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Transmission spectrum surveys have suggested the ubiquity of high-altitudeclouds in exoplanetary atmospheres. Theoretical studies have investigated theformation processes of the high-altitude clouds; however, cloud particles havebeen commonly approximated as compact spheres, which is not always true forsolid mineral particles that likely constitute exoplanetary clouds. Here, weinvestigate how the porosity of cloud particles evolve in exoplanetaryatmospheres and influence the cloud vertical profiles. We first construct aporosity evolution model that takes into account the fractal aggregation andthe compression of cloud particle aggregates. Using a cloud microphysical modelcoupled with the porosity model, we demonstrate that the particle internaldensity can significantly decrease during the cloud formation. As a result,fluffy-aggregate clouds ascend to altitude much higher than that forcompact-sphere clouds assumed so far. We also examine how the fluffy-aggregateclouds affect transmission spectra. We find that the clouds largely obscure themolecular features and produce a spectral slope originated by the scatteringproperties of aggregates. Finally, we compare the synthetic spectra with theobservations of GJ1214 b and find that its flat spectrum could be explained ifthe atmospheric metallicity is sufficiently high ($\ge100\times$ solar) and themonomer size is sufficiently small ($r_{\rm mon}<1~{\rm {\mu}m}$). Thehigh-metallicity atmosphere may offer the clues to explore the gas accretionprocesses onto past GJ1214b.

Clouds of Fluffy Aggregates: How They Form in Exoplanetary Atmospheres and Influence Transmission Spectra