: A golden mean approach to multispecies cloud modeling in atmospheric retrieval

Context. Clouds are ubiquitous in exoplanets’ atmospheres and play an important role in setting the opacity and chemical inventory of the atmosphere. Understanding clouds is a critical step in interpreting exoplanets’ spectroscopic data. Aims. The aim is to model the multispecies nature of clouds in atmospheric retrieval studies. To this end, we developed <ns0:monospace>ExoLyn</ns0:monospace> – a 1D cloud model that balances physical consistency with computational efficiency. Methods. <ns0:monospace>ExoLyn</ns0:monospace> solves the transport equation of cloud particles and vapor under cloud condensation rates that are self-consistently calculated from thermodynamics. Exolyn is a standalone, open source package capable of being combined with <ns0:monospace>optool</ns0:monospace> to calculate solid opacities and with <ns0:monospace>petitRADTRANS</ns0:monospace> to generate transmission or emission spectra. Results. With <ns0:monospace>ExoLyn</ns0:monospace> we find that the compositional structure of clouds in hot Jupiter planets’ atmospheres is layered with a cloud dominated by magnesiumsilicates on top of an iron cloud. This finding is consistent with more complex cloud formation models but can be obtained with <ns0:monospace>ExoLyn</ns0:monospace> in only a few seconds. The composition of the cloud particles can be constrained from the spectrum, for example, MgSiO3 and Mg2SiO4 components give rise to an absorption feature at 8–10 μm. We investigate the dependence of the cloud structure on the bulk elemental composition of the planet and find that SiO2-dominated clouds form on metal-rich planets and Fe clouds with a strong extinction effect form on C-rich planets. Conclusions. Designed toward maximum flexibility, <ns0:monospace>ExoLyn</ns0:monospace> can also be used in retrieval analysis of sub-Neptunes and self-luminous planets. The efficiency of <ns0:monospace>ExoLyn</ns0:monospace> opens the possibility of joint retrieval of exoplanets’ gas and cloud components.