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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 (2.7 σ ) 8 and G395H (4.5 σ ) 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. Observations from the JWST show the presence of a spectral absorption feature at 4.05 μm arising from SO 2 in the atmosphere of the gas giant exoplanet WASP-39b, which is produced by photochemical processes and verified by numerical models.

The James Webb Space Telescope (JWST) presents the opportunity to transform our understanding of planets and the origins of life by revealing the atmospheric compositions, structures, and dynamics of transiting exoplanets in unprecedented detail. However, the high-precision, timeseries observations required for such investigations have unique technical challenges, and prior experience with Hubble, Spitzer, and other facilities indicates that there will be a steep learning curve when JWST becomes operational. In this paper, we describe the science objectives and detailed plans of the Transiting Exoplanet Community Early Release Science (ERS) Program, which is a recently approved program for JWST observations early in Cycle 1. We also describe the simulations used to establish the program. The goal of this project, for which the obtained data will have no exclusive access period, is to accelerate the acquisition and diffusion of technical expertise for transiting exoplanet observations with JWST, while also providing a compelling set of representative data sets that will enable immediate scientific breakthroughs. The Transiting Exoplanet Community ERS Program will exercise the timeseries modes of all four JWST instruments that have been identified as the consensus highest priorities, observe the full suite of transiting planet characterization geometries (transits, eclipses, and phase curves), and target planets with host stars that span an illustrative range of brightnesses. The observations in this program were defined through an inclusive and transparent process that had participation from JWST instrument experts and international leaders in transiting exoplanet studies. The targets have been vetted with previous measurements, will be observable early in the mission, and have exceptional scientific merit. Community engagement in the project will be centered on a two-phase Data Challenge that culminates with the delivery of planetary spectra, timeseries instrument performance reports, and open-source data analysis toolkits in time to inform the agenda for Cycle 2 of the JWST mission.

Atoms and molecules have long been thought to be versatile tracers of the cold neutral gas in the universe, from high-redshift galaxies to star forming regions and proto-planetary disks, because their internal degrees of freedom bear the signature of the physical conditions where these species reside. However, the promise that molecular emission has a strong diagnostic power of the underlying physical and chemical state is still hampered by the difficulty to combine sophisticated chemical codes with gas dynamics. It is therefore important 1) to acquire self-consistent data sets that can be used as templates for this theoretical work, and 2) to reveal the diagnostic capabilities of molecular lines accurately. The advent of sensitive wideband spectrometers in the (sub)- millimeter domain (e.g., IRAM-30m/EMIR, NOEMA, …) during the 2010s has allowed us to image a significant fraction of a Giant Molecular Cloud with enough sensitivity to detect tens of molecular lines in the 70 – 116 GHz frequency range. Machine learning techniques applied to these data start to deliver the next generation of molecular line diagnostics of mass, density, temperature, and radiation field.

The ionization fraction in neutral interstellar clouds is a key physical parameter controlling multiple physical and chemical processes, and varying by orders of magnitude from the UV irradiated surface of the cloud to its cosmic-ray dominated central regions. Traditional observational tracers of the ionization fraction, which mostly rely on deuteration ratios of molecules like HCO + , suffer from the fact that the deuterated molecules are only detected in a tiny fraction of a given Giant Molecular Cloud (GMC). In [1], we propose a machine learning -based, semi-automatic method to search in a large dataset of astrochemical model results for new tracers of the ionization fraction, and propose several new tracers relevant in different ranges of physical conditions.

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. The transmission spectrum of the exoplanet WASP-39b is obtained using observations from the Single-Object Slitless Spectroscopy mode of the Near Infrared Imager and Slitless Spectrograph instrument aboard the JWST.

Transmission spectroscopy 1 – 3 of exoplanets has revealed signatures of water vapour, aerosols and alkali metals in a few dozen exoplanet atmospheres 4 , 5 . However, these previous inferences with the Hubble and Spitzer Space Telescopes were hindered by the observations’ relatively narrow wavelength range and spectral resolving power, which precluded the unambiguous identification of other chemical species—in particular the primary carbon-bearing molecules 6 , 7 . Here we report a broad-wavelength 0.5–5.5 µm atmospheric transmission spectrum of WASP-39b 8 , a 1,200 K, roughly Saturn-mass, Jupiter-radius exoplanet, measured with the JWST NIRSpec’s PRISM mode 9 as part of the JWST Transiting Exoplanet Community Early Release Science Team Program 10 – 12 . We robustly detect several chemical species at high significance, including Na (19 σ ), H 2 O (33 σ ), CO 2 (28 σ ) and CO (7 σ ). The non-detection of CH 4 , combined with a strong CO 2 feature, favours atmospheric models with a super-solar atmospheric metallicity. An unanticipated absorption feature at 4 µm is best explained by SO 2 (2.7 σ ), which could be a tracer of atmospheric photochemistry. These observations demonstrate JWST’s sensitivity to a rich diversity of exoplanet compositions and chemical processes. A broad-wavelength 0.5–5.5 µm atmospheric transmission spectrum of WASP-39b, a 1,200 K, roughly Saturn-mass, Jupiter-radius exoplanet, demonstrates JWST’s sensitivity to a rich diversity of exoplanet compositions and chemical processes.

