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.
This is a preview of subscription content, access via your institution
Access options
Access Nature and 54 other Nature Portfolio journals
Get Nature+, our best-value online-access subscription
$29.99 / 30 days
cancel any time
Subscribe to this journal
Receive 12 digital issues and online access to articles
$119.00 per year
only $9.92 per issue
Buy this article
- Purchase on SpringerLink
- Instant access to full article PDF
Prices may be subject to local taxes which are calculated during checkout
Similar content being viewed by others
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.
References
Kreidberg, L. et al. Clouds in the atmosphere of the super-Earth exoplanet GJ1214b. Nature 505, 69–72 (2014).
Knutson, H. A., Benneke, B., Deming, D. & Homeier, D. A featureless transmission spectrum for the Neptune-mass exoplanet GJ436b. Nature 505, 66–68 (2014).
Knutson, H. A. et al. Hubble space telescope near-IR transmission spectroscopy of the super-Earth HD 97658b. Astrophys. J. 794, 155 (2014).
May, E. M., Gardner, T., Rauscher, E. & Monnier, J. D. MOPSS. II. Extreme optical scattering slope for the inflated super-Neptune HATS-8b. Astron. J. 159, 7 (2019).
Dragomir, D. et al. Rayleigh scattering in the atmosphere of the warm exo-Neptune GJ 3470b. Astrophys. J. 814, 102 (2015).
JWST Transiting Exoplanet Community Early Release Science Team Identification of carbon dioxide in an exoplanet atmosphere. Nature 614, 649–652 (2023).
Gao, P. et al. Aerosol composition of hot giant exoplanets dominated by silicates and hydrocarbon hazes. Nat. Astron. 4, 951–956 (2020).
Morley, C. V. et al. Quantitatively assessing the role of clouds in the transmission spectrum of GJ 1214b. Astrophys. J. 775, 33 (2013).
He, C. et al. Photochemical haze formation in the atmospheres of super-Earths and mini-Neptunes. Astron. J. 156, 38 (2018).
Hörst, S. M. et al. Haze production rates in super-Earth and mini-Neptune atmosphere experiments. Nat. Astron. 2, 303–306 (2018).
He, C. et al. Sulfur-driven haze formation in warm CO2-rich exoplanet atmospheres. Nat. Astron. 4, 986–993 (2020).
He, C. et al. Haze formation in warm H2-rich exoplanet atmospheres. Planet. Sci. J. 1, 51 (2020).
Gao, P., Wakeford, H. R., Moran, S. E. & Parmentier, V. Aerosols in exoplanet atmospheres. J. Geophys. Res. Planets 126, e06655 (2021).
Khare, B. N. et al. Optical constants of organic tholins produced in a simulated Titanian atmosphere: from soft X-ray to microwave frequencies. Icarus 60, 127–137 (1984).
Chang, H. & Charalampopoulos, T. T. Determination of the wavelength dependence of refractive indices of flame soot. Proc. R. Soc. Lond. A 430, 577–591 (1990).
Lavvas, P. & Koskinen, T. Aerosol properties of the atmospheres of extrasolar giant planets. Astrophys. J. 847, 32 (2017).
Morley, C. V. et al. Thermal emission and reflected light spectra of super Earths with flat transmission spectra. Astrophys. J. 815, 110 (2015).
Moran, S. E. et al. Chemistry of temperate super-Earth and mini-Neptune atmospheric hazes from laboratory experiments. Planet. Sci. J. 1, 17 (2020).
Tsiaras, A., Waldmann, I. P., Tinetti, G., Tennyson, J. & Yurchenko, S. N. Water vapour in the atmosphere of the habitable-zone eight-Earth-mass planet K2-18b. Nat. Astron. 3, 1086–1091 (2019).
Benneke, B. et al. Water vapor and clouds on the habitable-zone sub-Neptune exoplanet K2-18b. Astrophys. J. Lett. 887, L14 (2019).
Mulders, G., Ciesla, F., Min, M. & Pascucci, I. The snow line in viscous disks around low-mass stars: implications for water delivery to terrestrial planets in the habitable zone. Astrophys. J. 807, 9–15 (2015).
Kite, E. S. & Ford, E. B. Habitability of exoplanet waterworlds. Astrophys. J. 864, 75–102 (2018).
Zeng, L. et al. Growth model interpretation of planet size distribution. Proc. Natl Acad. Sci. USA 116, 9723–9728 (2019).
Kite, E. S. & Schaefer, L. Water on hot rocky exoplanets. Astrophys. J. Lett. 909, L22 (2021).
Luque, R. & Pallé, E. Density, not radius, separates rocky and water-rich small planets orbiting M dwarf stars. Science 377, 1211–1214 (2022).
Chachan, Y. et al. A featureless infrared transmission spectrum for the super-puff planet Kepler-79d. Astrophys. J. 160, 201 (2020).
Libby-Roberts, J. E. et al. The featureless transmission spectra of two super-puff planets. Astron. J. 159, 57 (2020).
Adams, D., Gao, P., de Pater, I. & Morley, C. V. Aggregate hazes in exoplanet atmospheres. Astrophys. J. 874, 61 (2019).
