General Circulation Models of Hycean Worlds

TL;DR

Hycean planets with liquid oceans and hydrogen-rich atmospheres may sustain stable climates, but the runaway greenhouse threshold depends on both atmospheric mass and albedo. The study implements a 3D ExoCAM GCM with updated convection, a TOA-albedo forcing and a haze-based Rayleigh-scattering approach to map climate stability for K2-18b at 1 and 5 bar surface pressures, identifying IHZ boundaries and haze-assisted stabilization. Key findings show IHZ thresholds near albedos of 0.525–0.55 (1 bar) and 0.7–0.8 (5 bar) under TOA forcing, while haze-enhanced scattering yields lower effective thresholds around 0.25–0.27 (1 bar) and 0.35 (5 bar), extending stability and supporting Hycean habitability under plausible atmospheric conditions. The results reveal a slow-rotator dynamical regime with equatorial super-rotation or mid-latitude jets, convective inhibition near the substellar region, and stratospheric temperature inversions driven by haze and greenhouse effects, with transit spectra indicating hazes could be present without contradicting observations.

Abstract

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 ($A_b$) by modifying the incident stellar flux, we find that the threshold for K2-18~b to not enter a runaway greenhouse state is $A_b \geq 0.55$ for a 1 bar atmosphere, consistent with previous studies, and $A_b \geq 0.8$ 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 $A_b \geq 0.27$, 5 bar atmospheres for $A_b \geq 0.35$, and 10 bar atmospheres for $A_b \geq 0.48$. 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.

Figures (16)

• Figure 1: Various climate features in our imposed TOA $A_b=0.6$ case. Left: The surface temperature in our simulations. The higher temperatures on the dayside are immediately apparent, steady increasing going towards substellar point, with the exception of a slightly cooler equatorial band around the substellar band. The maximum temperature difference is about 30 K, and the coldest regions are approximately at the freezing point of water. Middle: The outgoing longwave radiation (OLR) representing the net thermal energy emitted by the planet. It is balanced by the incoming stellar radiation. The global OLR is generally very uniform, with the exception of a very slight decrease in a colder regions in the nightside gyres and and a small decrease around the substellar region as a result of the clouds there. However, this only leads to a difference of 5% in OLR. Right: The global cloud cover. Clouds are concentrated on the substellar point and the region east of it, and streams of clouds entrained eastwards by the high-latitude jets can also be seen.
• Figure 2: Pressure-Temperature profiles for the imposed TOA $A_b=0.6$ case at a range of locations across the surface: the Substellar and Antistellar points, the North and South poles, and a global average. A significant temperature inversion can be between 10 and 100 mbar driven by CH$_4$ and CO$_2$ absorption. There are minimal temperature variations throughout the stratosphere and free troposphere, as expected from the planet largely being in a weak temperature gradient (WTG) regime. However, near the surface we see significant temperature gradients emerge.
• Figure 3: Wind fields in our imposed TOA $A_b=0.6$ case. Top left: horizontal wind field at 250 hPa. Top right: Divergent component of the circulation, and the vertical pressure velocity $\omega$ 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.
• Figure 4: Dynamical features in our imposed TOA $A_b=0.6$ case. Left: Mean meridional mass streamfunction in the standard latitude-longitude coordinates, in units of $kg$$s^{-1}$. Right: Mean meridional mass streamfunction in Tidally-locked latitude-longitude coordinates, in units of $kg$$s^{-1}$. A TL latitude of 90 (-90) $\degree$ corresponds to the substellar (antistellar) point. Average circulation is anticlockwise around negative values and clockwise around positive values. We see both equator-pole and a more general dayside-nightside overturning circulation. The dayside-nightside overturning circulation is stronger than the pole-equator overturning circulation, but of the same order of magnitude.
• Figure 5: Dynamical features in our imposed TOA $A_b=0.6$ case. Left: The contributions of stellar heating and atmospheric circulation to the large-scale heating patterns, as shown for each longitude slice, averaging over all altitudes and latitudes. The divergent circulation makes up the majority of the day-night heat flux despite its much smaller magnitude. The jet and eddy rotational components carry a significant amount of heat but largely cancel each other out, and the latent heat flux also carries some energy, but the low H$_2$O abundances in the free troposphere limit its magnitude. There are some small residuals (not shown) left from regridding approximations. Right: The zonally averaged zonal wind u. The atmosphere as a whole is super-rotating with no westerly winds anywhere, which is possible given the presence of surface drag. We see twin stratospheric jets at high latitudes, with one set in the troposphere and then stronger stratospheric twin jets. There is a single broad equatorial jet in the troposphere.