Planetary Climate Dynamics

Abstract:

Transmission spectroscopy has provided unprecedented insight into the makeup of exoplanet atmospheres. A transmission spectrum contains contributions from a planet’s morning and evening limbs, which can differ in temperature, composition, and aerosol properties due to atmospheric circulation. While high-resolution ground-based observations have identified limb asymmetry in several ultrahot/hot exoplanets, space-based studies of limb asymmetry are still in their early stages. The prevalence of limb asymmetry across a broad range of exoplanets remains largely unexplored. We conduct a comparative analysis of retrievals on transmission spectra, including traditional one-dimensional (1D) approaches and four 2D models that account for limb asymmetry. Two of these 2D models include our newly proposed dynamical constraints derived from shallow-water simulations to provide physically-motivated temperature differences between limbs. Our analysis of WASP-39 b using JWST observations and previous combined data sets (HST, VLT, and Spitzer) strongly favors 2D retrievals over traditional 1D approaches, confirming significant limb asymmetry in this hot Jupiter. Within our 2D framework, unconstrained models recover larger temperature contrasts than dynamically-constrained models, with improved fits to specific spectral features, although Bayesian evidence cannot definitively distinguish between these 2D approaches. Our results support the presence of homogeneous C/O in both the morning and evening atmospheres, but with temperature differences leading to variations in clouds and hazes. Using this treatment, we can study a larger sample of hot Jupiters to gain insights into atmospheric limb asymmetries on these planets.

Abstract:

We present a new epoch of JWST spectroscopic variability monitoring of the benchmark binary brown dwarf WISE 1049AB, the closest, brightest brown dwarfs known. Our 8-h JWST/MIRI low resolution spectroscopy and 7-h JWST/NIRSpec prism observations extended variability measurements for any brown dwarfs beyond 11  μm for the first time, reaching up to 14 μm. Combined with the previous epoch in 2023, they set the longest JWST weather monitoring baseline to date. We found that both WISE 1049AB show wavelength-dependent light-curve behaviours. Using a robust k-means clustering algorithm, we identified several clusters of variability behaviours associated with three distinct pressure levels. By comparing to a general circulation model, we identified the possible mechanisms that drive the variability at these pressure levels: patchy clouds rotating in and out of view likely shaped the dramatic light curves in the deepest layers between 1–2.5 μm, whereas hotspots arising from temperature/chemical variations of molecular species likely dominate the high-altitude levels between 2.5–3.6 μm and 4.3–8.5 μm. Small-grain silicates potentially contributed to the variability of WISE 1049A at 8.5–11 μm. While distinct atmospheric layers are governed by different mechanisms, we confirmed for the first time that each variability mechanism remains consistent within its layer over the long term. Future multiperiod observations will further test the stability of variability mechanisms on this binary, and expanded JWST variability surveys across the L-T-Y sequence will allow us to trace and understand variability mechanisms across a wider population of brown dwarfs and planetary-mass objects.