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Optical properties of organic haze analogues in water-rich exoplanet atmospheres observable with JWST

Nature Astronomy volume 8, pages 182–192 (2024)Cite this article

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Abstract

The James Webb Space Telescope (JWST) has begun its scientific mission, which includes the atmospheric characterization of transiting exoplanets. Some of the first exoplanets to be observed by JWST have equilibrium temperatures below 1,000 K, which is a regime where photochemical hazes are expected to form. The optical properties of these hazes, which control how they interact with light, are critical for interpreting exoplanet observations, but relevant experimental data are not available. Here we measure the density and optical properties of organic haze analogues generated in water-rich exoplanet atmosphere experiments. We report optical constants (0.4 to 28.6 μm) of organic haze analogues for current and future observational and modelling efforts covering the entire wavelength range of JWST instrumentation and a large part of Hubble. We use these optical constants to generate hazy model atmospheric spectra. The synthetic spectra show that differences in haze optical constants have a detectable effect on the spectra, impacting our interpretation of exoplanet observations. This study emphasizes the need to investigate the optical properties of hazes formed in different exoplanet atmospheres and establishes a practical procedure for determining such properties.

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Fig. 1: Simplified schematic of the experimental set-up, the simulated atmospheric compositions and conditions, the measurements and the analytical method for the current study.
Fig. 2: Transmittance spectra of two exoplanet haze analogues formed in water-rich gas mixtures at 300 or 400 K.
Fig. 3: Optical constants of two exoplanet haze analogues along with those of the Titan haze analogue from Khare et al.
Fig. 4: Model spectra of a water-rich atmosphere around a GJ 1214 b-like planet, showing the effect of our newly measured haze analogue optical properties.

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Data availability

The data resulting from this study are provided in the article and supplementary information. Data associated with Figs. 3 and 4 are available in the Johns Hopkins University Data Archive from https://doi.org/10.7281/T1/NEACHP. Source data are provided with this paper. Other data that support the plots within this paper and other findings of this study are available from the corresponding author upon reasonable request.

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Acknowledgements

This work was supported by the NASA Exoplanets Research Program 80NSSC20K0271 (C. He) and the NASA Astrophysics Research and Analysis Program NNX17AI87G (S. M. Hörst).

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Authors and Affiliations

  1. Department of Earth and Planetary Sciences, Johns Hopkins University, Baltimore, MD, USA

    Chao He, Michael Radke, Sarah E. Moran & Sarah M. Hörst

  2. School of Earth and Space Sciences, University of Science and Technology of China, Hefei, China

    Chao He

  3. Lunar and Planetary Laboratory, University of Arizona, Tucson, AZ, USA

    Sarah E. Moran & Mark S. Marley

  4. Space Telescope Science Institute, Baltimore, MD, USA

    Sarah M. Hörst & Jeff A. Valenti

  5. Department of Astronomy and Carl Sagan Institute, Cornell University, Ithaca, NY, USA

    Nikole K. Lewis

  6. Space Science Institute, Boulder, CO, USA

    Julianne I. Moses

  7. NASA Ames Research Center, Moffett Field, CA, USA

    Natasha E. Batalha

  8. Department of Astronomy, University of Maryland, College Park, MD, USA

    Eliza M.-R. Kempton

  9. Department of Astronomy, The University of Texas at Austin, Austin, TX, USA

    Caroline V. Morley

  10. Univ. Grenoble Alpes, CNRS, IPAG, Grenoble, France

    Véronique Vuitton

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Contributions

C.H., M.R., S.E.M., S.M.H., N.K.L., M.S.M. and J.I.M. conceived the study. J.I.M. calculated the starting gas mixtures. C.H. carried out the experiments. C.H. and M.R. performed the optical measurements. S.E.M. and C.H. simulated the synthetic transmission spectra. C.H. conducted the data analysis and prepared the manuscript. All authors participated in discussions regarding the interpretation of the results and in editing the manuscript.

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Correspondence to Chao He.

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Nature Astronomy thanks Alexandria Johnson and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.

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Extended data

Extended Data Fig. 1 Model spectra of a water-rich atmosphere around a GJ 1214 b -like planet with 10 nm haze particles.

We show the effect of our newly measured haze optical properties using small radii (10 nm) haze particles, focusing on the wavelength range accessible to Hubble. The method and settings for generating the spectra here are the same as described in 4.5, except the haze particle radii (10 nm) and haze mass loading (4 particles/cm3). With sufficiently small particles, the large scattering slopes between different haze compositions are differentiable with Hubble’s ultraviolet/visible capabilities even if such hazes less strongly impact the NIR.

Supplementary information

Supplementary Information

Supplementary Tables 1–3 and Figs. 1–6.

Source data

Source Data for Fig. 2

The transmittance measured for two samples.

Source Data for Fig. 3

Derived n and k values with uncertainties.

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He, C., Radke, M., Moran, S.E. et al. Optical properties of organic haze analogues in water-rich exoplanet atmospheres observable with JWST. Nat Astron 8, 182–192 (2024). https://doi.org/10.1038/s41550-023-02140-4

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