Catastrophic atmospheric loss event induced by the giant impact

Recent observations, especially space telescope missions, reveal that there are many kinds of exoplanets. Especially, Earth-mass to several Earth-mass planets has two types of planets: the bare-rocky planets and planets possessing significant atmosphere. They are divided by the planetary radius since the hydrogen-helium atmosphere expands its radius. Focusing on the planets whose radius larger than typically 1.8 Earth radius, those planets have 1-30 % of atmospheres by mass. Such diversities are essential to understand the diversities of the formation processes of exoplanets. Here we focus on the late stage of the formation process. Planets have experienced giant impact events that cause atmospheric escape. We perform the smoothed particle hydrodynamic simulation to reveal the impact-induced atmospheric escape. We find that the kinetic energy of escaped atmospheric mass is simply proportional to the kinetic energy of the giant impact. We demonstrate the relationship between the kinetic energy of the escaped mass and the escaped atmospheric mass fraction. We find two regimes that determine the atmospheric escape: the momentum-driven regime and the other is the energy-driven regime. Combined the relationships among the kinetic impact energy, kinetic escape energy, and the escaped atmospheric mass, we can derive an analytic expression for the atmospheric escape as a function of the impact energy. The present study provides strong constraints on the formation scenarios of observed rocky planets. Since the giant impact removes the primordial atmosphere, rocky planets expected to have atmospheres should have formed before the protoplanetary disk dissipation.