Photochemical Hazes in Sub-Neptunian Atmospheres with a Focus on GJ 1214b

Lavvas, P.; Koskinen, Tommi; Steinrueck, Maria E.; Muñoz, A. García; Showman, Adam P.

doi:10.3847/1538-4357/ab204e

Cited by 56 publications

(66 citation statements)

References 68 publications

(124 reference statements)

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“…formation peaks are similar between our work and those relying on more sophisticated photochemical models (∼1 µbar, e.g. Kawashima et al 2019;Lavvas et al 2019) that take into account the full stellar spectrum and the shielding effects of other molecules, including photodissocation products. However, what we do not capture is the more extended region of haze formation at higher pressures.…”

Section: Additional Photochemical Considerationssupporting

confidence: 77%

“…Kawashima & Ikoma (2018) computed a haze production rate for GJ 1214b by scaling the haze production rate of Titan by the ratio of the Lyman-α flux of GJ 1214 to that received at Titan. Lavvas & Koskinen (2017); Lavvas et al (2019); Kawashima et al (2019); Kawashima & Ikoma (2019) all used similar approaches, where the haze production rate is set equal to the photolysis rates of methane, nitrogen, and major hydrocarbon and nitrile species, reduced by an efficiency factor.…”

Section: Photochemistry and Haze Microphysicsmentioning

confidence: 99%

“…Ohno et al (2019) showed that compression forces in exoplanet atmospheres are unlikely to strongly impact aggregates, and that they will maintain a fractal dimension of ∼2, rather than 2.4 as assumed in Adams et al (2019). This leads to a wavelengthdependence of the opacity more similar to that of the individual monomers, which are small enough to create spectral slopes (Lavvas et al 2019). Given the uncertainties regarding how aggregates form and evolve in exoplanet atmospheres, it is difficult to deduce how they would affect the observed planet radius.…”

Section: Additional Microphysical Considerationsmentioning

confidence: 99%

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Deflating Super-puffs: Impact of Photochemical Hazes on the Observed Mass–Radius Relationship of Low-mass Planets

Gao

Zhang

2020

ApJ

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The observed mass-radius relationship of low-mass planets informs our understanding of their composition and evolution. Recent discoveries of low mass, large radii objects ("super-puffs") have challenged theories of planet formation and atmospheric loss, as their high inferred gas masses make them vulnerable to runaway accretion and hydrodynamic escape. Here we propose that high altitude photochemical hazes could enhance the observed radii of low-mass planets and explain the nature of super-puffs. We construct model atmospheres in radiative-convective equilibrium and compute rates of atmospheric escape and haze distributions, taking into account haze coagulation, sedimentation, diffusion, and advection by an outflow wind. We develop mass-radius diagrams that include atmospheric lifetimes and haze opacity, which is enhanced by the outflow, such that young (∼0.1-1 Gyr), warm (T eq ≥ 500 K), low mass objects (M c < 4M ⊕ ) should experience the most apparent radius enhancement due to hazes, reaching factors of three. This reconciles the densities and ages of the most extreme super-puffs. For Kepler-51b, the inclusion of hazes reduces its inferred gas mass fraction to <10%, similar to that of planets on the large radius side of the sub-Neptune radius gap. This suggests that Kepler-51b may be evolving towards that population, and that some warm sub-Neptunes may have evolved from super-puffs. Hazes also render transmission spectra of super-puffs and sub-Neptunes featureless, consistent with recent measurements. Our hypothesis can be tested by future observations of super-puffs' transmission spectra at mid-infrared wavelengths, where we predict that the planet radius will be half of that observed in the near-infrared.

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Section: Additional Photochemical Considerationssupporting

confidence: 77%

Section: Photochemistry and Haze Microphysicsmentioning

confidence: 99%

Section: Additional Microphysical Considerationsmentioning

confidence: 99%

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Deflating Super-puffs: Impact of Photochemical Hazes on the Observed Mass–Radius Relationship of Low-mass Planets

Gao

Zhang

2020

ApJ

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“…Another way to distinguish water clouds from photochemical hazes would be transmission spectroscopy in the visible range. Water clouds should produce a flat-absorption in-transit spectrum of the cloudy part, while sub-micrometric haze particles should produce a slope due to Rayleigh scattering (Lavvas et al 2019).…”

Section: Discussionmentioning

confidence: 99%

Formation and dynamics of water clouds on temperate sub-Neptunes: the example of K2-18b

Charnay

Blain

Bézard

et al. 2021

A&A

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Context. Hubble Space Telescope (HST) spectroscopic transit observations of the temperate sub-Neptune K2-18b were interpreted as the presence of water vapour with potential water clouds. 1D modelling studies also predict the formation of water clouds in K2-18b’s atmosphere in some conditions. However, such models cannot predict the cloud cover, which is driven by atmospheric dynamics and thermal contrasts, and thus neither can they predict the real impact of clouds on spectra. Aims. The main goal of this study is to understand the formation, distribution, and observational consequences of water clouds on K2-18b and other temperate sub-Neptunes. Methods. We simulated the atmospheric dynamics, water cloud formation, and spectra of K2-18b for a H2-dominated atmosphere using a 3D general circulation model. We analysed the impact of atmospheric composition (with metallicity from 1× solar to 1000× solar), concentration of cloud condensation nuclei, and planetary rotation rate. Results. Assuming that K2-18b has a synchronous rotation, we show that the atmospheric circulation in the upper atmosphere essentially corresponds to a symmetric day-to-night circulation with very efficient heat redistribution. This regime preferentially leads to cloud formation at the sub-stellar point or at the terminator. Clouds form at metallicity ≥100× solar with relatively large particles (radius = 30–450 μm). At 100–300× solar metallicity, the cloud fraction at the terminators is small with a limited impact on transit spectra. At 1000× solar metallicity, very thick clouds form at the terminator, greatly flattening the transit spectrum. The cloud distribution appears very sensitive to the concentration of cloud condensation nuclei and to the planetary rotation rate, although the impact on transit spectra is modest in the near-infrared. Fitting HST transit data with our simulated spectra suggests a metallicity of ~100–300× solar, which is consistent with the mass-metallicity trend of giant planets in the Solar System. In addition, we found that the cloud fraction at the terminator can be highly variable in some conditions, leading to a potential variability in transit spectra that is correlated with spectral windows. This effect could be common on cloudy exoplanets and could be detectable with multiple transit observations. Finally, the complex cloud dynamics revealed in this study highlight the inherent 3D nature of clouds shaped by couplings between microphysics, radiation, and atmospheric circulation.

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“…In addition, several models have simulated exoplanet haze particles as fractal aggregates (G. N. Arney et al, 2017;G. Arney et al, 2018;Adams et al, 2019;Lavvas et al, 2019), much like haze particles on Titan . Several cloud models have also considered aggregates composed of condensates Samra et al, 2020).…”

Section: Accepted Articlementioning

confidence: 99%