2024RNAAS...8..320Q

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.