  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.
