2025ApJ...989L..17F

Aerosols are an old topic in the young field of exoplanet atmospheres. Understanding what they are, how they form, and where they go has long provided a fertile playground for theorists. For observers, however, aerosols have been a multidecade migraine, as their chronic presence hides atmospheric features. For hot Jupiters, the large day-night temperature contrast drives inhomogeneous thermal structures and aerosol distribution, leading to different limb properties probed by transit spectra. We present JWST NIRISS/SOSS spectra of morning and evening limbs for nine gas giants with equilibrium temperatures of ∼800-1700 K. By measuring feature size of the 1.4 μm water band for both limbs, we found three planets (WASP-39 b, WASP-94 Ab, and WASP-17 b) show prominent (>5σ) limb-limb atmospheric opacity difference with muted morning and clear evening limbs. The heavily muted water features on morning limbs indicate high-altitude (0.1-0.01 mbar) aerosols. To simultaneously have clear evening limbs requires processes with timescales (∼day) comparable to advection to remove these lofted grains, and we found that both downwelling flow and dayside cloud evaporation could be plausible mechanisms. We hypothesize an empirical boundary-termed the “asymmetry horizon”-in temperature-gravity space that marks the transition where inhomogeneous aerosol coverage begins to emerge. Heterogeneous aerosol coverage is common among hot Jupiters. If unrecognized, limb averaging suppresses spectral features, mimicking high-mean-molecular-weight atmospheres, inflating inferred metallicity by up to 2 dex, and underestimating limb temperatures by as much as half. Finally, we introduce the Limb Spectroscopy Metric to predict limb spectral feature size based on planet parameters.