# The carbon-to-oxygen ratio: implications for the spectra of hydrogen-dominated exoplanet atmospheres

## TL;DR

This study explores how varying the carbon-to-oxygen ratio affects the thermal, chemical, and spectral properties of hydrogen-dominated exoplanet atmospheres, highlighting the importance of individual element abundances for accurate modeling.

## Contribution

It demonstrates that individual element abundances significantly influence atmospheric properties and spectra, emphasizing the need to consider full elemental compositions rather than just ratios.

## Key findings

- Differences in chemical composition affect spectra observably.
- Current and future telescopes can detect these compositional effects.
- Full elemental abundances are crucial for accurate atmospheric modeling.

## Abstract

We present results from one-dimensional atmospheric simulations investigating the effect of varying the carbon-to-oxygen (C/O) ratio on the thermal structure, chemical composition and transmission and emission spectra, for irradiated hydrogen-dominated atmospheres. We find that each of these properties of the atmosphere are strongly dependent on the individual abundances (relative to hydrogen) of carbon and oxygen. We confirm previous findings that different chemical equilibrium compositions result from different sets of element abundances but with the same C/O ratio. We investigate the effect of this difference in composition on the thermal structure and simulated spectra. We also simulate observations using the PandExo tool and show that these differences are observationally significant with current (i.e. Hubble Space Telescope) and future (i.e. James Webb Space Telescope) instruments. We conclude that it is important to consider the full set of individual element abundances, with respect to hydrogen, rather than the ratios of only two elements, such as the C/O ratio, particularly when comparing model predictions with observed transmission and emission spectra.