Evidence for morning-to-evening limb asymmetry on the cool low-density exoplanet WASP-107 b
Fortney, Jonathan J.; Bell, Taylor J.; Line, Michael R.; Parmentier, Vivien; Rieke, Marcia; Beatty, Thomas G.; Rauscher, Emily; Welbanks, Luis; Greene, Thomas P.; Schlawin, Everett; Murphy, Matthew M.
United States, France
Abstract
The atmospheric properties of hot exoplanets are expected to be different between the morning and the evening limbs due to their global atmospheric circulation. Ground-based observations at high spectral resolution have detected this limb asymmetry in several ultra-hot (>2,000 K) exoplanets, but the prevalence of the phenomenon in the broader exoplanetary population remains unexplored. Here we use JWST/NIRCam transmission spectra between 2.5 and 4.0 μm to find evidence of limb asymmetry on exoplanet WASP-107 b. With its equilibrium temperature of 770 K and low density of 0.126 g cm−3, WASP-107 b probes a very different regime compared to ultra-hot giant planets and was not expected to exhibit substantial spatial heterogeneity according to atmospheric models. We infer instead a morning-to-evening temperature difference of the order of 100 K with a hotter evening limb. Further observations of other cooler exoplanets are needed to determine whether WASP-107 b is an outlier or whether the models have underestimated the presence of limb asymmetry in exoplanets.