  The atmospheres of late M stars represent a significant challenge in the
characterization of any transiting exoplanets due to the presence of strong
molecular features in the stellar atmosphere. TRAPPIST-1 is an ultra-cool
dwarf, host to seven transiting planets, and contains its own molecular
signatures which can potentially be imprinted on planetary transit lightcurves
due to inhomogeneities in the occulted stellar photosphere. We present a case
study on TRAPPIST-1g, the largest planet in the system, using a new observation
together with previous data, to disentangle the atmospheric transmission of the
planet from that of the star. We use the out-of-transit stellar spectra to
reconstruct the stellar flux based on one-, two-, and three-temperature
components. We find that TRAPPIST-1 is a 0.08 M$_*$, 0.117 R$_*$, M8V star with
a photospheric effective temperature of 2400 K, with ~35% 3000 K spot coverage
and a very small fraction, <3%, of ~5800 K hot spot. We calculate a planetary
radius for TRAPPIST-1g to be Rp = 1.124 R$_\oplus$ with a planetary density of
$\rho_p$ = 0.8214 $\rho_\oplus$. Based on the stellar reconstruction there are
eleven plausible scenarios for the combined stellar photosphere and planet
transit geometry; in our analysis we are able to rule out 8 of the 11
scenarios. Using planetary models we evaluate the remaining scenarios with
respect to the transmission spectrum of TRAPPIST-1g. We conclude that the
planetary transmission spectrum is likely not contaminated by any stellar
spectral features, and are able to rule out a clear solar H2/He-dominated
atmosphere at greater than 3-sigma.
