Observations of exoplanets during the transit of their host star allow us to probe the structure and composition of their atmospheres. While the visible and infrared wavelength domains give access to low altitudes in the atmosphere, space-borne ultraviolet observations can reveal their highest atmospheric layers. Transit observations of strongly irradiated gaseous giants led to the detection of evaporation, when massive amounts of gas escape from the planet and form an extended exosphere shaped by interactions with the star. Recently, the observation of a giant hydrogen exosphere around the warm Neptune GJ436b showed that much larger atmospheric signals can be retrieved from the upper atmosphere of moderately irradiated, lower-mass planets. This opens thrilling perspectives for the use of the LUVOIR to characterize the many small planets that will be discovered in the coming years by missions like TESS, CHEOPS, and PLATO. I will show how high-resolution UV spectroscopy can be used to probe the high-energy environment and upper atmosphere of Earth-size planets close to their stars, in particular in the habitable zone of M dwarfs, to determine their nature, evolution and habitability.