Cloud Parameterizations and their Effect on Retrievals of Exoplanet Reflection Spectroscopy

TL;DR

This study investigates how different cloud models affect the retrieval of atmospheric properties from low-resolution reflected spectra of exoplanets, highlighting the importance of model choice for accurate atmospheric characterization.

Contribution

It systematically compares four cloud parameterizations in retrievals of reflected spectra from cool giant exoplanets, revealing their impact on inferred atmospheric and cloud properties.

Findings

Simplistic models overestimate molecular abundances.

Retrievals struggle to accurately determine planetary gravity.

Low SNR and resolution spectra provide only basic cloud information.

Abstract

Future space-based direct imaging missions will perform low-resolution (R$<$100) optical (0.3-1~$\mu$m) spectroscopy of planets, thus enabling reflected spectroscopy of cool giants. Reflected light spectroscopy is encoded with rich information about the scattering and absorbing properties of planet atmospheres. Given the diversity of clouds and hazes expected in exoplanets, it is imperative we solidify the methodology to accurately and precisely retrieve these scattering and absorbing properties that are agnostic to cloud species. In particular, we focus on determining how different cloud parameterizations affect resultant inferences of both cloud and atmospheric composition. We simulate mock observations of the reflected spectra from three top priority direct imaging cool giant targets with different effective temperatures, ranging from 135 K to 533 K. We perform retrievals of cloud structure and molecular abundances on these three planets using four different parameterizations, each with increasing levels of cloud complexity. We find that the retrieved atmospheric and scattering properties strongly depend on the choice of cloud parameterization. For example, parameterizations that are too simplistic tend to overestimate the abundances. Overall, we are unable to retrieve precise/accurate gravity beyond $\pm$50\%. Lastly, we find that even low SNR=5, low R=40 reflected light spectroscopy gives cursory zeroth order insights into cloud deck position relative to molecular and Rayleigh optical depth level.