The Most Extreme Case of Atmospheric Escape Detected on the Warm Neptune GJ 3470b with HST

Observations of exoplanets during the transit of their host star allow probing the structure and composition of their atmosphere. The intense stellar energy input into exoplanets orbiting close to their star can lead to a dramatic expansion of their upper atmosphere, and the 'evaporation' of large amounts of gas into space. UV observations of hot Jupiters revealed the extended exospheres formed by this escaping gas, and showed that these planets are too massive to lose a substantial fraction of their atmosphere. Lower-mass planets are expected to be much more sensitive to evaporation, which has long been thought to play a role in forming the desert of hot Neptunes (a deficit of Neptune-size exoplanets on very short orbits). I will present the discovery of a giant hydrogen exosphere around GJ3470b, a warm Neptune located at the border of the desert. This is the first UV result of the Panchromatic Comparative Exoplanet Treasury (PanCET) survey, a Hubble program targeting 20 exoplanets across the entire spectrum. Our numerical simulations of the resolved exospheric transit show that GJ3470b is subjected to mass losses comparable to that of hot Jupiters, making it the most extreme case of evaporation observed to date. GJ3470b could already have lost up to 40% of its mass over its 2 Gyr lifetime, bringing direct observational confirmation that evaporation shaped the population of close-in exoplanets. I will compare GJ3470b with other known evaporating planets and discuss the reasons for its dramatic escape. Our results strengthen the interest of observing the upper atmosphere of exoplanets to determine their properties and understand how they depend on their past evolution. This is particularly important for super-Earth and Earth-size planets, whose lower atmosphere could be hidden by clouds. The development of new tracers of atmospheric escape at optical/infrared wavelengths opens thrilling perspectives for the characterization of exoplanets via their upper atmosphere.