The importance of ground-based transmission spectra in characterising stellar activity for JWST

The James Webb Space Telescope’s unprecedented precision and wavelength coverage have already led to many new discoveries in the field of exoplanets. However, this increased precision means contamination of the data by the host star's stellar activity is also observed more clearly. This phenomenon is highly variable and can introduce false positives of molecular features, such as those of water, which is also found in sunspots. This makes optical transmission spectra more important than ever, as their coverage of bluer wavelengths can not only give a measure of the amount of activity going on, but also distinguish between different types of activity such as spots and faculae, which are degenerate in photometric monitoring. In this talk I will present an optical transmission spectrum of the hot Jupiter WASP-69 b taken with the FORS2 optical spectrograph on the VLT. I will identify the signatures of spots and faculae, which would not be distinguishable with JWST data alone, but are nonetheless needed to distinguish between molecule detections in the planet atmosphere and on the stellar surface.