Methane presence indicates specific atmospheric conditions.

Synthetic color diagrams reveal two distinct planetary populations.

Abstract

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