Evidence for morning-to-evening limb asymmetry on the cool low-density exoplanet WASP-107 b

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.