Using Transmission Spectroscopy to Measure the Masses of Young Exoplanets in the Era of the James Webb Space Telescope

Measuring the masses of young (JWST) as an alternative method to constrain the masses of a representative sample of young, highly inflated exoplanets. We generate synthetic JWST NIRISS/SOSS and NIRSpec/G395H spectra for five known young planets and fit grids of forward climate models to determine the accuracy and precision of measured masses. Specifically, we investigate the impact of degeneracies that have historically plagued this technique, including atmospheric composition, unocculted stellar spots, and cloud cover. We find that the low bulk densities and extended scale heights of these young planets produce strong spectral features that effectively break the mass-composition degeneracy. Furthermore, stellar spots do not significantly degrade mass constraints. However, high altitude, opaque clouds mute spectral features and currently pose the largest barrier to precise mass determinations. Ultimately, for planets with clear atmospheres, we demonstrate that NIRISS/SOSS observations can routinely achieve accurate mass measurements with better than 20% precision— the precision necessary to successfully constrain planetary formation pathways. We opt not to use an atmospheric retrieval framework to analyze the synthetic JWST spectra, due to concerns over their simplifying physical assumptions and disagreeing opacity databases.