2020ApJ...889...78I

The impact of unocculted stellar surface heterogeneities in the form of cool spots and hot faculae on the spectrum of a transiting planet has been a daunting problem for the characterization of exoplanet atmospheres. The wavelength-dependent nature of stellar surface heterogeneities imprinting their signatures on planetary transmission spectra are of concern particularly for systems of sub-Neptunes orbiting M-dwarfs. Here we present a systematic exploration of the impact of this spot-contamination on simulated near-infrared transmission spectra of sub-Neptune planets. From our analysis, we find that improper correction for stellar surface heterogeneities on transmission spectra can lead to significant bias when inferring planetary atmospheric properties. However, this bias is negligible for lower fractions of heterogeneities (<1%). Additionally, we find that acquiring a priori knowledge of stellar heterogeneities does not improve precision in constraining planetary parameters if the heterogeneities are appropriately marginalized within a retrieval; however, these are conditional on our confidence of stellar atmospheric models being accurate representations of the true photosphere. In sum, to acquire unbiased constraints when characterizing planetary atmospheres with the James Webb Space Telescope, we recommend performing joint retrievals of both the disk-integrated spectrum of the star and the stellar-contamination-corrected transmission spectrum.