  Transmission spectroscopy during planetary transits is expected to be a major
source of information on the atmospheres of small (approximately Earth-sized)
exoplanets in the next two decades. This technique, however, is intrinsically
affected by stellar spectral contamination caused by the fact that stellar
photo- and chromospheres are not perfectly homogeneous. Such stellar
contamination will often reach or exceed the signal introduced by the planetary
spectral features. Finding effective methods to correct stellar contamination
-- or at least to quantify its possible range -- for the most important
exoplanets is a necessary step for our understanding of exoplanet atmospheres.
This will require significantly deepening our understanding of stellar
heterogeneity, which is currently limited by the available data.
