An Observational Diagnostic for Distinguishing between Clouds and Haze in Hot Exoplanet Atmospheres

We present an atmospheric transmission spectrum for the ultra-hot Jupiter WASP-121b, measured using the Space Telescope Imaging Spectrograph (STIS) onboard the Hubble Space Telescope (HST). Across the 0.47-1 µm wavelength range, the data imply an atmospheric opacity comparable to -and in some spectroscopic channels exceeding -that previously measured at near-infrared wavelengths (1.15-1.65 µm). Wavelength-dependent variations in the opacity rule out a gray cloud deck at a confidence level of 3.8σ and may instead be explained by VO spectral bands. We find a cloud-free model assuming chemical equilibrium for a temperature of 1500 K and metal enrichment of 10-30× solar matches these data well. Using a free-chemistry retrieval analysis, we estimate a VO abundance of −6.6 +0.2 −0.3 dex. We find no evidence for TiO and place a 3σ upper limit of −7.9 dex on its abundance, suggesting TiO may have condensed from the gas phase at the day-night limb. The opacity rises steeply at the shortest wavelengths, increasing by approximately five pressure scale heights from 0.47 to 0.3 µm in wavelength. If this feature is caused by Rayleigh scattering due to uniformly-distributed aerosols, it would imply an unphysically high temperature of 6810±1530 K. One alternative explanation for the short-wavelength rise is absorption due to SH (mercapto radical), which has been predicted as an important product of non-equilibrium chemistry in hot Jupiter atmospheres. Irrespective of the identity of the NUV absorber, it likely captures a significant amount of incident stellar radiation at low pressures, thus playing a significant role in the overall energy budget, thermal structure, and circulation of the atmosphere.

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“…Subsequent modeling of these data confirmed such an interpretation to be plausible (e.g. Kempton et al 2017;Parmentier et al 2018).…”

Section: Introductionmentioning

confidence: 73%

An Optical Transmission Spectrum for the Ultra-hot Jupiter WASP-121b Measured with the Hubble Space Telescope

et al. 2018

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“…Mac-Donald & Madhusudhan 2017). In particular, Kempton et al (2017) shows that non-uniform clouds/hazes on WASP-121 b can cause observable spectral gradients in the HST/WFC3 wavelengths.…”

Section: Molecular Opacities Detectedmentioning

confidence: 97%

A Population Study of Gaseous Exoplanets

et al. 2018

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We present here the analysis of 30 gaseous extrasolar planets, with temperatures between 600 and 2400 K and radii between 0.35 and 1.9 R Jup . The quality of the HST/WFC3 spatially scanned data combined with our specialized analysis tools allow us to study the largest and most self-consistent sample of exoplanetary transmission spectra to date and examine the collective behavior of warm and hot gaseous planets rather than isolated case-studies. We define a new metric, the Atmospheric Detectability Index (ADI) to evaluate the statistical significance of an atmospheric detection and find statistically significant atmospheres around 16 planets out of the 30 analysed. For most of the Jupiters in our sample, we find the detectability of their atmospheres to be dependent on the planetary radius but not on the planetary mass. This indicates that planetary gravity plays a secondary role in the state of gaseous planetary atmospheres. We detect the presence of water vapour in all of the statistically detectable atmospheres, and we cannot rule out its presence in the atmospheres of the others. In addition, TiO and/or VO signatures are detected with 4 σ confidence in WASP-76 b, and they are most likely present in WASP-121 b. We find no correlation between expected signal-to-noise and atmospheric detectability for most targets. This has important implications for future large-scale surveys.

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“…Here we propose an alternative explanation: as shown by the green curve, when the presence of CaTiO 3 cloud opacity is included in the model, the amplitude of the water feature is reduced while the shape of the water feature is kept similar, leading to a much better fit of the spectra. This is a direct consequence of the presence of a partially cloudy atmosphere (Line & Parmentier 2016;Kempton et al 2017). Interestingly, since TiO is still present as a gaseous species above the 0.1mbar cloud top, and the spectral band of TiO is still apparent in the transmission spectrum and is not strongly affected by the presence of condensed CaTiO 3 below it.…”

Section: Clouds and Transmission Spectrummentioning

confidence: 99%

From thermal dissociation to condensation in the atmospheres of ultra hot Jupiters: WASP-121b in context

Parmentier

Line

Bean

et al. 2018

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Context.A new class of exoplanets has emerged: the ultra hot Jupiters, the hottest close-in gas giants. The majority of them have weaker-than-expected spectral features in the 1.1 − 1.7µm bandpass probed by HST/WFC3 but stronger spectral features at longer wavelengths probed by Spitzer. This led previous authors to puzzling conclusions about the thermal structures and chemical abundances of these planets. Aims. We investigate how thermal dissociation, ionization, H − opacity, and clouds shape the thermal structures and spectral properties of ultra hot Jupiters. Methods. We use the SPARC/MITgcm to model the atmospheres of four ultra hot Jupiters and discuss more thoroughly the case of WASP-121b. We expand our findings to the whole population of ultra hot Jupiters through analytical quantification of the thermal dissociation and its influence on the strength of spectral features. Results. We predict that most molecules are thermally dissociated and alkalies are ionized in the dayside photospheres of ultra hot Jupiters. This includes H 2 O, TiO, VO, and H 2 but not CO, which has a stronger molecular bond. The vertical molecular gradient created by the dissociation significantly weakens the spectral features from H 2 O while the 4.5µm CO feature remains unchanged. The water band in the HST/WFC3 bandpass is further weakened by the continuous opacity of the H − ions. Molecules are expected to recombine before reaching the limb, leading to order of magnitude variations of the chemical composition and cloud coverage between the limb and the dayside. Conclusions. Molecular dissociation provides a qualitative understanding of the lack of strong spectral features of water in the 1 − 2µm bandpass observed in most ultra hot Jupiters. Quantitatively, our model does not provide a satisfactory match to the WASP-121b emission spectrum. Together with WASP-33b and Kepler-33Ab, they seem the outliers among the population of ultra hot Jupiters, in need of a more thorough understanding.

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An Observational Diagnostic for Distinguishing between Clouds and Haze in Hot Exoplanet Atmospheres

Cited by 62 publications

References 48 publications

An Optical Transmission Spectrum for the Ultra-hot Jupiter WASP-121b Measured with the Hubble Space Telescope

An Optical Transmission Spectrum for the Ultra-hot Jupiter WASP-121b Measured with the Hubble Space Telescope

A Population Study of Gaseous Exoplanets

From thermal dissociation to condensation in the atmospheres of ultra hot Jupiters: WASP-121b in context

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