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Aerosol composition of hot giant exoplanets dominated by silicates and hydrocarbon hazes

Nature Astronomy volume 4, pages 951–956 (2020)Cite this article

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Abstract

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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Fig. 1: Exoplanet cloudiness as a function of equilibrium temperature, gravity and atmospheric metallicity.
Fig. 2: Evolution of exoplanet aerosols with temperature.
Fig. 3: Nucleation rates of exoplanet condensates.
Fig. 4: Predictions for the amplitude of the 10 μm condensed silicate spectral feature in transmission.

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Data availability

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.

Code availability

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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Acknowledgements

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. E.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.

Author information

Authors and Affiliations

  1. Department of Astronomy, University of California Berkeley, Berkeley, CA, USA

    Peter Gao

  2. Institute for Research on Exoplanets, Universite ‘de Montreal, Montreal, Quebec, Canada

    Daniel P. Thorngren

  3. Department of Physics, University of Oxford, Oxford, UK

    Elspeth K. H. Lee

  4. Department of Astronomy and Astrophysics, University of California Santa Cruz, Santa Cruz, CA, USA

    Jonathan J. Fortney & Diana K. Powell

  5. Department of Astronomy, University of Texas at Austin, Austin, TX, USA

    Caroline V. Morley

  6. School of Physics, HH Wills Physics Laboratory, University of Bristol, Bristol, UK

    Hannah R. Wakeford

  7. Johns Hopkins Applied Physics Laboratory, Laurel, MD, USA

    Kevin B. Stevenson

  8. Department of Earth and Planetary Sciences, University of California Santa Cruz, Santa Cruz, CA, USA

    Xi Zhang

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Contributions

P.G. conceived the research, performed the calculations and wrote the manuscript. D.P.T. provided necessary parameters for constructing the background atmospheres. E.K.H.L. supplied the equilibrium gas abundances for the various condensate species. J.J.F. and X.Z. imparted much needed guidance on the scope and structure of the manuscript. C.V.M. provided the refractive indices for most of the aerosol species considered here. H.R.W. contributed to manuscript writing and, along with K.B.S., provided guidance on data handling. D.K.P. provided insights on the aerosol microphysics model. All authors contributed to editing the manuscript.

Corresponding author

Correspondence to Peter Gao.

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Supplementary information

Supplementary Information

Supplementary Figs. 1–7 and Tables 1–4.

Supplementary Software

Python code that computes nucleation rates; can be used to recreate Fig. 3 in the main text. Python 3 required.

Source data

Source Data Fig. 1

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.

Source Data Fig. 2

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.

Source Data Fig. 4

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, E.K.H. et al. Aerosol composition of hot giant exoplanets dominated by silicates and hydrocarbon hazes. Nat Astron 4, 951–956 (2020). https://doi.org/10.1038/s41550-020-1114-3

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