Evolution of atmospheric escape in close-in giant planets and their associated Ly α and H α transit predictions

Abstract

ABSTRACT Strong atmospheric escape has been detected in several close-in exoplanets. As these planets consist mostly of hydrogen, observations in hydrogen lines, such as Ly α and H α, are powerful diagnostics of escape. Here, we simulate the evolution of atmospheric escape of close-in giant planets and calculate their associated Ly α and H α transits. We use a 1D hydrodynamic escape model to compute physical properties of the atmosphere and a ray tracing technique to simulate spectroscopic transits. We consider giant (0.3 and 1 Mjup) planets orbiting a solar-like star at 0.045 au, evolving from 10 to 5000 Myr. We find that younger giants show higher rates of escape, owing to a favourable combination of higher irradiation fluxes and weaker gravities. Less massive planets show higher escape rates (1010–1013 g s−1) than those more massive (109–1012 g s−1) over their evolution. We estimate that the 1-Mjup planet would lose at most 1 per cent of its initial mass due to escape, while the 0.3-Mjup planet, could lose up to 20 per cent. This supports the idea that the Neptunian desert has been formed due to significant mass-loss in low-gravity planets. At younger ages, we find that the mid-transit Ly α line is saturated at line centre, while H α exhibits transit depths of at most 3–4 per cent in excess of their geometric transit. While at older ages, Ly α absorption is still significant (and possibly saturated for the lower mass planet), the H α absorption nearly disappears. This is because the extended atmosphere of neutral hydrogen becomes predominantly in the ground state after ∼1.2 Gyr.