Atmospheres, Climate, Weather

General Circulation Models Of Hycean Worlds

By Keith Cowing

November 12, 2025

Filed under albedo, atmosphere, climate, ExoCAM GCM, exoplanet, general circulation model, habitability, Habitable Zone, Hycean world, K2-18 b, sub-Neptune, water world

General Circulation Models Of Hycean Worlds — Wind fields in our imposed TOA 𝐴𝑏 = 0.6 case. Top left: horizontal wind field at 250 hPa. Top right: Divergent component of the circulation, and the vertical pressure velocity 𝜔 at 100 hPa. Bottom left: Zonal-mean rotational (Jet) component Bottom right: Eddy rotational component, and deviations of temperature from the zonal mean at 600 hPa. The eastward nature of the overall winds can be seen, although it is strongest moving from the substellar to the antistellar point. The wind’s jet rotational component makes up the largest single component of the wind and shows twin high-latitude jets. The eddy rotational component is also significant and shows equatorial Rossby waves. In the western hemisphere these increase the eastward wind magnitude at the equator and reduce it at the pole, whereas the opposite is true in the eastern hemisphere. Compared to the rotational component, the divergent circulation is small and shows roughly strong divergence at the substellar point motion, although this is compensated by a returning circulation at lower pressures. Despite its small magnitude, the divergent circulation is still responsible for the majority of the net dayside-nightside heat transport. The jet and eddy rotational components carry a significant amount of heat but largely cancel each other out. — astro-ph.EP

Sub-Neptunes represent the current frontier of exoplanet atmospheric characterisation. A proposed subset, Hycean planets, would have liquid water oceans and be potentially habitable, but there are many unanswered questions about their atmospheric dynamics and 3D climate states.

To explore such climates in detail, we report a General Circulation Model (GCM) for Hycean worlds, building on a modified version of the ExoCAM GCM. Considering the temperate sub-Neptune K2-18 b as a Hycean candidate, we implement GCMs with different surface pressures and albedos.

We find dynamical structures similar to those of tidally-locked terrestrial planets as `slow rotators’ with either one equatorial or twin mid-latitude zonal jets. We see moist convective inhibition that matches high resolution models, although in hotter cases the inhibited zone is subsaturated.

When imposing a top-of-the-atmosphere (TOA) Bond albedo (Ab) by modifying the incident stellar flux, we find that the threshold for K2-18~b to not enter a runaway greenhouse state is Ab≥0.55 for a 1 bar atmosphere, consistent with previous studies, and Ab≥0.8 for a 5 bar atmosphere.

However, a more realistic treatment of the albedo, by modelling scattering within the atmosphere using an enhanced Rayleigh parametrisation, leads to lower lapse rates and stronger thermal inversions.

We find that 1 bar atmospheres are stable for an albedo of Ab≥0.27, 5 bar atmospheres for Ab≥0.35, and 10 bar atmospheres for Ab≥0.48. Moderate albedos such as these are typical of the solar system planets and the required scattering is consistent with observational constraints for K2-18~b, supporting its plausibility as a Hycean world.

Edouard Barrier, Nikku Madhusudhan

Comments: Accepted for publication in MNRAS
Subjects: Earth and Planetary Astrophysics (astro-ph.EP)
Cite as: arXiv:2511.07546 [astro-ph.EP] (or arXiv:2511.07546v1 [astro-ph.EP] for this version)
https://doi.org/10.48550/arXiv.2511.07546
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Submission history
From: Edouard Barrier
[v1] Mon, 10 Nov 2025 19:01:25 UTC (2,251 KB)
https://arxiv.org/abs/2511.07546
Astrobiology,

Keith Cowing

Explorers Club Fellow, ex-NASA Space Station Payload manager/space biologist, Away Teams, Journalist, Lapsed climber, Synaesthete, Na’Vi-Jedi-Freman-Buddhist-mix, ASL, Devon Island and Everest Base Camp veteran, (he/him) 🖖🏻

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