Letter to the Editor

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

¹ Faculty of Science, Tohoku University, Sendai, Miyagi 980-8578, Japan
² Department of Earth Sciences, University of Hawai’i at Mänoa, Honolulu, Hawai’i 96822, USA
³ Institute for Astrophysics, University of Vienna, 1180 Vienna, Austria

^⋆ Corresponding authors; tatsuya@tohoku.ac.jp, gaidos@hawaii.edu

Received: 3 January 2025
Accepted: 26 March 2025

Abstract

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.