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

In this work my colleague Tatsuya and I use a self-consistent hydrodynamic-radiative-chemical model of atmospheric outflow to show that sub-Neptunes with water vapor in their upper atmospheres can retain their envelopes against stellar radiation-driven escape. Since the water-vapor in the atmosphere is sensitive to the planet’s equilibrium temperature through the Clausius-Claperyon relation, this creates an ``oasis” for sub-Neptunes at intermediate distances and explains a puzzling feature of the demographics of kepler sub-Neptunes around M dwarfs. It also predicts that water-poor systems like those seen around M dwarfs will host fewer sub-Neptunes relative to rocky super-Earths, in accord with available observations.

Citation: Yoshida, T. and Gaidos, E. (2025) Astronomy & Astrophysics 696, L13.
Download Paper | Download Bibtex