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

Gao, Peter; Thorngren, Daniel; Lee, Elspeth K. H.; Fortney, Jonathan J.; Morley, Caroline; Wakeford, Hannah R.; Powell, Diana; Stevenson, Kevin B.; Zhang, Xi

doi:10.1038/s41550-020-1114-3

Cited by 206 publications

(227 citation statements)

References 72 publications

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“…Common organic molecules are not stable at very high temperature (>1000 K, Johns et al 1962, Smay 1985, so we would not necessarily expect significant organic haze in the atmospheres of Jupiters hotter than 1000 K. This is consistent with a recent modeling study (Gao et al 2020), which found that aerosol composition is dominated by silicates for hot giant exoplanets with planetary equilibrium temperatures above 950 K, while is dominated by hydrocarbon aerosols below 950 K. However, it is possible to form organic hazes at temperatures above 1000 K as reported by Fleury et al (2019) who found that photochemistry can lead to the formation of an organic solid condensate at 1500K. We also need to consider that temperatures on the night sides of these hot planets can be significantly lower than on the day side, so there may be situations where gases that are photochemically produced on the day side are transported by winds to the night side, where they can form condensates.…”

Section: Haze Production Ratesupporting

confidence: 90%

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Sulfur-driven haze formation in warm CO2-rich exoplanet atmospheres

He¹,

Hörst²,

Lewis³

et al. 2020

Nat Astron

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Sulfur gases significantly affect the photochemistry of planetary atmospheres in ourSolar System, and are expected to be important components in exoplanet atmospheres. However, sulfur photochemistry in the context of exoplanets is poorly understood due to a lack of chemical-kinetics information for sulfur species under relevant conditions.Here, we study the photochemical role of hydrogen sulfide (H2S) in warm CO2-rich exoplanet atmospheres (800 K) by carrying out laboratory simulations. We find that H2S plays a significant role in photochemistry, even when present in the atmosphere at relatively low concentrations (1.6%). It participates in both gas and solid phase chemistry, leading to the formation of other sulfur gas products (CH3SH/SO, C2H4S/OCS, SO2/S2, and CS2) and to an increase in solid haze particle production and compositional complexity. Our study shows that we may expect thicker haze with small particle sizes (20 to 140 nm) for warm CO2-rich exoplanet atmospheres that possess H2S.Observations 1-5 and laboratory simulations 6,7 have shown that clouds and/or hazes are likely ubiquitous in the atmospheres of exoplanets. These clouds and hazes play an important role in exoplanet atmospheres and affect the spectra of planets, therefore impacting our ability to observe their atmosphere and assess their habitability. Although a variety of atmospheric gases can condense at specific temperature and pressure conditions to form clouds, haze particles may be produced photochemically over a range of temperatures, pressures, and atmospheric compositions. 6−9 Among the various atmospheric components, sulfur gases significantly influence the photochemistry and haze formation in the atmospheres of Solar System bodies, such as Earth, Venus 10,11 , Jupiter 12,13 , and its moon, Io.

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Section: Haze Production Ratesupporting

confidence: 90%

“…Liang et al 2004, Zahnle et al 2009, Moses et al 2011Miller-Ricci Kempton et al 2012), and exoplanet studies usually estimate the production rate of organic haze only from CH4 photodissociation (e.g. Morley et al 2013, Gao et al 2020). The production rates in the 800 K experiments (without CH4)…”

Section: Mass Spectra Of Gas Phase Productsmentioning

confidence: 99%

Sulfur-driven haze formation in warm CO2-rich exoplanet atmospheres

He¹,

Hörst²,

Lewis³

et al. 2020

Nat Astron

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“…The PT profile is also expected to cross the condensation curve of ZnS (Figure 7), which was suggested by Morley et al (2012) as another potentially important cloud species. However, recent modeling work by Gao et al (2020) suggests that the formation of significant ZnS cloud mass is challenging, owing to high nucleation energy barriers. In the same study, Gao et al note that although the formation of KCl clouds should be efficient, at temperatures below 950 K photochemical haze is likely to be the dominant aerosol opacity source.…”

Section: Discussionmentioning

confidence: 99%

Transmission Spectroscopy for the Warm Sub-Neptune HD 3167c: Evidence for Molecular Absorption and a Possible High-metallicity Atmosphere

Evans

Crossfield

Benneke

et al. 2020

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We present a transmission spectrum for the warm (500−600 K) sub-Neptune HD 3167c obtained using the Hubble Space Telescope Wide Field Camera 3 infrared spectrograph. We combine these data, which span the 1.125–1.643 μm wavelength range, with broadband transit measurements made using Kepler/K2 (0.6–0.9 μm) and Spitzer/IRAC (4–5 μm). We find evidence for absorption by at least one of H2O, HCN, CO2, and CH4 (Bayes factor 7.4; 2.5σ significance), although the data precision does not allow us to unambiguously discriminate between these molecules. The transmission spectrum rules out cloud-free hydrogen-dominated atmospheres with metallicities ≤100× solar at >5.8σ confidence. In contrast, good agreement with the data is obtained for cloud-free models assuming metallicities >700× solar. However, for retrieval analyses that include the effect of clouds, a much broader range of metallicities (including subsolar) is consistent with the data, due to the degeneracy with cloud-top pressure. Self-consistent chemistry models that account for photochemistry and vertical mixing are presented for the atmosphere of HD 3167c. The predictions of these models are broadly consistent with our abundance constraints, although this is primarily due to the large uncertainties on the latter. Interior structure models suggest that the core mass fraction is >40%, independent of a rock or water core composition, and independent of atmospheric envelope metallicity up to 1000× solar. We also report abundance measurements for 15 elements in the host star, showing that it has a very nearly solar composition.

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“…The computation time prohibits these models from being coupled to a retrieval algorithm; however, as a purely forward modelling tool these simulations can be very informative. Gao et al (2020) apply the Community Aerosol Radiation Model for Atmospheres (CARMA) to hot Jupiter atmospheres, to investigate haze and cloud formation over a range of temperatures. CARMA explicitly models: the nucleation of cloud particles, both homogeneous (self-nucleation) and heterogeneous (nucleating onto a particle of another substance); condensational growth and evaporation; and coagulation.…”

Section: Physical Modelsmentioning

confidence: 99%