- Muted molecular-band amplitudes in the near-infrared, for example weak water structure around : an optically thick exoplanet cloud deck raises the continuum and hides part of the gas column.
- A nearly grey transit-radius continuum across optical/near-infrared wavelengths, roughly --, when particles are large relative to the wavelengths. Suppressed optical atomic-line wings provide a related indication that deep high-pressure layers are hidden.
- A radius increasing toward blue/ultraviolet wavelengths, roughly --, for small-particle scattering of light. In a simple Rayleigh scattering limit, , and the scattering slope of a transmission spectrum is .
A second approach uses the dayside spectrum and brightness obtained with exoplanet secondary eclipses, optionally supported by orbital phase variations. Its three useful aerosol signatures are:
- Enhanced reflected-light geometric albedo or characteristic colour in the visible, roughly --, for a scattering exoplanet cloud. Absorbing atmospheric haze can instead lower the reflected flux, so high albedo is not a universal aerosol rule.
- Weak near-/mid-infrared gas features and a cloud-set continuum brightness temperature, roughly --, when an opaque cloud moves the thermal photosphere to a different pressure. Changes in the exoplanet thermal phase curve can support a spatially patchy cloud interpretation, though circulation also affects it.
- Broad condensate-dependent infrared spectral structure, for example a silicate resonance around , if particles are sufficiently small and the optical-depth/temperature geometry permits the feature. Its presence and sign depend on particle composition, size and vertical thermal structure.
Transmission measures aerosol extinction along the limb; eclipse/reflection measurements test dayside scattering and emission. No single flattened feature proves clouds: small atmospheric scale height, low molecular abundances, stellar heterogeneity and thermal-gradient degeneracies must be considered. Joint wavelength coverage and geometries make the inference stronger.
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