Aerosols are common in the atmospheres of exoplanets across a wide swath of temperatures, masses and ages1,2,3. These aerosols strongly impact observations of transmitted, reflected and emitted light from exoplanets, obfuscating our understanding of exoplanet thermal structure and composition4,5,6. Knowing the dominant aerosol composition would facilitate interpretations of exoplanet observations and theoretical understanding of their atmospheres. A variety of compositions have been proposed, including metal oxides and sulfides, iron, chromium, sulfur and hydrocarbons7,8,9,10,11. However, the relative contributions of these species to exoplanet aerosol opacity is unknown. Here we show that the aerosol composition of giant exoplanets observed in transmission is dominated by silicates and hydrocarbons. By constraining an aerosol microphysics model with trends in giant exoplanet transmission spectra, we find that silicates dominate aerosol opacity above planetary equilibrium temperatures of 950 K due to low nucleation energy barriers and high elemental abundances, while hydrocarbon aerosols dominate below 950 K due to an increase in methane abundance. Our results are robust to variations in planet gravity and atmospheric metallicity within the range of most giant transiting exoplanets. We predict that spectral signatures of condensed silicates in the mid-infrared are most prominent for hot (>1,600 K), low-gravity (<10 m s−2) objects.
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Observed AH values shown in Fig. 1 are published in ref. 15 and presented in Supplementary Table 1. Model results that support Figs. 1, 2 and 4 within the main text are provided as Source Data files. Model results that support the plots in the Supplementary Information, including the temperature–pressure and eddy diffusion coefficient profiles, composition profiles, compilation of refractive indices of exoplanet aerosol materials, and synthetic transmission spectra are available from the corresponding author upon reasonable request.
Many of the numerical models used in this work, including CARMA, the thermal structure model, the planetary interior model and the transmission spectrum model are not public. However, they are available from the corresponding author upon reasonable request. GGchem is public at https://github.com/pw31/GGchem; pymiecoated is public at https://github.com/jleinonen/pymiecoated. We include a code to compute heterogeneous and homogeneous nucleation rates, which we used to generate Fig. 3 in the main text, as a Supplementary Software file (gao_fig3_nucrate.py).
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We thank M. S. Marley for his valuable insights and G. Fu for enlightening discussions. P.G. acknowledges support from the NASA Postdoctoral Program and the 51 Pegasi b Fellowship from the Heising-Simons Foundation. G.K.H.L. acknowledges support from the University of Oxford and CSH Bern through the Bernoulli Fellowship, and funding from the European community through the ERC advanced grant exocondense (number 740963). X.Z. is supported by NASA Solar System Workings grant 80NSSC19K0791. H.R.W. acknowledges support from the Giacconi Prize Fellowship at STScI, operated by AURA.
The authors declare no competing interests.
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Text file showing the model AH values that we computed for clear and cloudy atmospheres across the temperature–gravity–metallicity parameter space that we considered.
Excel spreadsheet showing (sheet 1) the fractional optical depth contributions of different aerosol species at the pressures probed by transmission spectroscopy in the J or H band for the g = 10 m s−2, 10× solar atmospheric metallicity case, (sheets 2–5) pressures probed by transmission spectroscopy in the 1.4 μm water band and J or H band that we computed for clear and cloudy atmospheres across the temperature–gravity–metallicity parameter space that we considered, and (sheet 6) mean aerosol particle radius at the pressures probed by transmission spectroscopy in the J of H band that we computed for cloudy atmospheres across the temperature–gravity–metallicity parameter space that we considered.
Text file showing the predicted 10 μm silicate feature amplitudes in transmission that we computed for cloudy atmospheres across the temperature–gravity–metallicity parameter space that we considered. Data are in ascii format.
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Gao, P., Thorngren, D.P., Lee, G.K.H. et al. Aerosol composition of hot giant exoplanets dominated by silicates and hydrocarbon hazes. Nat Astron (2020). https://doi.org/10.1038/s41550-020-1114-3