Laboratory Investigation of Haze Formation in Exoplanet Atmospheres: Implication for Habitability and Biosignatures

Abstract:

More than 4000 exoplanets have been discovered in last two decades, including a sample of terrestrial planets in the habitable zone of their host stars. New observing capabilities coming online over the next few years will provide opportunities for characterizing their atmospheres and assessing their potential habitability. Photochemistry could play an important role in their potential atmospheres and lead to the formation of haze that could obscure the detection of major atmospheric constituents. However, the photochemical processes are poorly understood in these exoplanets because their atmospheric parameters are different from that in Solar System bodies.

We conducted a series of laboratory experiments that simulate photochemistry in a broad range of temperate (<800 K) exoplanet atmospheres. We investigated three types of atmospheric metallicities (100, 1,000, or 10,000 times solar) at four temperatures (300, 400, 600, and 800 K) using the PHAZER chamber at JHU with one of two energy sources (AC glow plasma and UV photons). We studied the size distributions (He et al. 2018a, 2018b) and the production rates (He et al. 2018a; Hörst et al. 2018a) of solid haze particles that formed, as well as the gas and solid phase chemistry (He et al. 2019, Moran et al. 2020). We find that the particle size is dependent on the experimental conditions, while the production rates are sensitive to atmospheric compositions (He et al. 2018a, 2018b, 2020; Hörst et al. 2018). Even a small amount of H₂S can enrich the photochemistry and enhance haze production rate significantly (He et al. 2020). We observed the photochemical formation of O₂, sulfur and organic products in the gas phase, which have been considered as potential biosignatures but are produced abiotically in our experiments. Organic molecules are detected in both gas and solid phases, including potential prebiotic precursors (HCHO and HCN) and compounds with prebiotic molecular formulas (sugars, amino acids, and nucleobases), which could provide a source of organic materials for life to arise.

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Hörst, S. M., et al. 2018, Nat. Astron., 2, 303

Moran, S. E., et al. 2020, PSJ, 1, 17