Gao, P. & Zhang, X. Deflating super-puffs: impact of photochemical hazes on the observed mass–radius relationship of low-mass planets. Astrophys. J. 890, 93 (2020).
Ohno, K. & Tanaka, Y. A. Grain growth in escaping atmospheres: implications for the radius inflation of super-puffs. Astrophys. J. 920, 124 (2021).
He, C. et al. Carbon monoxide affecting planetary atmospheric chemistry. Astrophys. J. Lett. 841, L31 (2017).
Moran, S. E. et al. Triton haze analogs: the role of carbon monoxide in haze formation. J. Geophys. Res. Planets 127, e2021JE006984 (2022).
Rao, C. N. R. (ed.) Ultra-violet and Visible Spectroscopy; Chemical Applications (Butterworth, 1975).
van Krevelen D. W. & te Nijenhuis K. in Properties of Polymers 4th edn, Ch. 10 (Elsevier, 2009).
Lin-Vien, D., Colthup, N. B., Fateley, W. G. & Grasselli, J. G. The Handbook of Infrared and Raman Characteristic Frequencies of Organic Molecules, 503 (Academic Press, 1991).
Socrates, G. Infrared and Raman Characteristic Group Frequencies, 347 (Wiley, 2001).
Duvernay, F. et al. Carbodiimide production from cyanamide by UV irradiation and thermal reaction on amorphous water ice. J. Phys. Chem. A 109, 603–608 (2005).
Khare, B. N. et al. Analysis of the time-dependent chemical evolution of Titan haze tholin. Icarus 160, 172–182 (2002).
Imanaka, H. et al. Laboratory experiments of Titan tholin formed in cold plasma at various pressures: implications for nitrogen-containing polycyclic aromatic compounds in Titan haze. Icarus 168, 344–366 (2004).
Vinatier, S. et al. Optical constants of Titan’s stratospheric aerosols in the 70–1500 cm−1 spectral range constrained by Cassini/CIRS observations. Icarus 219, 5–12 (2012).
Ohno, K., Zhang, X., Tazaki, R. & Okuzumi, S. Haze formation on Triton. Astrophys. J. 912, 37 (2021).
Zhang, X., Strobel, D. F. & Imanaka, H. Haze heats Pluto’s atmosphere yet explains its cold temperature. Nature 551, 352–355 (2017).
Arney, G. N. et al. Pale orange dots: the impact of organic haze on the habitability and detectability of Earthlike exoplanets. Astrophys. J. 836, 49 (2017).
Cloutier, R. et al. A more precise mass for GJ 1214 b and the frequency of multiplanet systems around mid-M dwarfs. Astron. J. 162, 174 (2021).
Lora, J. M. et al. Atmospheric circulation, chemistry, and infrared spectra of Titan-like exoplanets around different stellar types. Astrophys. J. 853, 58 (2018).
Teal, D. J. et al. Effects of UV stellar spectral uncertainty on the chemistry of terrestrial atmospheres. Astrophys. J. 927, 90 (2022).
Ackerman, A. S. & Marley, M. S. Precipitating condensation clouds in substellar atmospheres. Astrophys. J. 556, 872–884 (2001).
Rooney, C. M. et al. A new sedimentation model for greater cloud diversity in giant exoplanets and brown dwarfs. Astrophys. J. 925, 33 (2022).
Batalha, N. E. et al. Exoplanet reflected-light spectroscopy with PICASO. Astrophys. J. 878, 70 (2019).
Ohno, K. & Kawashima, Y. Super-Rayleigh slopes in transmission spectra of exoplanets generated by photochemical haze. Astrophys. J. Lett. 895, L47 (2020).
Rustamkulov, Z. et al. A panchromatic spectrum of the exoplanet WASP-39b with JWST NIRSpec PRISM. Nature 614, 659–663 (2023).
Sing, D. K. et al. A continuum from clear to cloud hot-Jupiter exoplanets without primordial water depletion. Nature 529, 7584 (2016).
Corrales, L. et al. Photochemical hazes can trace the C/O ratio in exoplanet atmospheres. Astrophys. J. Lett. 943, L26 (2023).
Lupu, R. E. et al. Developing atmospheric retrieval methods for direct imaging spectroscopy of gas giants in reflected light. I. Methane abundances and basic cloud properties. Astron. J. 152, 217 (2016).
Steinrueck, M. E. et al. 3D simulations of photochemical hazes in the atmosphere of hot Jupiter HD 189733b. Mon. Not. R. Astron. Soc. 504, 2783–2799 (2021).
Moses, J. I. et al. Compositional diversity in the atmospheres of hot Neptunes, with application to GJ 436b. Astrophys. J. 777, 34 (2013).
Ramirez, S. I. et al. Complex refractive index of Titan’s aerosol analogues in the 200–900 nm domain. Icarus 156, 515–529 (2002).
Tran, B. N. et al. Simulation of Titan haze formation using a photochemical flow reactor: the optical constants of the polymer. Icarus 165, 379–390 (2003).
