Connecting JWST Silicate Cloud Observations to Exoplanet Cloud Microphysics with Nimbus

TL;DR

This paper introduces Nimbus, a microphysical cloud model, to interpret JWST silicate cloud observations in exoplanets, revealing insights into cloud particle size, formation processes, and the importance of broad spectral data.

Contribution

We developed Nimbus and Virga models to connect JWST observations with microphysical cloud formation, providing new constraints on particle sizes and growth efficiencies in exoplanet atmospheres.

Findings

All four studied exoplanets have nanometer-sized silicate particles at high altitudes.

Cloud particle settling is highly inefficient, with fsed < 0.1, or growth is limited by very low sticking coefficients.

Sticking coefficients are consistent with laboratory experiments and influence cloud vertical extent.

Abstract

The unprecedented accuracy of JWST has led to the detection of silicate clouds in exoplanet atmospheres, allowing for the first time to probe cloud formation in extreme environments. While parametrized cloud descriptions can fit these observations, the results do not fully agree with microphysical models. To bridge this gap, we developed Nimbus, a fast microphysical cloud model that can constrain cloud formation processes from observations and utilize Virga, an equilibrium condensation model balancing gravitational settling and diffusion. Using both models, we investigate WASP-107 b, WASP-17 b, VHS-1256 b, and YSES-1 c to determine their cloud structure and constrain cloud formation processes. Our results show that all four planets have cluster-sized silicate particles (r ~ 1 nm) at high altitudes. Within Nimbus and Virga, these particles can only be explained by highly inefficient cloud particle settling (fsed < 0.1) or by inefficient growth rates due to low sticking coefficients (s < 0.0001). Our results also show that the sticking coefficient is directly linked to the vertical extent of clouds and can therefore be constrained using the broad shape of the spectral energy distribution. The sticking coefficients found for VHS-1256 b and YSES-1 c are in agreement with expectations from laboratory experiments under Earth-like conditions (0.01 < s < 0.3). Panchromatic observations were crucial to achieve these constraints. Future cloud studies should therefore aim to combine observational data from 1 micron to 10 micron whenever possible.