Learning about exoplanets' exospheres from the Lyman-alpha light curve.

Earth's exosphere extends further than 38 Earth radii. The most abundant element in the exosphere is Hydrogen which is very sensitive to Lyman α (Lyα) radiation; a gas column of N_H < 10^{17} cm^{-2} suffices to block the stellar radiation.As a result Earth analogues orbiting around nearby M-type stars could be detectable by monitoring Lyα variability. In spite of the interstellar, heliospheric and astrospheric absorption, the transit signature would be detectable in M5 V type stars with a dedicated Lyα flux monitor implemented in a 4-8 m class space telescope.In this work, we present simulations of the Lyα profile for various models of planetary outflows and show how the contrast between the core and the wings of the line could be usedto derive the properties of exoplanets' exospheres.