Fractal Aggregate Aerosols in the Virga Cloud Code II: Exploring the Effects of Key Cloud Parameters in Warm Neptune, Hot Jupiter and Brown Dwarf Atmospheres

TL;DR

This paper extends the Virga cloud-model framework to include fractal aggregate aerosols and investigates how particle shape, parameterized by the fractal dimension $D_{\rm f}$, alters transmission and emission spectra in warm Neptune, hot Jupiter, and brown dwarf atmospheres across $0.3-15~\mu$m. Using MMF-based optics (via optool) and PICASO atmosphere models, it demonstrates that aggregates form larger particles and exhibit distinctive optical properties compared to spheres, with regimes set by the Rayleigh and geometric limits. The study yields two practical rules for purely scattering aggregates: in the small-particle regime elongated aggregates have higher opacities, while in the large-particle regime elongated aggregates become less opaque than compact ones, and it shows these effects can produce measurable changes in spectral slopes ($\alpha$). It argues for including particle shape as a free parameter in retrievals and future atmosphere models to better capture the spectral signatures of exoplanet and brown dwarf aerosols. Overall, the work highlights the need to account for fractal aggregate morphology to accurately interpret exoplanetary spectra and informs observational strategies with JWST and future facilities.

Abstract

Aerosols and clouds are expected to be ubiquitous in exoplanet and brown dwarf atmospheres, where they can have a significant impact on transmission and emission spectra. The cloud code Virga is capable of quickly modeling cloud particle sizes as a function of altitude, and has recently been updated to include functionality for aggregates (ranging from very fluffy chains to compact fractals). We analyze the effect that these aggregates have on transmission spectra for typical warm Neptune and hot Jupiter environments, as well as their effect on emission spectra for an L-type brown dwarf, over the wavelength range 0.3 - 15 um. We find significant, measurable differences in spectra when particle shape is changed (particularly the shortest wavelengths where particle morphology strongly affects the scattering slope). We provide some intuitive rules for how non-absorbing aggregates impact spectra: when particle sizes are small compared to the wavelength of light, the most elongated and chain-like particles have the highest opacities. When particles are large, the inverse is true (the most compact shapes have the highest opacities). We present an explanation for these effects in terms of the dynamics of how the particles form and move through the atmosphere, as well as in terms of fundamental optics theory. Given the significant impact that particle shape can have on spectra, we strongly encourage the community to include shape as a free parameter in future case studies, atmospheric models, and retrievals.

Figures (13)

• Figure 1: Three possible shapes for aerosol particles (from left to right, fractal dimensions $D_{\rm{f}}$ of 1.6, 2.8, and a sphere). In this illustration each shape has the same mass, and therefore also the same compact radius $r$ (the radius of a sphere made from the same volume as each shape). However, the radius of gyration $R_{\rm{gyr}}$ of the aggregates (used to calculate the optical and dynamical properties) can differ, and is largest for shapes with the lowest fractal dimensions.
• Figure 2: Comparison of the number densities and sizes of particles formed within a single pressure layer for different condensate particle shapes. In the actual Virga model, distributions follow a lognormal size distribution, but here we only show the mean particle size in each case for clarity in showing how the sizes of particles within the distributions compare.
• Figure 3: Diagram describing a toy model in which we consider the difference in opacity of a single sphere of radius $R$, versus the same mass but distributed into a group of much smaller spheres of radius $r$.
• Figure 4: Refractive indices of KCl (light blue) and Mg$_2$SiO$_4$ (dark blue) as a function of wavelength, plotted on (a) linear and (b) logarithmic scales. The imaginary component $k$ (dotted lines) determines how absorptive each material is at a particular wavelength.
• Figure 5: Extinction efficiency as a function of compact radius $r$ for KCl particles of a variety of different shapes (with $\lambda=0.3~\mu$m), plotted on both a logarithmic (a) and linear (b) scale. The Rayleigh regime (where $r \ll \lambda$ and therefore the approximation $Q_{\rm{ext}}\propto r^4$ can be used) is shown in (a).