The Days Drag On on WASP-121 b: Interpreting Its NIRISS Spectroscopic Phase Curve with General Circulation Models

Ultra-hot Jupiters (UHJs) present extreme atmospheric phenomena not found in the solar system. These planets’ daysides experience strong temperature inversions, molecular species (including H 2 ) dissociate, and magnetism effects disrupt atmospheric circulation. On their nightsides, H 2 can recombine and clouds may form. Spectroscopic phase curves allow us to measure these spatially inhomogeneous conditions, which can then be interpreted with three-dimensional (3D) models. In this work, we compare the JWST Near Infrared Imager and Slitless Spectrograph (or NIRISS) spectroscopic phase curve of the UHJ WASP-121 b to state-of-the-art 3D models with varying modeling assumptions, including the aforementioned physical phenomena. We demonstrate the importance of accurately accounting for the planet’s radius when comparing data and models, as it changes the implied overall planetary emission. We find that the 3D models predict planetary emission that is ∼12% higher than observed, contributing to the continued tension between measured and predicted hot Jupiter albedos. We identify multiple lines of evidence confirming a strong source of drag operating in this planet’s atmosphere. In addition, the nightside emission spectrum is devoid of strong absorption features, which may be best explained by nightside clouds. One feature of the dataset that is not reproduced by the 3D models is a trend toward increasingly eastward phase offsets at wavelengths shorter than ∼1.4 μ m. This result is inconsistent with reflection from dayside clouds, nor can it be explained by removing atmospheric opacity sources. Our analysis highlights the complexities involved in generating 3D models and interpreting observations of UHJs in the JWST era.