Chemical diversity of the atmospheres and interiors of sub-Neptunes: a case study of GJ 436 b

TL;DR

This study explores the chemical diversity of sub-Neptune atmospheres, using GJ 436 b as a case, highlighting the importance of self-consistent atmosphere-interior models and the limitations of current spectral fits.

Contribution

It introduces a combined atmosphere-interior modeling approach for sub-Neptunes and assesses the constraints from spectral data, emphasizing the need for self-consistent analysis with JWST.

Findings

High-metallicity models fit GJ 436 b's spectrum but are inconsistent with interior structures.

Current models cannot adequately fit the 4.5-micron Spitzer measurement, likely due to disequilibrium chemistry.

Emission and transmission spectra provide different constraints on atmospheric composition.

Abstract

The atmospheres of sub-Neptunes are expected to exhibit considerable chemical diversity, beyond what is anticipated for gas-giant exoplanets. In the current study, we construct self-consistent radiative transfer and equilibrium chemistry models to explore this chemical diversity. We use GJ 436 b as a case study to further study joint atmosphere-interior models. In particular, we constrain the properties of the interior and atmosphere of the planet based on the available Spitzer measurements. While it is possible to fit the emission spectrum of GJ 436 b using a high-metallicity model, we demonstrate that such an atmosphere is inconsistent with physically plausible interior structures. It remains the case that no existing study can adequately fit the 4.5-micron Spitzer secondary eclipse measurement, which is probably caused by chemical disequilibrium. Finally, an information content analysis reveals that emission and transmission spectra constrain the carbon-to-oxygen ratio and metallicity at different wavelengths, but the former are less susceptible to flat spectra stemming from highly metal-enriched atmospheres. With the recently-launched JWST, we recommend that future analysis of emission and transmission spectra of sub-Neptune planets are carried out self-consistently using both the atmospheric and interior structure models.