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Transmission spectroscopy during planetary transits, which is based on themeasurements of the variations of planet-to-star radius ratio as a function ofwavelength, is a powerful technique to study the atmospheric properties oftransiting planets. One of the main limitation of this technique is the effectsof stellar activity, which up until now, have been taken into account only byassessing the effect of non-occulted stellar spots on the estimates ofplanet-to-star radius ratio. In this paper, we study, for the first time, theimpact of the occultation of a stellar spot and plage on the transmissionspectra of transiting exoplanets. We simulated this effect by generating alarge number of transit light curves for different transiting planets, stellarspectral types, and for different wavelengths. Results of our simulationsindicate that the anomalies inside the transit light curve can lead to asignificant underestimation or overestimation of the planet-to-star radiusratio as a function of wavelength. At short wavelengths, the effect can reachto a difference of up to 10% in the planet-to-star radius ratio, mimicking thesignature of light scattering in the planetary atmosphere. Atmosphericscattering has been proposed to interpret the increasing slopes of transmissionspectra toward blue for exoplanets HD 189733b and GJ 3470b. Here we show thatthese signatures can be alternatively interpreted by the occultation of stellarplages. Results also suggest that the best strategy to identify and quantifythe effects of stellar activities on the transmission spectrum of a planet isto perform several observations during the transit epoch at the samewavelength. This will allow for identifying the possible variations in transitdepth as a function of time due to stellar activity variability.

Impact of occultations of stellar active regions on transmission spectra: Can occultation of a plage mimic the signature of a blue sky?