The intense UV radiation can drive atmospheric escape of close-in planets. The escape process is a key process in the evolution of close-in exoplanets. Photoionization heating of hydrogen atoms by extreme-ultraviolet (EUV) radiation with energies above 13.6 eV drives hydrodynamic atmospheric escape. Such atmospheric escape has been observed in some short-period planets using the transit method.
Hydrodynamic escape is determined by photo heating, gravity, and gas expansion. A characteristic temperature to equilibrium temperature ratio, which represents how fast the photoheating is compared to the gas expansion, is considered to characterize the system. We find that the ratio can be used to determine the regime of the escape.
Planetary masses and radii evolve with atmospheric escape, and the EUV flux from the host star is also time-dependent. As the EUV flux from the host star decreases, the main physics determining atmospheric escape changes. We calculate the evolution of short-period planets taking atmospheric escape into account. We find that atmospheric escape in planets evolves from recombination-limited, where radiative cooling is dominant, to energy-limited, where cooling due to gas expansion is dominant, in a few Gyr. We also calculate the effect of the interaction between planets and torus around the star which may be formed by the escaping outflow. We find that the system changes from the recombination-limited to energy-limited as the system without torus in early stage. The interaction causes the migration of planets and the system turns to be recombination-limited due to the intense radiation. We discuss the effect of atmospheric escape on the evolution of planets including the orbital evolution.