A Systematic North-South asymmetry in the Steady-state Climate of rapidly-rotating Oblique Water Worlds

TL;DR

This study uses ExoSPEEDY, a general circulation model of intermediate complexity, to simulate the steady-state climates of aqua-planets across a range of masses, obliquities, and rotation rates. The results show that obliquity and rotation strongly control seasonal variability and, at high obliquity, drive a robust north–south polar temperature asymmetry that intensifies with faster rotation and remains largely mass-independent. These hemispheric temperature and cloud-pattern asymmetries persist across atmospheric pressures (1013–30 mbar) and provide a potential observational handle on obliquity and rotation for exoplanets, with implications for habitability assessments of ocean worlds such as K2-18b and TOI-1452b. The work also discusses bifurcation and multi-equilibrium dynamics in planetary atmospheres, and situates Uranus as a Solar System analogue to motivate and contextualize the exoplanet results, while noting current modeling limitations and directions for future improvements.

Abstract

Planetary obliquity (axial tilt) plays an important role in regulating the climate evolution and habitability of water-covered planets. Despite the suspicion of large obliquities in several exoplanetary systems, this phenomenon remains hard to observe directly. We aimed to study the effect of mass, obliquity, and rotation on the steady state climate of water-covered planets. We simulated the climate evolution of such planets with varying obliquities, rotational speed, and mass using a general circulation model (GCM) of intermediate complexity, assuming aqua-planet configurations. High obliquity supports an asymmetry between the equilibrium climatological conditions in the northern and southern hemispheres. The polar temperature ratio deviates further from unity with increasing obliquity and rotation rate. Cloud coverage patterns also shift with obliquity, displaying distinct latitudinal bands and increased cloudiness in the warmer hemisphere. The climate of habitable-zone aqua-planets turns out to be sensitive to changes in obliquity and rotation rate, but are independent of planet mass. Our results highlight the importance of considering these factors when assessing the surface conditions of exoplanets. As a consequence, surface condition asymmetries in water-world exo-planets can be used to infer the planet's obliquity and rotation rate.

Figures (9)

• Figure 1: Zonal-average pressure-latitude for the climate of water-Earth with 23.4$^\circ$ obliquity. Latitude 0$^\circ$ is at the equator with positive values for the Northern Hemisphere. These are the time-average results from the last three years of the model simulation. Left panel: Color-map gives the average temperature, overplotted with zonal wind velocity contour lines (m s$^{-1}$). Dashed lines indicates easterlies whereas solid lines represents westerlies. Right panel: Color-map of the relative humidity.
• Figure 2: Mass and radius for a number of potentially habitable ocean worlds. The bullet points show identified ocean worlds: listed from bottom to top: HD 11505b, WASP 35, TOI 157, TOI 1452b, KELT 10b, HD 80883b, HD 221585b, HIP 67537b, K2-18b, Kepler 1980b, HD 50499b, Kepler 1766b, HD 50499c. The green curve gives the mass-radius relation for planets with a water content of $\ {\hbox{$\buildrel>\over\sim$}}\ 30$ % (by mass) from 2024AA...686A.296M. The arrows (and black crosses [right-most one is white]) give the simulation atmosphere models.
• Figure 3: Time evolution of the global average sea-level temperature for K2-18b over 180 months. Each frame represents a specific value of obliquity. Line colors represent different values of $\alpha$. The effect of its orbital eccentricity (of only $e=0.09$) is noticeable as the 12-month periodic variations in temperature.
• Figure 4: Global average values of the sea-level temperature (left panel), deep cloud coverage (middle), and specific humidity (right) as a function of obliquity and $\alpha$. Each panel has several curves for each value of $\alpha$; the legend is in the left-most panel.
• Figure 5: Latitudinal profiles of sea-level temperature for different values of the obliquity and $\alpha$. Each panel represents a specific obliquity and the curves are colored according to $\alpha$ using the same color scheme for each panel. The legend is presented along the bottom of the figure. Negative latitudes are in the Southern hemisphere and positive values represents the Northern hemisphere; the equator is at 0$^{\circ}$. The presented data gives the time average of the last two orbital periods in the calculation of the planet's atmosphere.