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 (28.5 σ ) and H 2 O (21.5 σ ), and identify SO 2 as the source of absorption at 4.1 μm (4.8 σ ). 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 . The medium-resolution transmission spectrum of the exoplanet WASP-39b, described using observations from the Near Infrared Spectrograph G395H grating aboard JWST, shows significant absorption from CO 2 and H 2 O and detection of SO 2 .

WASP-107b is a warm (approximately 740 K) transiting planet with a Neptune-like mass of roughly 30.5  M ⊕ and Jupiter-like radius of about 0.94  R J (refs.  1 , 2 ), whose extended atmosphere is eroding 3 . Previous observations showed evidence for water vapour and a thick, high-altitude condensate layer in the atmosphere of WASP-107b (refs.  4 , 5 ). Recently, photochemically produced sulfur dioxide (SO 2 ) was detected in the atmosphere of a hot (about 1,200 K) Saturn-mass planet from transmission spectroscopy near 4.05 μm (refs.  6 , 7 ), but for temperatures below about 1,000 K, sulfur is predicted to preferably form sulfur allotropes instead of SO 2 (refs.  8 – 10 ). Here we report the 9 σ detection of two fundamental vibration bands of SO 2 , at 7.35 μm and 8.69 μm, in the transmission spectrum of WASP-107b using the Mid-Infrared Instrument (MIRI) of JWST. This discovery establishes WASP-107b as the second irradiated exoplanet with confirmed photochemistry, extending the temperature range of exoplanets exhibiting detected photochemistry from about 1,200 K down to about 740 K. Furthermore, our spectral analysis reveals the presence of silicate clouds, which are strongly favoured (around 7 σ ) over simpler cloud set-ups. Furthermore, water is detected (around 12 σ ) but methane is not. These findings provide evidence of disequilibrium chemistry and indicate a dynamically active atmosphere with a super-solar metallicity. The JWST MIRI transmission spectrum of WASP-107b, a transiting planet with Neptune-like mass and Jupiter-like radius, shows observations of sulfur dioxide and silicate clouds but no methane in its atmosphere, providing evidence of disequilibrium chemistry and active photochemistry.

Observations of transiting gas giant exoplanets have revealed a pervasive depletion of methane 1 – 4 , which has only recently been identified atmospherically 5 , 6 . The depletion is thought to be maintained by disequilibrium processes such as photochemistry or mixing from a hotter interior 7 – 9 . However, the interiors are largely unconstrained along with the vertical mixing strength and only upper limits on the CH 4 depletion have been available. The warm Neptune WASP-107b stands out among exoplanets with an unusually low density, reported low core mass 10 , and temperatures amenable to CH 4 , though previous observations have yet to find the molecule 2 , 4 . Here we present a JWST-NIRSpec transmission spectrum of WASP-107b that shows features from both SO 2 and CH 4 along with H 2 O, CO 2 , and CO. We detect methane with 4.2 σ significance at an abundance of 1.0 ± 0.5 ppm, which is depleted by 3 orders of magnitude relative to equilibrium expectations. Our results are highly constraining for the atmosphere and interior, which indicate the envelope has a super-solar metallicity of 43 ± 8 × solar, a hot interior with an intrinsic temperature of T int  = 460 ± 40 K, and vigorous vertical mixing which depletes CH 4 with a diffusion coefficient of K zz  = 10 11.6±0.1  cm 2  s −1 . Photochemistry has a negligible effect on the CH 4 abundance but is needed to account for the SO 2 . We infer a core mass of 11.5 − 3.6 + 3.0 M ⊕ , which is much higher than previous upper limits 10 , releasing a tension with core-accretion models 11 . Observations of the warm Neptune WASP-107b show that the planet has a hot interior with an intrinsic temperature of about 460 K.

A correlation between giant-planet mass and atmospheric heavy elemental abundance was first noted in the past century from observations of planets in our own Solar System and has served as a cornerstone of planet-formation theory. Using data from the Hubble and Spitzer Space Telescopes from 0.5 to 5 micrometers, we conducted a detailed atmospheric study of the transiting Neptune-mass exoplanet HAT-P-26b. We detected prominent H₂O absorption bands with a maximum base-to-peak amplitude of 525 parts per million in the transmission spectrum. Using the water abundance as a proxy for metallicity, we measured HAT-P-26b’s atmospheric heavy element content ( 4.8 − 4.0 + 21.5 times solar). This likely indicates that HAT-P-26b’s atmosphere is primordial and obtained its gaseous envelope late in its disk lifetime, with little contamination from metal-rich planetesimals.