WASP-39b is a hot Saturn-mass exoplanet with a predicted clear atmosphere based on observations in the optical and infrared. Here, we complete the transmission spectrum of the atmosphere with observations in the near-infrared (NIR) over three water absorption features with the Hubble Space Telescope (HST) Wide Field Camera 3 (WFC3) G102 (0.8-1.1 μm) and G141 (1.1-1.7 μm) spectroscopic grisms. We measure the predicted high-amplitude H 2 O feature centered at 1.4 μm and the smaller amplitude features at 0.95 and 1.2 μm, with a maximum water absorption amplitude of 2.4 planetary scale heights. We incorporate these new NIR measurements into previously published observational measurements to complete the transmission spectrum from 0.3 to 5 μm. From these observed water features, combined with features in the optical and IR, we retrieve a well constrained temperature T eq =1030 20 30 -+ K, and atmospheric metallicity 151 46 48-+ solar, which is relatively high with respect to the currently established mass-metallicity trends. This new measurement in the Saturn-mass range hints at further diversity in the planet formation process relative to our solar system giants.

Broad absorption signatures from alkali metals, such as the sodium (Na I) and potassium (K I) resonance doublets, have long been predicted in the optical atmospheric spectra of cloud-free irradiated gas giant exoplanets. However, observations have revealed only the narrow cores of these features rather than the full pressure-broadened profiles. Cloud and haze opacity at the day-night planetary terminator are considered to be responsible for obscuring the absorption-line wings, which hinders constraints on absolute atmospheric abundances. Here we report an optical transmission spectrum for the 'hot Saturn' exoplanet WASP-96b obtained with the Very Large Telescope, which exhibits the complete pressure-broadened profile of the sodium absorption feature. The spectrum is in excellent agreement with cloud-free, solar-abundance models assuming chemical equilibrium. We are able to measure a precise, absolute sodium abundance of logε = [Formula: see text], and use it as a proxy for the planet's atmospheric metallicity relative to the solar value (Z/Z = [Formula: see text]). This result is consistent with the mass-metallicity trend observed for Solar System planets and exoplanets.

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We present a study of the effect of wind-driven advection on the chemical composition of hot Jupiter atmospheres using a fully-consistent 3D hydrodynamics, chemistry and radiative transfer code, the Met Office Unified Model (UM). Chemical modelling of exoplanet atmospheres has primarily been restricted to 1D models that cannot account for 3D dynamical processes. In this work we couple a chemical relaxation scheme to the UM to account for the chemical interconversion of methane and carbon monoxide. This is done consistently with the radiative transfer meaning that departures from chemical equilibrium are included in the heating rates (and emission) and hence complete the feedback between the dynamics, thermal structure and chemical composition. In this letter we simulate the well studied atmosphere of HD 209458b. We find that the combined effect of horizontal and vertical advection leads to an increase in the methane abundance by several orders of magnitude; directly opposite to the trend found in previous works. Our results demonstrate the need to include 3D effects when considering the chemistry of hot Jupiter atmospheres. We calculate transmission and emission spectra, as well as the emission phase curve, from our simulations. We conclude that gas-phase non-equilibrium chemistry is unlikely to explain the model-observation discrepancy in the 4.5 µm Spitzer/IRAC channel. However, we highlight other spectral regions, observable with the James Webb Space Telescope, where signatures of wind-driven chemistry are more prominant.

WASP-121b is a transiting gas giant exoplanet orbiting close to its Roche limit, with an inflated radius nearly double that of Jupiter and a dayside temperature comparable to a late M dwarf photosphere. Secondary eclipse observations covering the 1.1-1.6 µm wavelength range have revealed an atmospheric thermal inversion on the dayside hemisphere, likely caused by high altitude absorption at optical wavelengths. Here we present secondary eclipse observations made with the Hubble Space Telescope Wide Field Camera 3 spectrograph that extend the wavelength coverage from 1.1 µm down to 0.8 µm. To determine the atmospheric properties from the measured eclipse spectrum, we performed a retrieval analysis assuming chemical equilibrium, with the effects of thermal dissociation and ionization included. Our best-fit model provides a good fit to the data with reduced χ 2 ν = 1.04. The data diverge from a blackbody spectrum and instead exhibit emission due to H − shortward of 1.1 µm. The best-fit model does not reproduce a previously reported bump in the spectrum at 1.25µm, possibly indicating this feature is a statistical fluctuation in the data rather than a VO emission band as had been tentatively suggested. We estimate an atmospheric metallicity of [M/H] = 1.09 +0.57 −0.69 , and fit for the carbon and oxygen abundances separately, obtaining [C/H] = −0.29 +0.61 −0.48 and [O/H] = 0.18 +0.64 −0.60 . The corresponding carbon-tooxygen ratio is C/O = 0.49 +0.65 −0.37 , which encompasses the solar value of 0.54, but has a large uncertainty.