# On Signatures of Clouds in Exoplanetary Transit Spectra

## TL;DR

This paper develops and applies three observable metrics—optical slope, its uniformity, and infrared features—to constrain cloud properties in exoplanet atmospheres, using model spectra and real observations, highlighting the potential and limitations of current methods.

## Contribution

It introduces a comprehensive framework using optical and infrared spectral features to characterize clouds in exoplanet atmospheres, supported by publicly available Mie scattering data.

## Key findings

- Steep optical slopes suggest sulphide clouds like MnS and ZnS.
- Non-uniform optical slopes indicate certain condensates such as TiO2 and Fe2O3.
- Infrared features can identify condensates like SiO2 and MgSiO3 with JWST.

## Abstract

Transmission spectra of exoplanetary atmospheres have been used to infer the presence of clouds/hazes. Such inferences are typically based on spectral slopes in the optical deviant from gaseous Rayleigh scattering or low-amplitude spectral features in the infrared. We investigate three observable metrics that could allow constraints on cloud properties from transmission spectra, namely, the optical slope, the uniformity of this slope, and condensate features in the infrared. We derive these metrics using model transmission spectra considering Mie extinction from a wide range of condensate species, particle sizes, and scale heights. Firstly, we investigate possible degeneracies among the cloud properties for an observed slope. We find, for example, that spectra with very steep optical slopes suggest sulphide clouds (e.g. MnS, ZnS, Na$_2$S) in the atmospheres. Secondly, (non)uniformities in optical slopes provide additional constraints on cloud properties, e.g., MnS, ZnS, TiO$_2$, and Fe$_2$O$_3$ have significantly non-uniform slopes. Thirdly, infrared spectra provide an additional powerful probe into cloud properties, with SiO$_2$, Fe$_2$O$_3$, Mg$_2$SiO$_4$, and MgSiO$_3$ bearing strong infrared features observable with the James Webb Space Telescope. We investigate observed spectra of eight hot Jupiters and discuss their implications. In particular, no single or composite condensate species considered here conforms to the steep and non-uniform optical slope observed for HD 189733b. Our work highlights the importance of the three above metrics to investigate cloud properties in exoplanetary atmospheres using high-precision transmission spectra and detailed cloud models. We make our Mie scattering data for condensates publicly available to the community.