Evidence of energy-, recombination-, and photon-limited escape regimes in giant planet H/He atmospheres

TL;DR

This study provides observational evidence for the three theoretically predicted hydrodynamic escape regimes—energy-limited, recombination-limited, and photon-limited—in giant planet atmospheres, using helium and hydrogen absorption data.

Contribution

It is the first to empirically confirm the existence of these escape regimes through detailed spectral analysis and hydrodynamic modeling of exoplanet atmospheres.

Findings

HD 209458 b is in the energy-limited regime

HD 189733 b is in the recombination-limited regime

GJ 3470 b is in the photon-limited regime

Abstract

Hydrodynamic escape is the most efficient atmospheric mechanism of planetary mass loss and has a large impact on planetary evolution. Three hydrodynamic escape regimes have been identified theoretically: energy-limited, recombination-limited, and photon-limited. However, no evidence of these regimes had been reported until now. Here, we report evidence of these three regimes via an analysis of helium I triplet at 10830 angstroms and Ly-$\alpha$ absorption involving a 1D hydrodynamic model that allows us to estimate hydrogen recombination and advection rates. In particular, we show that HD 209458 b is in the energy-limited regime, HD 189733 b is in the recombination-limited regime, and GJ 3470 b is in the photon-limited regime. These exoplanets can be considered as benchmark cases for their respective regimes.