Planets that orbit stars other than our sun are called exoplanets. Over 6,000 exoplanets have been confirmed in our galaxy. Hot Jupiters are a type of exoplanet that orbit very close to their star and are tidally locked, with a permanent daytime and nighttime side. Brown Dwarfs are a type of planet that are failed stars. Being the hottest exoplanets, these emit the most radiation and thus are a prime target for the James Webb Space Telescope. Silicates are a ubiquitous feature of aerosols on hot giant exoplanets 1. WASP 17-b is a hot Jupiter with an orbital period of 3.7 days whose atmosphere was recently observed by James Webb Space Telescope to be dominated by quartz (SiO2) nanocrystals, although magnesium-rich silicates were expected to be seen 2. In the brown dwarf VHS 1256-1257b, the best fit models for spectroscopic observations were clouds of enstatite (MgSiO3), forsterite (Mg2SiO4), and quartz (SiO2) 3. Despite key silicate features in spectroscopy, it is not possible to determine complete atmospheric composition and cloud formation by astronomical observations alone, and particle formation in atmospheres must be modeled. A major factor in modeling atmospheres is condensation and nucleation, which is exponentially dependent on the species’ surface energy, with higher surface energies drastically hindering nucleation rates. Although the significance of surface energy measurements is evident, surface energies of several key species in hot giant exoplanets are not yet constrained by experiment. In this work, surface energies of likely exoplanet atmosphere condensates, including zinc sulfide (ZnS), crystalline and amorphous enstatite (MgSiO3) 4,5 were measured using oxide melt solution calorimetry of appropriate nanoparticles. These are then input into a nucleation code that gives nucleation rates for these species. The surface energy of crystalline SiO2 is much lower than that of the crystalline magnesium-rich silicates 4,6, supporting the observation of silica in the atmosphere of WASP-17b, while the surface energies measured in our lab of amorphous enstatite and amorphous forsterite are much lower than their crystalline counterparts and closer to the surface energy of quartz. This suggests that the initial nucleation of MgSiO3 in VHS 1256-1257b forms the amorphous phase. This research is of significant importance to the interpretation of observations of exoplanets. In particular, our research provides laboratory data of high relevance to a broad range of exoplanet atmospheres.
[1] Adams, D.; Gao, P.; Pater, I. de; Morley, C. V. Aggregate Hazes in Exoplanet Atmospheres. Astrophys J 2019, 874 (1), 61. https://doi.org/10.3847/1538-4357/ab074c.
[2] Grant, D.; Lewis, N. K.; Wakeford, H. R.; Batalha, N. E.; Glidden, A.; Goyal, J.; Mullens, E.; MacDonald, R. J.; May, E. M.; Seager, S.; Stevenson, K. B.; Valenti, J. A.; Visscher, C.; Alderson, L.; Allen, N. H.; Cañas, C. I.; Colón, K.; Clampin, M.; Espinoza, N.; Gressier, A.; Huang, J.; Lin, Z.; Long, D.; Louie, D. R.; Peña-Guerrero, M.; Ranjan, S.; Sotzen, K. S.; Valentine, D.; Anderson, J.; Balmer, W. O.; Bellini, A.; Hoch, K. K. W.; Kammerer, J.; Libralato, M.; Mountain, C. M.; Perrin, M. D.; Pueyo, L.; Rickman, E.; Rebollido, I.; Sohn, S. T.; Marel, R. P. van der; Watkins, L. L. JWST-TST DREAMS: Quartz Clouds in the Atmosphere of WASP-17b. Astrophys J Lett 2023, 956 (2), L29. https://doi.org/10.3847/2041-8213/ACFC3B.
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[4] Householder, M. A.; Subramani, T.; Lilova, K.; Lyons, J. R.; Stroud, R. M.; Navrotsky, A. Calorimetric Measurement of the Surface Energy of Enstatite, MgSiO3. The Journal of Physical Chemistry C 2023, 127 (40), 20106–20112. https://doi.org/10.1021/ACS.JPCC.3C04211.
[5] Subramani, T.; Lilova, K.; Householder, M.; Yang, S.; Lyons, J.; Navrotsky, A. Surface Energetics of Wurtzite and Sphalerite Polymorphs of Zinc Sulfide and Implications for Their Formation in Nature. Geochim Cosmochim Acta 2022, 340, 99–107. https://doi.org/10.1016/j.gca.2022.11.003.
[6] Chen, S.; Navrotsky, A. Calorimetric Study of the Surface Energy of Forsterite. American Mineralogist 2010, 95 (1), 112–117. https://doi.org/10.2138/AM.2010.3339.
