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The near-infrared transmission spectrum of the warm sub-Neptune exoplanet GJ 1214 b has been observed to be flat and featureless, implying a high metallicity atmosphere with abundant aerosols. Recent JWST MIRI Low Resolution Spectrometer observations of a phase curve of GJ 1214 b showed that its transmission spectrum is flat out into the mid-infrared. In this paper, we use the combined near- and mid-infrared transmission spectrum of GJ 1214 b to constrain its atmospheric composition and aerosol properties. We generate a grid of photochemical haze models using an aerosol microphysics code for a number of background atmospheres spanning metallicities from 100 to 1000× solar, as well as a steam atmosphere scenario. The flatness of the combined data set largely rules out atmospheric metallicities ≤300× solar due to their large corresponding molecular feature amplitudes, preferring values ≥1000× solar and column haze production rates ≥10−10 g cm−2 s−1. The steam atmosphere scenario with similarly high haze production rates also exhibits sufficiently small molecular features to be consistent with the transmission spectrum. These compositions imply that atmospheric mean molecular weights ≥15 g mol−1 are needed to fit the data. Our results suggest that haze production is highly efficient on GJ 1214 b and could involve non-hydrocarbon, non-nitrogen haze precursors. Further characterization of GJ 1214 b's atmosphere would likely require multiple transits and eclipses using JWST across the near- and mid-infrared, potentially complemented by ground-based high-resolution transmission spectroscopy. [ABSTRACT FROM AUTHOR]

Transmission spectroscopy has been a workhorse technique used over the past two decades to constrain the physical and chemical properties of exoplanet atmospheres 1-5 . One of its classical key assumptions is that the portion of the atmosphere it probes-the terminator region-is homogeneous. Several works from the past decade, however, have put this into question for highly irradiated, hot (T eq  ≳ 1,000 K) gas giant exoplanets, both empirically 6-10 and through three-dimensional modelling 11-17 . While models have predicted clear differences between the evening (day-to-night) and morning (night-to-day) terminators, direct morning and evening transmission spectra in a wide wavelength range have not been reported for an exoplanet so far. Under the assumption of precise and accurate orbital parameters for the exoplanet WASP-39 b, here we report the detection of inhomogeneous terminators on WASP-39 b, which has allowed us to retrieve its morning and evening transmission spectra in the near-infrared (2-5 μm) using the James Webb Space Telescope. We have observed larger transit depths in the evening, which are, on average, 405 ± 88 ppm larger than the morning ones, and also have qualitatively larger features than the morning spectrum. The spectra are best explained by models in which the evening terminator is hotter than the morning terminator by 17 7 - 57 + 65  K, with both terminators having C/O ratios consistent with solar. General circulation models predict temperature differences broadly consistent with the above value and point towards a cloudy morning terminator and a clearer evening terminator., (© 2024. The Author(s).)

As the closest transiting hot Jupiter to Earth, HD 189733b has been the benchmark planet for atmospheric characterization 1-3 . It has also been the anchor point for much of our theoretical understanding of exoplanet atmospheres from composition 4 , chemistry 5,6 , aerosols 7 to atmospheric dynamics 8 , escape 9 and modelling techniques 10,11 . Previous studies of HD 189733b have detected carbon and oxygen-bearing molecules H 2 O and CO (refs.  12,13 ) in the atmosphere. The presence of CO 2 and CH 4 has been claimed 14,15 but later disputed 12,16,17 . The inferred metallicity based on these measurements, a key parameter in tracing planet formation locations 18 , varies from depletion 19,20 to enhancement 21,22 , hindered by limited wavelength coverage and precision of the observations. Here we report detections of H 2 O (13.4σ), CO 2 (11.2σ), CO (5σ) and H 2 S (4.5σ) in the transmission spectrum (2.4-5.0 μm) of HD 189733b. With an equilibrium temperature of about 1,200 K, H 2 O, CO and H 2 S are the main reservoirs for oxygen, carbon and sulfur. Based on the measured abundances of these three main volatile elements, we infer an atmospheric metallicity of three to five times stellar. The upper limit on the methane abundance at 5σ is 0.1 ppm, which indicates a low carbon-to-oxygen ratio (<0.2), suggesting formation through the accretion of water-rich icy planetesimals. The low oxygen-to-sulfur and carbon-to-sulfur ratios also support the planetesimal accretion formation pathway 23 ., (© 2024. The Author(s), under exclusive licence to Springer Nature Limited.)

