A sub-Neptune exoplanet with a low-metallicity methane-depleted atmosphere and Mie-scattering clouds

Benneke, Björn; Knutson, Heather A.; Lothringer, Joshua D.; Crossfield, Ian J. M.; Moses, Julianne I.; Morley, Caroline; Kreidberg, Laura; Fulton, Benjamin J.; Dragomir, Diana; Howard, Andrew W.; Wong, Ian; Désert, Jean-Michel; McCullough, P. R.; Kempton, Eliza M.-R.; Fortney, Jonathan J.; Gilliland, Ronald L.; Deming, Drake; Kammer, Joshua A.

doi:10.1038/s41550-019-0800-5

Cited by 219 publications

(299 citation statements)

References 72 publications

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“…While the metallicity of the atmosphere may be higher (see §4.2.2), current observations do not provide sufficient constraints on the atmospheric metallicity of exo-Neptunes and sub-Neptunes (e.g. Fraine et al 2014;Wakeford et al 2017;Benneke et al 2019;Chachan et al 2019). The resulting temperature-pressure profiles all have the same general features: A deep adiabat in the convective region, a transition region around the RCB, and 0 200 400 600 800 1000 1200 1400 Temperature (K) 10 9 10 7 10 5 10 3 10 1 10 1 10 3…”

Section: Atmospheric Structurementioning

confidence: 79%

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: Atmospheric Structurementioning

confidence: 79%

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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“…We (Tennyson et al 2012), as well as the collision-induced absorption of H 2 and He. Following Benneke et al (2019), we use a threeparameter Mie-scattering cloud description for the retrieval analysis defining the mean particle size R part , the pressure level P τ =1 at which the clouds become optically opaque to grazing starlight at 1.5 µm, and the scale height of the cloud profile relative to the gas pressure scale height H part /H gas as free parameters. All free parameters are allowed to vary independently in the retrieval.…”

Section: Atmospheric Retrievalmentioning

confidence: 99%

“…When calculating the cloud opacity, the retrieval is agnostic to the particular composition of the spherical cloud particles, considering only their size and vertical distribution; the former is assumed to be a logarithmic Gaussian distribution with a fixed width of σ R = 1.5. This three-parameter cloud description is motivated by the information content of transmission spectra and captures the wavelength-dependent opacities of a wide range of finite-sized cloud particles near the cloud deck in a highly orthogonal way, ideal for retrieval (Benneke et al 2019). It reduces to Rayleigh hazes in the limit of small particles and a gray cloud deck for large particles while simultaneously allowing for any finite-sized Mie-scattering particles in between.…”

Section: Atmospheric Retrievalmentioning

confidence: 99%

Optical to Near-infrared Transmission Spectrum of the Warm Sub-Saturn HAT-P-12b

Wong

Benneke

Gao

et al. 2020

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We present the transmission spectrum of HAT-P-12b through a joint analysis of data obtained from the Hubble Space Telescope Space Telescope Imaging Spectrograph (STIS) and Wide Field Camera 3 (WFC3) and Spitzer, covering the wavelength range 0.3-5.0 µm. We detect a muted water vapor absorption feature at 1.4 µm attenuated by clouds, as well as a Rayleigh scattering slope in the optical indicative of small particles. We interpret the transmission spectrum using both the state-ofthe-art atmospheric retrieval code SCARLET and the aerosol microphysics model CARMA. These models indicate that the atmosphere of HAT-P-12b is consistent with a broad range of metallicities between several tens to a few hundred times solar, a roughly solar C/O ratio, and moderately efficient vertical mixing. Cloud models that include condensate clouds do not readily generate the sub-micron particles necessary to reproduce the observed Rayleigh scattering slope, while models that incorporate photochemical hazes composed of soot or tholins are able to match the full transmission spectrum. From a complementary analysis of secondary eclipses by Spitzer, we obtain measured depths of 0.042% ± 0.013% and 0.045% ± 0.018% at 3.6 and 4.5 µm, respectively, which are consistent with a blackbody temperature of 890 +60 −70 K and indicate efficient day-night heat recirculation. HAT-P-12b joins the growing number of well-characterized warm planets that underscore the importance of clouds and hazes in our understanding of exoplanet atmospheres.

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“…One method of studying the atmospheric constituents of an exoplanet is by employing an atmospheric retrieval (Madhusudhan & Seager 2009;Line et al 2013;Waldmann et al 2015a,b;Blecic et al 2017;Cubillos et al 2017;Mollière et al 2019;Benneke et al 2019). Atmospheric retrieval is the act of obtaining the physical and chemical characteristics of an atmosphere based on observations.…”

Section: Introductionmentioning

confidence: 99%

Understanding and mitigating biases when studying inhomogeneous emission spectra with JWST

Taylor

Parmentier

Irwin

et al. 2020

Monthly Notices of the Royal Astronomical Society

106

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Exoplanet emission spectra are often modelled assuming that the hemisphere observed is well represented by a horizontally homogenised atmosphere. However this approximation will likely fail for planets with a large temperature contrast in the James Webb Space Telescope (JWST) era, potentially leading to erroneous interpretations of spectra.We first develop an analytic formulation to quantify the signal-to-noise ratio and wavelength coverage necessary to disentangle temperature inhomogeneities from a hemispherically averaged spectrum. We find that for a given signal-to-noise ratio, observations at shorter wavelengths are better at detecting the presence of inhomogeneities. We then determine why the presence of an inhomogeneous thermal structure can lead to spurious molecular detections when assuming a fully homogenised planet in the retrieval process.Finally, we quantify more precisely the potential biases by modelling a suite of hot Jupiter spectra, varying the spatial contributions of a hot and a cold region, as would be observed by the different instruments of JWST/NIRSpec. We then retrieve the abundances and temperature profiles from the synthetic observations. We find that in most cases, assuming a homogeneous thermal structure when retrieving the atmospheric chemistry leads to biased results, and spurious molecular detection. Explicitly modelling the data using two profiles avoids these biases, and is statistically supported provided the wavelength coverage is wide enough, and crucially also spanning shorter wavelengths. For the high contrast used here, a single profile with a dilution factor performs as well as the two-profile case, with only one additional parameter compared to the 1-D approach.

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A sub-Neptune exoplanet with a low-metallicity methane-depleted atmosphere and Mie-scattering clouds

Cited by 219 publications

References 72 publications

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

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

Optical to Near-infrared Transmission Spectrum of the Warm Sub-Saturn HAT-P-12b

Understanding and mitigating biases when studying inhomogeneous emission spectra with JWST

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