Water-cooled (sub)-Neptunes get better gas mileage

The demographics of sub-Jovian planets around low-mass stars is dominated by populations of sub-Neptunes and super-Earths, distinguished by the presence or absence of envelopes of volatiles with a low molecular weight, that is, H2, He, and H2O. The current paradigm is that sub-Neptunes on close-in orbits evolve into super-Earths via atmospheric escape driven by high-energy stellar irradiation. We used an integrated hydrodynamic-radiation-chemical network model of the outflow to demonstrate that this escape is modulated by the abundance of H2O, which is an efficient infrared coolant. Increasing the H2O/H2 at the base of the flow induces a 1 dex decline in the escape rate, with definitive consequences for the retention of envelopes over Gyr. We show that saturation limits on H2O in the upper atmospheres of temperate sub-Neptunes could explain the paradoxical observation that these objects disappear more rapidly than their counterparts closer to their host stars. We also propose that the scarcity of sub-Neptunes around very low-mass stars could be related to the water-poor chemistry of their antecedent protoplanetary disks. Observations of atmospheric H2O by JWST as well as searches for atmospheric escape from younger planets using H and He lines could test these predictions.