  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 of 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.
