By knowing the mass (radial velocity) and size (transits) of exosolar planets mass loss caused by hydrogen escape of hot Jupiters can be studied. A recent study by Lammer et al. (2009) was carried out at the IWF and the University of Graz (IGAM) with international cooperators on the possible atmospheric mass loss from 57 known transiting exoplanets (status: spring 2009) around F, G, K, and M-type stars over evolutionary time scales. The thermal mass loss of atomic hydrogen is calculated by a mass loss equation which is in agreement with hydrodynamic modeling and which uses a realistic heating efficiency, a radius-scaling law and a mass loss enhancement factor due to stellar tidal forces. The model takes into account the temporal evolution of the stellar EUV flux by applying power laws for F, G, K, and M-type stars.
By assuming a realistic heating efficiency η of about 10–25% we found that WASP-12b may have lost about 6–12% of its mass during life time. A few transiting low density gas giants at similar orbital location, like WASP-13b, WASP-15b, CoRoT-1b or CoRoT-5b may have lost up to about 1–4% of their initial mass. All other transiting exoplanets in our sample experience negligible thermal loss (≤ 1%) during their life time.
Depending on a stellar luminosity spectral type, planetary density, heating efficiency, orbital distance, and the related Roche lobe effect we expect that at distances between 0.015–0.02 AU Jupiter-class and sub-Jupiter-class exoplanets can loose several percent of their initial mass. At orbital distances ≤ 0.015 AU low density hot gas giants in orbits around solar type stars may even evaporate down to their core-size, while low density Neptune-class objects can loose their hydrogen envelopes at orbital distances ≤ 0.02 AU.