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Recent observations indicate potentially carbon-rich exoplanet atmospheres.Spectral fitting methods for brown dwarfs and exoplanets have invoked the C/Oratio as additional parameter but carbon-rich cloud formation modeling is achallenge for the models applied. The determination of the habitable zone forexoplanets requires the treatment of cloud formation in chemically differentregimes. Disk models show that carbon-rich or near-carbon-rich niches mayemerge and cool carbon planets may trace these particular stages of planetaryevolution. We extend our kinetic cloud formation model by including carbon seedformation and the formation of C[s], TiC[s], SiC[s], KCl[s], and MgS[s] bygas-surface reactions. The seed formation efficiency is lower in carbon-richatmospheres than in oxygen-rich gases due to carbon being a very effectivegrowth species. The consequence is that less particles will make up a cloud forC/O_0>1. The cloud particles will be smaller in size than in an oxygen-richatmosphere. An increasing initial C/O ratio does not revert this trend becausea much greater abundance of condensible gas species exists in a rich carbonenvironment. Cloud particles are generally made of a mix of materials: carbondominates if C/O_0>1 and silicates dominate if C/O_0<1. 80-90% carbon isreached only in extreme cases where C/O_0=3.0 or 10.0. Carbon-rich atmosphereswould form clouds that are made of particles of height-dependent mixedcompositions, sizes and numbers. The remaining gas-phase is far less depletedthan in an oxygen-rich atmosphere. Typical tracer molecules are HCN and C2H2 incombination with a featureless, smooth continuum due to a carbonaceous cloudcover, unless the cloud particles become crystalline.

Dust in brown dwarfs and extra-solar planets V. Cloud formation in carbon- and oxygen-rich environment