The Detection of Multiple Molecular Species in the Atmospheres of a Sub-Neptune and Super-Neptune

Understanding how Neptune and sub-Neptune exoplanets form is an important part of understanding planet formation as a whole. Measurements of their atmospheric compositions provide important boundary conditions on formation models - since the carbon, oxygen, and overall bulk metal abundances are expected to be strongly influenced by their specific formation histories. To date, precise measurements of exoplanets’s bulk metallicities and C/O have proven difficult in this mass regime due to the prevalence of optically thick clouds and the necessity of having to rely primarily on a single water abundance measurement from WFC3 observations. I will present new panchromatic transmission spectra of the sub-Neptune GJ 3470b and the super-Neptune WASP-107b from HST/WFC3, JWST/NIRCam, and JWST/MIRI (in the case of WASP-107b) data. These observations allow us to spectroscopically detect water, methane, sulfur dioxide, and carbon dioxide in the atmospheres of both planets and ammonia and carbon monoxide on WASP-107b. Using a combination of forward models and retrievals, we place constraints on the atmospheric pressure-temperature profiles, cloud structure, and the abundances of the detected molecules. Both planets show evidence of disequilibrium chemistry in their atmospheres, with one or more molecular species not present at the abundance one would expect. I will discuss how this illustrates the need to detect multiple molecular species to accurately estimate the metallicity and C/O ratio of exoplanets using JWST data, and what we can infer about the formation histories of both planets.