Observing planet-starspot crossings with the James Webb Space Telescope

Transiting exoplanets orbiting active stars frequently occult starspots and faculae on the visible stellar disc. Such occultations can provide useful constraints to retrieve the temperature of active features and their effect on transmission spectra. We analyse the constraints that the James Webb Space Telescope will be able to place on the spectra of starspots, despite its lack of optical wavelength instruments on board. Focusing on K and M spectral types, we find that starspot temperatures can be determined to within a few hundred kelvins using NIRSpec/Prism and the proposed NIRCam/F150W2+F322W2's broad wavelength capabilities.