Atmospheric Circulation of High-Obliquity Mini-Neptunes

TL;DR

The paper addresses how high obliquity and asynchronous rotation influence the atmospheric dynamics of close-in mini-Neptunes, focusing on K2-290 b. It employs the ADAM GCM with SPARC non-grey radiative transfer and cloud tracers, coupled to PICASO radiative post-processing, to compare synchronous and nonsynchronous spin states across baseline and enhanced metallicity and cloud scenarios. The key findings show a global Weak-Temperature-Gradient regime with slow rotation and modest thermal contrasts, eastward jets under synchronous rotation, and a QBO-like, seasonal cycle with long-term ~70-orbit variability under nonsynchronous rotation, all modulated by metallicity and clouds. Observational signatures in thermal emission and transmission spectra remain small (roughly 100 ppm), with obliquity-driven differences at the tens of ppm level, underscoring the challenge of detecting such planets yet providing a framework applicable to a broader class of high-obliquity exoplanets in the WTG regime.

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.

Figures (17)

• Figure 1: Atmospheric circulation pattern and mechanism of the formation of eastward jets in low- and mid-latitudes under synchronous rotation. (a) Zonal-mean zonal wind (shading, m s$^{-1}$) and temperature (contours, K) as functions of latitude and pressure. (b) Horizontal temperature map (K) near the photosphere ($p \sim 0.22$ bar), with horizontal winds indicated by arrows. (c) Zonal-mean horizontal eddy momentum flux (m$^2$ s$^{-2}$), with positive (negative) values indicating northward (southward) eddy momentum transport. (d) Zonal-mean vertical eddy momentum flux (m$^2$ s$^{-2}$), with positive (negative) values implying upward (downward) eddy momentum transport; (e) Zonal-mean horizontal and (f) vertical eddy momentum convergence (m s$^{-2}$), with positive (negative) values implying eastward (westward) eddy acceleration. Diagnoses in panels (c-f) were calculated using GCM outputs during the spin-up phase, and only values between $10^{-3}$ and $10^{-1}$ bar are shown, where the zonal jets are centered.
• Figure 2: Temporal variability of normalized light curve from GCM simulations under nonsynchronous rotation. (a) Long-term normalized light curves viewed 30N-on as a function of simulation time over 4800$\sim$6200 orbits. (b) A zoom-in light curve between 5500 and 5640 orbital periods, and the temporal range is marked by the black rectangle in the top panel. The inset shows a zoom-in over a single orbit. (c) Power spectrum of periodogram for the normalized light curves. From left to right, the red dotted lines correspond to the orbital period, the rotation period, and the period of the long-term oscillations shown in the top panel ($\sim70$ orbital periods), respectively. The inset shows a magnified view near the orbital and rotation periods.
• Figure 3: QBO-like oscillations under nonsynchronous rotation. (a) Timeseries of zonal-mean zonal wind at 1.41$^{\circ}$N as functions of pressure. (b) Timeseries of zonal-mean equator-to-pole temperature contrast. Positive/negative values of the equator-to-pole temperature contrast indicate higher/lower equatorial than polar temperatures. The onset time of each QBO-like oscillation is marked with dashed lines in both panels.
• Figure 4: Seasonal variations of circulation patterns during the eastward phase (the top two rows) and the westward phase (the bottom two rows) under nonsynchronous rotation. Each row, from left to right, corresponds to the snapshot at vernal equinox, summer solstice, fall equinox, and winter solstice. For each phase: the top panels show horizontal temperature maps (K) near the photosphere ($\sim$0.2 bar) with horizontal winds as arrows as functions of longitude and latitude. The intersection of the horizontal and vertical black lines (marked by red filled circles) denotes the substellar point. The bottom panels display zonal-mean zonal wind (colors, m s$^{-1}$) and temperature (contours, K) as functions of latitude and pressure.
• Figure 5: (a) Timeseries of zonal-mean temperature and substellar latitude versus latitude within three orbital periods at pressures of 0.02 bar (top), 0.12 bar (middle), and 0.43 bar (bottom) during the eastward phase, with the timeseries of substellar latitude indicated by black solid lines. (b) Time lag between the latitude of maximum zonal-mean temperature and the substellar latitude as a function of pressure, with zero time lag indicated by a dashed line. Positive values indicate that the temperature lags behind the insolation, while negative values indicate that it leads.