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 \sim 35 % 3000 K spot coverage and a very small fraction , < 3 % , of \sim 5800 K hot spot .
We calculate a planetary radius for TRAPPIST-1g to be R _ { p } = 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 H _ { 2 } /He-dominated atmosphere at greater than 3-sigma .