Silicate Sundogs: Probing the Effects of Grain Directionality in Exoplanet Observations

Abstract

Crystalline ice in Earth's atmosphere can produce spectacular phenomena due to orientation-dependent attenuation, such as sun dogs and halos, providing diagnostics of the external processes acting on the aerosol grains. Crystalline mineral aerosols, such as quartz (SiO${}_2$) and enstatite/forsterite (MgSiO${}_3$/Mg${}_2$SiO${}_4$), have long been predicted to form in hot Jupiter atmospheres with JWST MIRI LRS verifying the existence of crystalline quartz observationally. Due to the strong horizontal winds ($\sim$ 1 - 5 km s${}^{-1}$) and small aerosol grains ($<1$ $\mu$m) found in hot Jupiter atmospheres, we show that aerosols could be mechanically aligned with the winds. We then derive directional-dependent optical properties of quartz, enstatite, and forsterite and model transmission and emission spectra assuming random and mechanically aligned orientations, finding that the orientation of all three crystalline aerosols can impart $\ge$ 100 ppm differences in observed spectra (8 - 12 $\mu$m). We run retrievals on JWST MIRI LRS transmission and emission data of WASP-17b and find that directionality alone cannot physically explain the transmission data, pointing towards polymorphs or insufficient lab data, and find weak hints of directionality (1.0 - 1.3$\sigma$) in the emission data. This work demonstrates the power of JWST MIRI LRS in detecting aerosol directionality with future observations, and a technique by which to probe how aerosols interact with atmospheric dynamical processes. To foster the exploration of aerosols in exoplanet data, the open-source code POSEIDON has been updated (v1.3.1) to include 144 new directional and temperature aerosols with precomputed optical properties, alongside new aerosol models.

27 pages, 12 figures, 1 table. Published in ApJL (Volume 988, Issue 2, August 2025)

References in corpus (21)

• Array Programming with NumPy
• Gas- and dust evolution in protoplanetary disks
• On the radiative equilibrium of irradiated planetary atmospheres
• Tracing Magnetic Fields with Aligned Grains
• Transmission spectral properties of clouds for hot Jupiter exoplanets
• The JWST Early Release Science Program for Direct Observations of Exoplanetary Systems II: A 1 to 20 Micron Spectrum of the Planetary-Mass Companion VHS 1256-1257 b
• High temperature condensate clouds in super-hot Jupiter atmospheres
• SO2, silicate clouds, but no CH4 detected in a warm Neptune
• Optical constants of refractory oxides at high temperatures
• Silica in Protoplanetary Disks
• POSEIDON: A Multidimensional Atmospheric Retrieval Code for Exoplanet Spectra
• Quartz Clouds in the Dayside Atmosphere of the Quintessential Hot Jupiter HD 189733 b
• On Internal and External Alignment of Dust Grains in Protostellar Environments
• Millimeter-wave polarization due to grain alignment by the gas flow in protoplanetary disks
• Density, porosity, mineralogy, and internal structure of cosmic dust and alteration of its properties during high velocity atmospheric entry
• Hot Jupiter Diversity and the Onset of TiO/VO Revealed by a Large Grid of Non-Grey Global Circulation Models
• Multiple Clues for Dayside Aerosols and Temperature Gradients in WASP-69 b from a Panchromatic JWST Emission Spectrum
• Effects of grain alignment with magnetic fields on grain growth and the structure of dust aggregates
• JWST-TST DREAMS: A Precise Water Abundance for Hot Jupiter WASP-17b from the NIRISS SOSS Transmission Spectrum
• Why heterogeneous cloud particles matter: Iron-bearing species and cloud particle morphology affects exoplanet transmission spectra
• JWST-TST DREAMS: Non-Uniform Dayside Emission for WASP-17b from MIRI/LRS