H2O abundances and cloud properties in ten hot giant exoplanets

Abstract

Transmission spectroscopy of exoplanets has the potential to provide precise measurements of atmospheric chemical abundances, in particular of hot Jupiters whose large sizes and high temperatures make them conducive to such observations. To date, several transmission spectra of hot Jupiters have revealed low amplitude features of water vapour compared to expectations from cloud-free atmospheres of solar metallicity. The low spectral amplitudes in such atmospheres could either be due to the presence of aerosols that obscure part of the atmosphere or due to inherently low abundances of H 2 O in the atmospheres. A recent survey of transmission spectra of ten hot Jupiters used empirical metrics to suggest atmospheres with a range of cloud/haze properties but with no evidence for H 2 O depletion. Here, we conduct a detailed and homogeneous atmospheric retrieval analysis of the entire sample and report the H 2 O abundances, cloud properties, terminator temperature profiles, and detection significances of the chemical species. Our present study finds that the majority of hot Jupiters have atmospheres consistent with sub-solar H 2 O abundances at their day-night terminators. The best constrained abundances range from log(H 2 O) of −5.04 +0.46 −0.30 to −3.16 +0.66 −0.69 , which compared to expectations from solar-abundance equilibrium chemistry correspond to 0.018 +0.035 −0.009 × solar to 1.40 +4.97 −1.11 × solar. Besides H 2 O we report statistical constraints on other chemical species and cloud/haze properties, including cloud/haze coverage fractions which range from 0.18 +0.26 −0.12 to 0.76 +0.13 −0.15 . The retrieved H 2 O abundances suggest sub-solar oxygen and/or super-solar C/O ratios, and can provide important constraints on the formation and migration pathways of hot giant exoplanets. clouds/hazes (e.g., Deming et al. 2013) or due to inherently low H 2 O abundances (e.g., Madhusudhan et al. 2014a) Sing et al. (2016) interpreted the observed atmospheric spectra using empirical metrics based on chemical equilibrium atmospheric models to constrain the prominence of clouds/hazes vis-à-vis the H 2 O abundances in the atmospheres. The contribution of H 2 O was represented by the amplitude of the H 2 O feature at ∼1.4 µm, while the difference between the optical and infrared planetary radii was taken as representative of the cloud/haze contribution. These empirical metrics of the observations were evaluated with a grid of theoretical chemical equilibrium models to suggest that the atmospheres spanned a continuum from clear to cloudy with no evidence for sub-solar H 2 O abundances. However, a complementary study of the original Sing et al. (2016) survey by Barstow et al. (2017) suggested a general range of H 2 O abundances indicating sub-solar abundances in most of the targets.

HyDRA-H: Simultaneous Hybrid Retrieval of Exoplanetary Emission Spectra

“…We constrain an H 2 O abundance that is consistent with solar, but with a larger uncertainty than the hybrid retrieval. This result is consistent with previous low-resolution retrievals of the same dataset as well as previous retrievals of the planet with transmission spectra which indicate sub-solar abundances (Madhusudhan et al 2014b;Barstow et al 2017;Pinhas et al 2019).…”

Why Is it So Cold in Here? Explaining the Cold Temperatures Retrieved from Transmission Spectra of Exoplanet Atmospheres

“…∼ 250 K below the true terminator-averaged temperature. This is consistent with reported literature temperatures (Pinhas et al 2019;. The retrieved abundances of TiO, VO, and H 2 O agree with the true terminator-averaged values to 1σ.…”

The Interior and Atmosphere of the Habitable-zone Exoplanet K2-18b

“…The data include observations from the HST WFC3 G141 grism (1.1-1.7 µm), photometry in the Spitzer IRAC 3.6 µm and 4.5 µm bands, and optical photometry in the K2 band (0.4-1.0 µm). We perform the atmospheric retrieval using an adaptation of the AURA retrieval code (Pinhas et al 2019;. Our model solves line-byline radiative transfer in a plane-parallel atmosphere in transmission geometry.…”