Vuitton, V., Tran, B. N., Persans, P. D. & Ferris, J. P. Determination of the complex refractive indices of Titan haze analogs using photothermal deflection spectroscopy. Icarus 203, 663–671 (2009).
Imanaka, H., Cruikshank, D. P., Khare, B. N. & McKay, C. P. Optical constants of Titan tholins at mid-infrared wavelengths (2.5–25 μm) and the possible chemical nature of Titan’s haze particles. Icarus 218, 247–261 (2012).
Sciamma-O’Brien, E. et al. Optical constants from 370 nm to 900 nm of Titan tholins produced in a low pressure RF plasma discharge. Icarus 218, 356–363 (2012).
Mahjoub, A. et al. Influence of methane concentration on the optical indices of Titan’s aerosols analogues. Icarus 221, 670–677 (2012).
Gavilan, L., Carrasco, N., Hoffmann, S. V., Jones, N. C. & Mason, N. J. Organic aerosols in anoxic and oxic atmospheres of Earth-like exoplanets: VUV-MIR spectroscopy of CHON tholins. Astrophys. J. 861, 110 (2018).
Jovanović, L. et al. Optical constants of Pluto aerosol analogues from UV to near-IR. Icarus 362, 114398 (2021).
He, C., Hörst, S. M., Radke, M. & Yant, M. Optical constants of a Titan haze analog from 0.4 to 3.5 μm determined using vacuum spectroscopy. Planet. Sci. J. 3, 25 (2022).
Brassé, C., Muñoz, O., Coll, P. & Raulin, F. Optical constants of Titan aerosols and their tholins analogs: experimental results and modeling/observational data. Planet. Space Sci. 109, 159–174 (2015).
Myers, T. L., et al. Obtaining the complex optical constants n and k via quantitative absorption measurements in KBr pellets. In Proc. Chemical, Biological, Radiological, Nuclear, and Explosives (CBRNE) Sensing XX, Vol. 11010 (eds. Guicheteau, J. A. & Howle, C. R.) (SPIE, Bellingham, 2019).
Wood, B. E. & Roux, J. A. Infrared optical properties of thin H2O, NH3, and CO2 cryofilms. J. Opt. Soc. Am. 72, 720–728 (1982).
Toon, O. B. et al. Infrared optical constants of H2O ice, amorphous nitric acid solutions, and nitric acid hydrates. J. Geophys. Res. Atmos. 99, 25631–25654 (1994).
Padera, F. Measuring Absorptance (k) and Refractive Index (n) of Thin Films with the PerkinElmer Lambda 1050+ High Performance UV/Vis/NIR Spectrometers Application Note (PerkinElmer, 2019).
Sumlin, B. J., Heinson, W. R. & Chakrabarty, R. K. Retrieving the aerosol complex refractive index using PyMieScatt: a Mie computational package with visualization capabilities. J. Quant. Spectrosc. Radiat. Transf. 205, 127–134 (2018).
He, C. et al. Laboratory simulations of haze formation in the atmospheres of super-Earths and mini-Neptunes: particle color and size distribution. Astrophys. J. Lett. 856, 1 (2018).
Lavvas, P., Yelle, R. V. & Vuitton, V. The detached haze layer in Titan’s mesosphere. Icarus 201, 626–633 (2009).
Kawashima, Y. & Ikoma, M. Theoretical transmission spectra of exoplanet atmospheres with hydrocarbon haze: effect of creation, growth, and settling of haze particles. I. Model description and first results. Astrophys. J. 853, 7 (2018).
Trainer, M. G. et al. The influence of benzene as a trace reactant in Titan aerosol analogs. Astrophys. J. Lett. 766, L4 (2013).
Lavvas, P. et al. Aerosol growth in Titan’s Ionosphere. Proc. Natl Acad. Sci. USA 110, 8 (2013).
Yoon, Y. H. et al. The role of benzene photolysis in Titan haze formation. Icarus 233, 233–241 (2014).
Gao, P. & Benneke, B. Microphysics of KCl and ZnS clouds on GJ 1214 b. Astrophys. J. 863, 165 (2018).
Ohno, K. & Okuzumi, S. A condensation–coalescence cloud model for exoplanetary atmospheres: formulation and test applications to terrestrial and Jovian clouds. Astrophys. J. 835, 261 (2017).
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).
Author information
Authors and Affiliations
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.
Corresponding author
Ethics declarations
Competing interests
The authors declare no competing interests.
Peer review
Peer review information
Nature Astronomy thanks Alexandria Johnson and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.
Additional information
Publisher’s note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
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.
Rights and permissions
Springer Nature or its licensor (e.g. a society or other partner) holds exclusive rights to this article under a publishing agreement with the author(s) or other rightsholder(s); author self-archiving of the accepted manuscript version of this article is solely governed by the terms of such publishing agreement and applicable law.
About this article
Cite this article
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
Received:
Accepted:
Published:
Issue Date:
DOI: https://doi.org/10.1038/s41550-023-02140-4
This article is cited by
-
In an exoplanet atmosphere far, far away
Nature Reviews Chemistry (2024)