Today, more that 4000 exoplanets have been detected, and their number is increasing daily. From the results obtained with the main detection methods – i.e. transits and radial velocity – we can measure the radius, the mass and the temperature of a planet. However, as it can be deduced from studying the planets within our Solar System, these characteristics alone are not enough to infer the nature of a planet. The most distinct example is the comparison between Earth and Venus, two extremely different environments on two planets with similar masses and radii. To break such degeneracies for exoplanets we need to characterise their composition through remote-sensing observations of their atmospheres. In the past twenty years, transit spectroscopy with the Hubble Space Telescope (HST), the Spitzer Space Telescope and ground-based facilities, provided the first detections of ionic, atomic and molecular species and condensates in the atmospheres of exoplanets, revolutionising the field of exoplanet characterisation.
In this talk I will give an overview of the techniques used to characterise the atmospheres of exoplanets and present the results that have been achieved so far. Currently, the WFC3 camera on-board the HST is the most powerful instrument to perform infrared transit spectroscopy of exoplanets. Such observations have helped us study the atmospheres of more than 50 planets so far, including exceptional cases like the hot super-Earth 55 Cancri e, the habitable-zone mini-Neptune K2-18b and the habitable-zone super-Earth LHS1140b. Moreover, I will discuss the key questions around exoplanetary atmospheres today and how they will be addresses by the next generation of space and ground facilities that will become available this decade. Finally, I will present ESA’s M4 mission, Ariel, which has been recently adopted. Ariel will carry an 1-m telescope that will observe spectroscopically (0.5 to 8 um) 1000 exoplanets of various sizes and temperatures, with the scope of detecting and characterising their atmospheres.