  Transmission spectroscopy has enabled unprecedented insights into the makeup
of exoplanet atmospheres. A transmission spectrum combines contributions from a
planet's morning and evening limbs, but these limbs may have different
temperatures, compositions, and aerosol properties due to atmospheric
circulation. High-resolution ground-based observations have detected limb
asymmetry on several ultra-hot (>2000 K) exoplanets, but space-based
investigation into limb asymmetry is in its infancy, and limb asymmetry's
prevalence in the broader exoplanet population remains unexplored. We find
evidence for limb asymmetry on the exoplanet WASP-107b via transmission
spectroscopy from 2.5 to 4.0 micrometers with JWST/NIRCam. This is one of the
first low-resolution space-based measurements of limb asymmetry and is unique
because, at 770 K, WASP-107b is in a relatively cool regime where planetary
terminators are expected to be homogeneous. These observations imply a
difference in temperature and cloud properties between WASP-107b's limbs,
challenging our models of limb asymmetry in this cooler regime.
