Challenges to Constraining Exoplanet Masses via Transmission Spectroscopy

Abstract

MassSpec, a method for determining the mass of a transiting exoplanet from its transmission spectrum alone, was proposed by \citet{dew13}. The premise of this method relies on the planet's surface gravity being extracted from the transmission spectrum via its effect on the atmospheric scale height, which in turn determines the strength of absorption features. Here, we further explore the applicability of \textit{MassSpec} to low-mass exoplanets -- specifically those in the super-Earth size range for which radial velocity determinations of the planetary mass can be extremely challenging and resource intensive. Determining the masses of these planets is of the utmost importance because their nature is otherwise highly unconstrained. Without knowledge of the mass, these planets could be rocky, icy, or gas-dominated. To investigate the effects of planetary mass on transmission spectra, we present simulated observations of super-Earths with atmospheres made up of mixtures of H$_2$O and H$_2$, both with and without clouds. We model their transmission spectra and run simulations of each planet as it would be observed with \textit{JWST} using the NIRISS, NIRSpec, and MIRI instruments. We find that significant degeneracies exist between transmission spectra of planets with different masses and compositions, making it impossible to unambiguously determine the planet's mass in many cases.