  Study Analysis Group 21 (SAG21) of NASA's Exoplanet Exploration Program
Analysis Group (ExoPAG) was organized to study the effect of stellar
contamination on space-based transmission spectroscopy, a method for studying
exoplanetary atmospheres by measuring the wavelength-dependent radius of a
planet as it transits its star. Transmission spectroscopy relies on a precise
understanding of the spectrum of the star being occulted. However, stars are
not homogeneous, constant light sources but have temporally evolving
photospheres and chromospheres with inhomogeneities like spots, faculae,
plages, granules, and flares. This SAG brought together an interdisciplinary
team of more than 100 scientists, with observers and theorists from the
heliophysics, stellar astrophysics, planetary science, and exoplanetary
atmosphere research communities, to study the current research needs that can
be addressed in this context to make the most of transit studies from current
NASA facilities like HST and JWST. The analysis produced 14 findings, which
fall into three Science Themes encompassing (1) how the Sun is used as our best
laboratory to calibrate our understanding of stellar heterogeneities ("The Sun
as the Stellar Benchmark"), (2) how stars other than the Sun extend our
knowledge of heterogeneities ("Surface Heterogeneities of Other Stars") and (3)
how to incorporate information gathered for the Sun and other stars into
transit studies ("Mapping Stellar Knowledge to Transit Studies"). In this
invited review, we largely reproduce the final report of SAG21 as a
contribution to the peer-reviewed literature.
