2026ApJ...997..317L

Aerosols and clouds are expected to be ubiquitous in exoplanet and brown dwarf atmospheres, where they can have a significant impact on transmission and emission spectra. The cloud code Virga is capable of quickly modeling cloud particle sizes as a function of altitude, and has recently been updated to include functionality for aggregates (ranging from very fluffy chains to compact fractals). We analyze the effect that these aggregates have on transmission spectra for typical warm Neptune and hot Jupiter environments, as well as their effect on emission spectra for an L-type brown dwarf, over the wavelength range 0.3-15 μm. We find significant, measurable differences in spectra when particle shape is changed (particularly the shortest wavelengths, where particle morphology strongly affects the scattering slope). We provide some intuitive rules for how nonabsorbing aggregates impact spectra: When particle sizes are small compared to the wavelength of light, the most elongated and chain-like particles have the highest opacities; when particles are large, the inverse is true (the most compact shapes have the highest opacities). We present an explanation for these effects in terms of the dynamics of how the particles form and move through the atmosphere, as well as in terms of fundamental optics theory. Given the significant impact that particle shape can have on spectra, we strongly encourage the community to include shape as a free parameter in future case studies, atmospheric models, and retrievals.