Vivien Parmentier

Clouds and disequilibrium chemistry shape the observable properties of substellar and planetary atmospheres, and JWST time-resolved spectroscopy now enables direct tests of their three-dimensional (3D) structures. We present an update to the 3D atmospheric framework ADAM , coupling the dynamical core to a miniature kinetic chemistry scheme (mini-chem) and a saturation–relaxation-tracer cloud module. Using this framework, we investigate the atmosphere of the brown dwarf orbiting the white dwarf ZTFJ0038+2030 in a 10 hr orbit. The brown dwarf receives stellar irradiation comparable to its internal heat flux, placing the atmosphere in a regime where the circulation is characterized by a standing Matsuno–Gill wave response and equatorial superrotation. Rapid rotation confines these structures closer to the equator, producing more symmetric broadband light curves with smaller hotspot offsets than canonical hot Jupiters, in agreement with recent JWST observations of this system. Circulation-driven disequilibrium chemistry enhances global mean CO and CO ₂ while slightly reducing H ₂ O, CH ₄ , and NH ₃ relative to chemical equilibrium. Although most species are globally homogenized, rapid rotation limits global-scale horizontal mixing and allows circulation-driven vertical transport to maintain strong CO and CO ₂ inhomogeneities in the photosphere. These chemical inhomogeneities produce asymmetric phase curves in the CO and CO ₂ absorption bands, which could qualitatively explain the observed CO ₂ light-curve asymmetry. When clouds are included, Na ₂ S clouds dominate the photosphere and weaken the large-scale circulation, lowering photospheric CO and CO ₂ abundances, while MgSiO ₃ and Fe clouds rain out at deeper levels. Because the dominant Na ₂ S clouds reside mostly above the quench region, cloud-driven convection has limited impact on the quenched abundances, which are instead regulated mainly by large-scale circulation.

Context . The JWST has revealed a large diversity of warm Jupiter spectra, despite broadly similar planetary parameters across the population. The main driver of this diversity remains unclear. Aims . We aim to identify the mechanisms responsible for the spectral differences of three warm Jupiter-size exoplanets observed by JWST: whereas WASP-80b appears mostly cloud-free, both WASP-107b and WASP-69b have spectra dominated by clouds. Methods . We modeled each planet using the same framework, ADAM (formerly SPARC/MITgcm), which solves for the interactions among cloud transport, radiative transfer, and atmospheric circulation in 3D. We investigated three condensate species (Na ₂ S, KCl, and MgSiO ₃ ) and four particle sizes (0.1, 1, 5, and 10 µm). Results . Clouds settle deeper in the atmosphere of the higher-gravity planet WASP-80b than in WASP-107b, and the model reproduces their spectral difference naturally. For WASP-107b, three cloud models can reproduce the NIRCam observations: 5 µm Na ₂ S, 1 µm KCl, and 5 µm MgSiO ₃ . However, they cannot match the scattering slope observed at shorter wavelengths in NIRISS or the possible silicate feature in the MIRI bandpass, suggesting a multimodal cloud distribution. Our model predicts that small silicate particles are homogeneously distributed and thus cannot account for the difference between the two limb spectra in the MIRI bandpass. Finally, applying the same model to WASP-69b does not yield a partially cloudy dayside solution that fits the emission spectra, as proposed in a previous study. Conclusions . Coupling among 3D circulation, clouds, and radiative transfer can increase the spectral diversity of warm Jupiter exoplanets, particularly through changes in cloudiness with gravity. The combination of multiphase, wide-wavelength coverage and models that couple clouds, circulation, and radiative transfer is key to advancing our understanding of these new objects.

