Exoplanets atmospheres: the impact of stellar activity on the transmission spectra

Abstract

Exoplanet atmospheres are our only window to study the physical and chemical processes that occur on distant worlds. Such processes are important tracers of the origins of evolution of planetary systems. These are also paramount to better understand how common or unique are the conditions that lead to the emergence of life, which can leave precise spectroscopic traces in exoplanetary atmospheres. Transmission spectroscopy is a very effective technique for probing and measuring the composition and properties of exoplanet atmospheres. This technique consists of taking a spectra during a planetary transit in front of its host star. During the transit, a small fraction of the stellar radiation is filtered through the planet's atmosphere, where it is only partially absorbed. The absorption will be wavelength dependent due to the scattering properties of atoms and molecules in the planetary atmosphere. At the wavelength of a strong atomic or molecular transition, the atmosphere is more opaque and the radius of the planet is larger. This raises the prospect of measuring the transmission spectrum of the planet's atmosphere and thus learning about its composition. One of the main limitations of this technique is the stellar activity that can lead to a significant underestimation or overestimation of the ratio between the radius of the planet and the stellar radius as a function of the wavelength. Therefore, we will quantitatively study the impact of stellar activity, such as the presence of spots, on the transmission spectrum of exoplanets.