Transmission spectra are differential measurements that utilize stellar illumination to probe \deleted the structure and composition of transiting exoplanet atmospheres .
Any spectral difference between the illuminating light source and the disk-integrated stellar spectrum due to \deleted the presence of starspots and faculae will be imprinted in the observed transmission spectrum .
However , \deleted despite their clear importance , few constraints exist for the extent of photospheric heterogeneities in M dwarfs .
Here , we \replaced present a forward model to assess the range ofmodel spot and faculae covering fractions consistent with observed photometric variabilities for M dwarfs and \replaced theirthe associated \added 0.3–5.5 \micron stellar contamination spectra .
We find that large ranges of spot and faculae covering fractions are consistent with \replaced observed variability levels .
We also show thatobservations and corrections \replaced that assumeassuming a linear \replaced correlationrelation between variability amplitude and covering fractions generally underestimate the \deleted level of stellar contamination . \deleted present .
Using realistic estimates for spot and faculae covering fractions , we find \deleted the stellar contamination \deleted signal can be more than 10 \times larger than transit depth changes expected for atmospheric features in rocky exoplanets . \replaced Also , integrating across photometric bands , weWe also find that stellar spectral contamination can lead to \deleted significant systematic errors in radius and therefore the derived density of small planets .
In the \deleted specific case of the \object TRAPPIST-1 system , we show that \deleted instead of the assumed 1–2 % spot coverage , TRAPPIST-1 ’ s rotational variability is \deleted in fact consistent with spot covering fractions \replaced f _ { spot } = 12 ^ { +10 } _ { -11 } \% f _ { spot } = 8 ^ { +18 } _ { -7 } \% and faculae covering fractions \replaced f _ { fac } = 65 ^ { +7 } _ { -55 } \% f _ { fac } = 54 ^ { +16 } _ { -46 } \% .
The associated stellar contamination signals alter transit depths of the TRAPPIST-1 planets at wavelengths of interest for planetary atmospheric species by roughly 1–15 \times the strength of planetary features , significantly complicating \replaced James Webb Space Telescope JWST follow-up observations of this system .
Similarly , we find that stellar contamination can lead to underestimates of bulk densities of the TRAPPIST-1 planets of \replaced 12 ^ { +11 } _ { -12 } \% \Delta ( \rho ) = -3 ^ { +3 } _ { -8 } \% , thus leading to overestimates of their volatile contents .