Atmospheric Circulation of High-Obliquity Mini-Neptunes

Abstract

With the operation of JWST, atmospheric characterization has now extended to low-mass exoplanets. In compact multiplanetary systems, secular spin-orbital resonance may preserve high obliquities and asynchronous rotation even for tidally-despinning, low-mass planets, potentially leading to unique atmospheric circulation patterns. To understand the impact on the atmospheric circulation and to identify the potential atmospheric observational signatures of such high-obliquity planets, we simulate the three dimensional circulation of a representative mini-Neptune K2-290 b, whose obliquity may reach about 67 degrees. Whether synchronously rotating or not, the planet's slow rotation, moderate temperature and radius result in a global Weak-Temperature-Gradient (WTG) behavior with moderate horizontal temperature contrasts. Under synchronous rotation, broad eastward superrotating jets efficiently redistribute heat. Circulation in an asynchronous rotation exhibits a seasonal cycle driven by high obliquity, along with quasi-periodic oscillations in winds and temperatures with a period of about 70 orbital periods. These oscillations, driven by wave-mean flow interactions, extend from low to mid-latitudes due to the slow planetary rotation. Higher atmospheric metallicity strengthens radiative forcing, increasing temperature contrasts and jet speeds. Clouds have minimal impact under synchronous rotation but weaken jets under nonsynchronous rotation by reducing temperature contrasts. In all cases, both thermal emission and transmission spectra exhibit moderate observational signals at a level of 100 ppm, and high-obliquity effects contribute differences at the 10 ppm level. Our results are also applicable to a range of potential high-obliquity exoplanets, which reside in the WTG regime and likely exhibit nearly homogeneous horizontal temperature patterns.