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Haze evolution in temperate exoplanet atmospheres through surface energy measurements

Abstract

Photochemical hazes are important opacity sources in temperate exoplanet atmospheres, hindering current observations from characterizing exoplanet atmospheric compositions. The haziness of an atmosphere is determined by the balance between haze production and removal. However, the material-dependent removal physics of the haze particles are currently unknown under exoplanetary conditions. Here we provide experimentally measured surface energies for a grid of temperate exoplanet hazes to characterize haze removal in exoplanetary atmospheres. We found large variations of surface energies for hazes produced under different energy sources, atmospheric compositions and temperatures. The surface energies of the hazes were found to be the lowest around 400 K for the cold plasma samples, leading to the lowest removal rates. We show a suggestive correlation between haze surface energy and atmospheric haziness with planetary equilibrium temperature. We hypothesize that habitable-zone exoplanets could be less hazy, as they would possess high-surface-energy hazes that can be removed efficiently.

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Fig. 1: Experimental set-up for exoplanet haze production and surface energy measurement.
Fig. 2: Summary of the derived surface energies for the cold plasma and UV exoplanet haze samples.
Fig. 3: Summary of the measured mean contact angles between water and the haze samples using circle fitting results.
Fig. 4: Summary of derived mean refractive indices at visible wavelengths for the haze samples.
Fig. 5: Haze production and removal schematics for low-surface-energy and high-surface-energy hazes.
Fig. 6: Exoplanet atmosphere haziness and properties of exoplanet haze samples as a function of temperature.

Data availability

Source data are provided with this paper. The original sessile drop contact angle image files and the processed contact angle and surface energy data files can be found in the repository at https://doi.org/10.7291/D1BM2P. The original data for Fig. 6 can be found in refs. 5,6,7,8,44,45,46,47,48,65.

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Acknowledgements

X.Y. is supported by a 51 Pegasi b Fellowship from the Heising-Simons Foundation. X.Z. is supported by the NASA Solar System Workings Program (grant no. 80NSSC19K0791). C.H. and S.M.H. are supported by the NASA Exoplanets Research Program (grant no. NNX16AB45G). P.M. is supported by a 3M Non-Tenured Faculty Grant. S.E.M. is supported by a NASA Earth and Space Science Fellowship (grant no. 80NSSC18K1109).

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Contributions

X.Y., C.H., X.Z. and S.M.H. conceived the study. C.H. prepared the samples. X.Y. and C.H. performed the surface energy measurements. P.M. helped with interpretation of the surface energy measurements. X.Y. and A.H.D. performed the water amplitude calculations. X.Y., C.H., S.M.H. and J.I.M. discussed the chemistry for haze formation. X.Y. and X.Z. discussed the implication of the results for haze removal. X.Y. conducted the data analysis and prepared the manuscript. X.Y., C.H., X.Z., S.M.H., A.H.D., P.M., J.I.M., N.K.L., J.J.F., P.G., E.M.-R.K., S.E.M., C.V.M., D.P., J.A.V. and V.V. contributed to discussing the results and editing the manuscript.

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Correspondence to Xinting Yu.

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Peer review information Nature Astronomy thanks Erika Barth and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.

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Supplementary Fig. 1 and Tables 1–8.

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Yu, X., He, C., Zhang, X. et al. Haze evolution in temperate exoplanet atmospheres through surface energy measurements. Nat Astron (2021). https://doi.org/10.1038/s41550-021-01375-3

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