The Runaway Greenhouse Effect on Hycean Worlds

Abstract Hycean worlds are a proposed subset of sub-Neptune exoplanets with substantial water inventories, liquid surface oceans, and extended hydrogen-dominated atmospheres favorable for habitability. We aim to quantitatively define the inner edge habitable zone (HZ) using 1D radiative-convective model. As limiting case, we model dry hydrogen–helium envelope above ocean. For 1 bar (10,100 bar) atmosphere, hydrogen greenhouse effect alone sets HZ at 0.216 au (0.58, 3.71 au) Sun-like G star 0.0364 (0.110, 0.774 an 3500 K M star. Introducing vapor into runaway instellation limit is greatly reduced due presence superadiabatic layers where convection inhibited. This moves from ≈1 1.6 (3.85 world H 2 –He inventory (10 bar). star, equivalently moved 0.17–0.28 (0.54 au). Our results suggest that most current observational targets not likely sustain present analytical framework interpreting our results, finding maximum possible outgoing longwave radiation scales approximately inversely mass atmosphere. discuss limitations modeling recommend use 3D convection-resolving models explore robustness layers.