Probing the Formation Mechanisms and Vibrational Spectra of Silicate Cloud Particles on WASP-17b using Molecular Simulation

Grant et al (2023) recently detected an 8.6 micron absorption in the atmosphere of the exoplanet WASP-17b using the James Webb Space Telescope (JWST), an infrared feature proposed to be caused by the presence of ~10nm radius “Quartz Cloud” droplets. WASP-17b is a so-called “hot Jupiter” exoplanet: its upper atmosphere has a temperature of 1250 K and pressure of 10-3 bar. Such conditions are below the melting point of quartz but also within the region of the silica phase diagram where β-tridymite should be the most stable phase. 

In this work, the vibrational densities of states of silica nanoparticles (quartz, β-tridymite and amorphous silica) have been calculated using both traditional force field molecular dynamics (Heinz et al 2013) and extended tight binding (Bannwarth et al 2019) methods, showing generally good agreement with the JWST data. The degree of hydrogenation of dangling Si-O bonds is found to affect the absorption wavelength (and therefore the overlap with the planetary spectrum) more significantly than the phase of the nanoparticles, confirming solid Silica is likely present in the WASP-17b atmosphere, but is not necessarily quartz.

Under the assumption that such "droplets” form through a nucleation and growth mechanism akin to that of terrestrial aerosol, atomistic simulations were conducted using the new generation neural network potential MACE-MH-1 (Batatia et al 2025). Both molecular SiO2 nucleation and the proposed oxidation of silicon monoxide by water were studied. We report a novel exoplanetary aerosol formation mechanism, involving clusters that initially form as polymeric chains with tetrahedral arrangements of Silica units, before transitioning to larger interconnected rings as they grow.

References

David Grant et al, The Astrophysical Journal Letters, 2023, 956, L29

Hendrik Heinz et al, Langmuir, 2013, 29(6), 1754

Christoph Bannwarth et al , J. Chem. Theory Comput., 2019, 15(3), 1652

Ilyes Batatia et al, arXiv, 2025, 2510.25380