Forecasting the Impact of Stellar Activity on Transiting Exoplanet Spectra

Exoplanet host star activity, in the form of unocculted starspots or faculae, alters the observed transmission and emission spectra of the exoplanet. This effect can be exacerbated when combining data from different epochs if the stellar photosphere varies between observations due to activity. Here, we present a method to characterize and correct for relative changes due to stellar activity by exploiting multi-epoch (>= 2 visits/transits) observations to place them in a consistent reference frame. Using measurements from portions of the planet's orbit where negligible planet transmission or emission can be assumed, we determine changes to the stellar spectral amplitude. With the analytical methods described here, we predict the impact of stellar variability on transit observations. Supplementing these forecasts with Kepler-measured stellar variabilities for F-, G-, K-, and M-dwarfs, and predicted transit precisions by the James Webb Space Telescope's (JWST) NIRISS, NIRCam, and MIRI, we conclude that stellar activity does not impact infrared transiting exoplanet observations of most presently known or predicted TESS targets by current or near-future platforms, such as JWST, as activity-induced spectral changes are below the measurement precision.