Exoplanets at small orbital distances from their host stars are submitted to
intense levels of energetic radiations, X-rays and extreme ultraviolet (EUV).
Depending on the masses and densities of the planets and on the atmospheric
heating efficiencies, the stellar energetic inputs can lead to atmospheric mass
loss. These evaporation processes are observable in the ultraviolet during
planetary transits. The aim of the present work is to quantify the mass-loss
rates (dm/dt), heating efficiencies (eta), and lifetimes for the whole sample
of transiting exoplanets, now including hot jupiters, hot neptunes, and hot
super-earths. The mass-loss rates and lifetimes are estimated from an "energy
diagram" for exoplanets, which compares the planet gravitational potential
energy to the stellar X/EUV energy deposited in the atmosphere. We estimate the
mass-loss rates of all detected transiting planets to be within 10^6 to 10^13
g/s for various conservative assumptions. High heating efficiencies would imply
that hot exoplanets such the gas giants WASP-12b and WASP-17b could be
completely evaporated within 1 Gyr. We further show that the heating efficiency
can be constrained when dm/dt is inferred from observations and the stellar
X/EUV luminosity is known. This leads us to suggest that eta ~ 100% in the
atmosphere of the hot jupiter HD209458b, while it could be lower for HD189733b.
Simultaneous observations of transits in the ultraviolet and X-rays are
necessary to further constrain the exospheric properties of exoplanets.Comment: 10 pages, 5 figures, 2 tables. Accepted for publication in Astronomy
& Astrophysic
