Atmospheric compositions can provide powerful diagnostics of formation and migration histories of planetary systems. We investigate constraints on atmospheric abundances of H 2 O, Na, and K, in a sample of transiting exoplanets using latest transmission spectra and new H 2 broadened opacities of Na and K. Our sample of 19 exoplanets spans from cool mini-Neptunes to hot Jupiters, with equilibrium temperatures between ∼300 and 2700 K. Using homogeneous Bayesian retrievals we report atmospheric abundances of Na, K, and H 2 O, and their detection significances, confirming 6 planets with strong Na detections, 6 with K, and 14 with H 2 O. We find a mass-metallicity trend of increasing H 2 O abundances with decreasing mass, spanning generally substellar values for gas giants and stellar/superstellar for Neptunes and mini-Neptunes. However, the overall trend in H 2 O abundances, from mini-Neptunes to hot Jupiters, is significantly lower than the mass-metallicity relation for carbon in the solar system giant planets and similar predictions for exoplanets. On the other hand, the Na and K abundances for the gas giants are stellar or superstellar, consistent with each other, and generally consistent with the solar system metallicity trend. The H 2 O abundances in hot gas giants are likely due to low oxygen abundances relative to other elements rather than low overall metallicities, and provide new constraints on their formation mechanisms. The differing trends in the abundances of species argue against the use of chemical equilibrium models with metallicity as one free parameter in atmospheric retrievals, as different elements can be differently enhanced.

Exoplanets orbiting M dwarfs present a valuable opportunity for their detection and atmospheric characterisation. This is evident from recent inferences of H 2 O in such atmospheres, including that of the habitable-zone exoplanet K2-18b. With a bulk density between Earth and Neptune, K2-18b may be expected to possess a H/He envelope. However, the extent of such an envelope and the thermodynamic conditions of the interior remain unexplored. In the present work, we investigate the atmospheric and interior properties of K2-18b based on its bulk properties and its atmospheric transmission spectrum. We constrain the atmosphere to be H 2 -rich with a H 2 O volume mixing ratio of 0.02 − 14.8%, consistent with previous studies, and find a depletion of CH 4 and NH 3 , indicating chemical disequilibrium. We do not conclusively detect clouds/hazes in the observable atmosphere. We use the bulk parameters and retrieved atmospheric properties to constrain the internal structure and thermodynamic conditions in the planet. The constraints on the interior allow multiple scenarios between rocky worlds with massive H/He envelopes and water worlds with thin envelopes. We constrain the mass fraction of the H/He envelope to be 6%; spanning 10 −5 for a predominantly water world to ∼ 6% for a pure iron interior. The thermodynamic conditions at the surface of the H 2 O layer range from the super-critical to liquid phases, with a range of solutions allowing for habitable conditions on K2-18b. Our results demonstrate that the potential for habitable conditions is not necessarily restricted to Earth-like rocky exoplanets.

The class of ultra-hot Jupiters comprises giant exoplanets undergoing intense irradiation from their host stars. They have proved to be a particularly interesting population for their orbital and atmospheric properties. One such planet, WASP-121 b, is in a highly misaligned orbit close to its Roche limit, and its atmosphere exhibits a thermal inversion. These properties make WASP-121 b an interesting target for additional atmospheric characterization. In this paper, we present analysis of archival high-resolution optical spectra obtained during transits of WASP-121 b. Artifacts from the Rossiter-McLaughlin effect and Center-to-Limb Variation are deemed negligible. However, we discuss scenarios where these effects warrant more careful treatment by modeling the WASP-121 system and varying its properties. We report a new detection of atmospheric absorption from Hα in the planet with a transit depth of 1.87 ± 0.11%. We further confirm a previous detection of the Na I doublet, and report a new detection of Fe I via cross-correlation with a model template. We attribute the Hα absorption to an extended Hydrogen atmosphere, potentially undergoing escape, and the Fe I to equilibrium chemistry at the planetary photosphere. These detections help to constrain the composition and chemical processes in the atmosphere of WASP-121 b.

Ultra-hot Jupiters (UHJs), giant exoplanets with equilibrium temperatures above 2000 K, are ideal laboratories for studying metal compositions of planetary atmospheres. At these temperatures the thermal dissociation of metal-rich molecules into their constituent elements makes these atmospheres conducive for elemental characterization. Several elements, mostly ionized metals, have been detected in UHJs recently using high-resolution transit spectroscopy. Even though a number of neutral transition metals (e.g., Fe, Ti, V, Cr) are expected to be strong sources of optical/near-ultraviolet (NUV) opacity and, hence, influence radiative processes in the lower atmospheres of UHJs, only Fe i has been detected to date. We conduct a systematic search for atomic species in the UHJ WASP-121 b. Using theoretical models we present a metric to predict the atomic species likely to be detectable in such planets with high-resolution transmission spectroscopy. We search for the predicted species in observations of WASP-121 b and report the first detections of neutral transition metals Cr i and V i in an exoplanet at 3.6σ and 4.5σ significance, respectively. We confirm previous detections of Fe i and Fe ii. Whereas Fe ii was detected previously in the NUV, we detect it in the optical. We infer that the neutral elements Fe i, V i, and Cr i are present in the lower atmosphere, as predicted by thermochemical equilibrium, while Fe ii is a result of photoionization in the upper atmosphere. Our study highlights the rich chemical diversity of UHJs.

Measuring the abundances of carbon and oxygen in exoplanet atmospheres is considered a crucial avenue for unlocking the formation and evolution of exoplanetary systems1,2. Access to the chemical inventory of an exoplanet requires high-precision observations, often inferred from individual molecular detections with low-resolution space-based3–5 and high-resolution ground-based6–8 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 CO2 (28.5σ) and H2O (21.5σ), and identify SO2 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 SO2, 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 range10.