Photochemistry is a fundamental process of planetary atmospheres that regulates the atmospheric composition and stability 1 . However, no unambiguous photochemical products have been detected in exoplanet atmospheres so far. Recent observations from the JWST Transiting Exoplanet Community Early Release Science Program 2,3 found a spectral absorption feature at 4.05 μm arising from sulfur dioxide (SO 2 ) in the atmosphere of WASP-39b. WASP-39b is a 1.27-Jupiter-radii, Saturn-mass (0.28 M J ) gas giant exoplanet orbiting a Sun-like star with an equilibrium temperature of around 1,100 K (ref.  4 ). The most plausible way of generating SO 2 in such an atmosphere is through photochemical processes 5,6 . Here we show that the SO 2 distribution computed by a suite of photochemical models robustly explains the 4.05-μm spectral feature identified by JWST transmission observations 7 with NIRSpec PRISM (2.7σ) 8 and G395H (4.5σ) 9 . SO 2 is produced by successive oxidation of sulfur radicals freed when hydrogen sulfide (H 2 S) is destroyed. The sensitivity of the SO 2 feature to the enrichment of the atmosphere by heavy elements (metallicity) suggests that it can be used as a tracer of atmospheric properties, with WASP-39b exhibiting an inferred metallicity of about 10× solar. We further point out that SO 2 also shows observable features at ultraviolet and thermal infrared wavelengths not available from the existing observations., (© 2023. The Author(s).)

The Saturn-mass exoplanet WASP-39b has been the subject of extensive efforts to determine its atmospheric properties using transmission spectroscopy 1-4 . However, these efforts have been hampered by modelling degeneracies between composition and cloud properties that are caused by limited data quality 5-9 . Here we present the transmission spectrum of WASP-39b obtained using the Single-Object Slitless Spectroscopy (SOSS) mode of the Near Infrared Imager and Slitless Spectrograph (NIRISS) instrument on the JWST. This spectrum spans 0.6-2.8 μm in wavelength and shows several water-absorption bands, the potassium resonance doublet and signatures of clouds. The precision and broad wavelength coverage of NIRISS/SOSS allows us to break model degeneracies between cloud properties and the atmospheric composition of WASP-39b, favouring a heavy-element enhancement ('metallicity') of about 10-30 times the solar value, a sub-solar carbon-to-oxygen (C/O) ratio and a solar-to-super-solar potassium-to-oxygen (K/O) ratio. The observations are also best explained by wavelength-dependent, non-grey clouds with inhomogeneous coverageof the planet's terminator., (© 2023. The Author(s).)

Measuring the metallicity and carbon-to-oxygen (C/O) ratio in exoplanet atmospheres is a fundamental step towards constraining the dominant chemical processes at work and, if in equilibrium, revealing planet formation histories. Transmission spectroscopy (for example, refs. 1,2 ) provides the necessary means by constraining the abundances of oxygen- and carbon-bearing species; however, this requires broad wavelength coverage, moderate spectral resolution and high precision, which, together, are not achievable with previous observatories. Now that JWST has commenced science operations, we are able to observe exoplanets at previously uncharted wavelengths and spectral resolutions. Here we report time-series observations of the transiting exoplanet WASP-39b using JWST's Near InfraRed Camera (NIRCam). The long-wavelength spectroscopic and short-wavelength photometric light curves span 2.0-4.0 micrometres, exhibit minimal systematics and reveal well defined molecular absorption features in the planet's spectrum. Specifically, we detect gaseous water in the atmosphere and place an upper limit on the abundance of methane. The otherwise prominent carbon dioxide feature at 2.8 micrometres is largely masked by water. The best-fit chemical equilibrium models favour an atmospheric metallicity of 1-100-times solar (that is, an enrichment of elements heavier than helium relative to the Sun) and a substellar C/O ratio. The inferred high metallicity and low C/O ratio may indicate significant accretion of solid materials during planet formation (for example, refs. 3,4 , ) or disequilibrium processes in the upper atmosphere (for example, refs. 5,6 )., (© 2023. The Author(s).)

Measuring the abundances of carbon and oxygen in exoplanet atmospheres is considered a crucial avenue for unlocking the formation and evolution of exoplanetary systems 1,2 . Access to the chemical inventory of an exoplanet requires high-precision observations, often inferred from individual molecular detections with low-resolution space-based 3-5 and high-resolution ground-based 6-8 facilities. Here we report the medium-resolution (R ≈ 600) transmission spectrum of an exoplanet atmosphere between 3 and 5 μm covering several absorption features for the Saturn-mass exoplanet WASP-39b (ref.  9 ), obtained with the Near Infrared Spectrograph (NIRSpec) G395H grating of JWST. Our observations achieve 1.46 times photon precision, providing an average transit depth uncertainty of 221 ppm per spectroscopic bin, and present minimal impacts from systematic effects. We detect significant absorption from CO 2 (28.5σ) and H 2 O (21.5σ), and identify SO 2 as the source of absorption at 4.1 μm (4.8σ). Best-fit atmospheric models range between 3 and 10 times solar metallicity, with sub-solar to solar C/O ratios. These results, including the detection of SO 2 , underscore the importance of characterizing the chemistry in exoplanet atmospheres and showcase NIRSpec G395H as an excellent mode for time-series observations over this critical wavelength range 10 ., (© 2023. The Author(s).)