Atmospheric Heat Transport and Reflected Light in the Ultra-hot Jupiter WASP-121b using a NIRISS/SOSS Phase Curve

Ultra-hot Jupiters represent extreme laboratories for studying atmospheric circulation, radiative processes, and the interaction of chemistry and dynamics under intense stellar irradiation. Due to their high equilibrium temperatures, they exhibit large day–night temperature contrasts, often driven by short radiative timescales relative to atmospheric advection. Secondary processes, such as hydrogen dissociation/recombination and magnetic drag, can further influence thermal transport and atmospheric structure. Here, we present the first full-orbit spectroscopic phase curve of an ultra-hot Jupiter WASP-121b observed with the JWST NIRISS/SOSS.This time-series observation spans 0.6–2.85 μm, a wavelength range that probes both reflected light in the optical and thermal emission in the near-infrared. Our spectral coverage captures approximately 50–83% of the planet’s bolometric flux, enabling robust constraints on its longitudinally-resolved energy budget. We use a geometric model of the tidally distorted planetary shape to accurately compute the projected area at each orbital phase and convert emergent flux into a phase-resolved effective temperature map.From the emission spectra, we derive hemisphere-averaged brightness temperatures of T_{\rm day}=2762_{−19}^{+17} and T_{\rm night} = 1556^{+17}_{-18}. Accounting for the full bolometric flux using physically motivated extrapolations, we infer a Bond albedo of AB=0.24±0.016 and a heat recirculation efficiency of ϵ=0.23±0.012. These values support previous findings of low reflectivity and inefficient heat redistribution in ultra-hot Jupiter atmospheres.We also detect wavelength-dependent phase curve offsets. At longer wavelengths (λ ≳ 1.5 μm), phase offsets are consistent with zero, indicative of poor heat transport to the nightside. In contrast, we find significant eastward offsets at shorter wavelengths, up to ~25°. If physical in nature, this may be an indication of some unknown physics causing increased reflection on the hotter, Eastern side of the planet. This is supported when fitting the Order 2 (0.6-0.85 μm) wavelengths for both reflected light and thermal emission; we find an increased level of reflected light on the eastern side of the planet, regardless of model.Since this observation extends into optical wavelengths it allows for simultaneous fitting for the Bond albedo and the geometric albedo. The derived geometric albedo in the optical is Ag=0.059−0.018+0.031​, reinforcing the observed albedo paradox that hot Jupiters typically exhibit higher Bond albedo than geometric albedos. To further interpret the thermal structure and dynamical behavior, we compare our observations to a 1D Energy Balance Model (EBM) grid that incorporates hydrogen dissociation/recombination physics and accounts for molecular heat transport. The model allows us to constrain key atmospheric parameters such as the radiative timescale, wind speed, and effective mixed-layer depth. The best-fit EBM solution supports our observational inference of low heat redistribution and suggests modest zonal wind speeds, on the order of less than 1 km/s which may be consistent with magnetic drag in the atmosphere.Together, these findings offer a detailed empirical picture of WASP-121b’s thermal structure and energy transport. The results underscore the power of NIRISS/SOSS for multi-wavelength phase curve studies and demonstrate how future observations across the JWST suite can continue to refine our understanding of irradiated atmospheres under extreme conditions.

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