Contamination in Exoplanet Transit Spectroscopy with a Changing Stellar Surface

Abstract Inhomogeneities in the stellar surface can lead to differences between the actual and assumed stellar spectrum below the planet’s transit chord, causing the inferred planet transmission spectrum to have a contamination component. In this study, we present a simple analytical formula for calculating the contamination spectrum in transmission spectroscopy, taking into consideration the variable spot coverage fraction or spot occultation. We took the temperate sub-Neptune, K2-18 b, as an example to demonstrate the range of the possible contamination spectra under fiducial spot coverage and variability assumptions, which is typically around a few to 15%, which is considerably more than when surface changes were ignored, illustrating the importance of considering these higher-order effects in exoplanet transmission spectroscopic observations.