We present the first JWST phase curve of a white dwarf–brown dwarf (WD–BD) binary, a NIRSpec PRISM observation of ZTF J0038+2030AB (ZTF0038). Short-period WD–BD binaries provide unique laboratories to probe substellar atmospheres. Tidal locking drives hot Jupiter–like atmospheric dynamics in the brown dwarf. The system’s formation history offers a window into planetary systems around post-main-sequence stars. We obtain a full-orbit phase curve of ZTF0038, including a total eclipse of the white dwarf, which enables us to separate the two components’ emission throughout the entire orbit, and we model the brown dwarf’s phase-resolved emission spectra using substellar-atmosphere forward models and atmospheric retrievals. The PRISM spectrum covers ∼80% of the brown dwarf’s bolometric emission, enabling a nearly model-independent energy-balance calculation, which yields a dayside-to-nightside heat transport efficiency of ≲10%. Inefficient heat redistribution is further supported by the phase-curve shape and the nightside spectrum closely resembling nonirradiated mid- to late-T dwarfs. The spectroscopic phase curves reveal a stark nightside asymmetry associated with a strong CO ₂ absorption feature at 4.2 μ m, while the retrieved abundances indicate a longitudinally homogeneous distribution of CO ₂ , as well as all other key species detected in the atmosphere. The precise internal luminosity measurement of the brown dwarf informs both the age of the WD–BD system (7.5–8.8 Gyr) and indicates a low common-envelope ejection efficiency. These data illustrate the exquisite opportunity to probe the three-dimensional processes of substellar atmospheres, connect substellar and exoplanet atmospheres, and probe the evolution of post-main-sequence planetary systems using WD–BDs.

Sulfur chemistry has emerged as an important probe of exoplanet atmospheres in the JWST era, although observational constraints have thus far been largely limited to SO ₂ and H ₂ S in warm and hot exoplanets. Recent JWST observations have revealed CS ₂ in several cooler gas giant exoplanets, yielding a new tracer of sulfur chemistry. However, the detailed chemical pathways responsible for the formation of CS ₂ remain poorly understood. Here, we use TOI-6894 b, a temperate gas giant with evidence for CS ₂ , as a test case for one-dimensional photochemical kinetic-transport modeling and sensitivity analyses of CS ₂ chemistry. We show that CS ₂ is produced through coupled thermochemical and photochemical processes involving CH ₄ and H ₂ S as the primary carbon and sulfur reservoirs, with S ₂ photolysis driving disequilibrium sulfur chemistry. Our models provide a physically consistent explanation for the observed CS ₂ feature in TOI-6894 b. Extending our analysis to gas giant exoplanets spanning a wide range of T _eq , we find that CS ₂ abundance peaks in temperate to warm atmospheres ( T _eq ∼ 500–700 K), and declines toward both lower and higher temperatures. This temperature dependence provides a unified framework for interpreting current CS ₂ observations, accounting for reported detections in temperate to warm planets and the lack of detections in colder and hotter giant exoplanets. Our results establish CS ₂ as a complementary probe of sulfur inventories and atmospheric metallicity in cool gas giant exoplanets.

We report the JWST NIRSpec/PRISM transit spectrum of TOI-6894 b, an exceptional 420 K sub-Saturn that is one of the rare giant planets transiting a late M dwarf. Remarkably, both the light curve and the transit spectrum exhibit almost no stellar contamination. The spectrum is dominated by prominent absorption features from CH ₄ and the photochemical product CS ₂ . For the first time in a transit spectrum, NH ₃ is visually evident, while subtler features from H ₂ O and CO ₂ can also be seen. We significantly improve upon state-of-the-art photochemical reaction networks, and use our new network to run radiative-convective photochemical (“RCP”) models at different metallicities. These models show that the spectrum—in particular the size of the NH ₃ and CO ₂ features relative to the CH ₄ and H ₂ O features—is most consistent with a metallicity of 3–10× solar. Using a semi-free retrieval framework that perturbs the RCP model’s abundance and temperature profiles to fit the data, we find that the planet’s C/O, N/O, and S/O ratios are consistent with solar values. A grid retrieval on 1D RCP equilibrium models reveals a similar result: [M/H] = 0.46 ± 0.08 and C/O = 0.69 ± 0.06. The planet’s atmospheric metallicity, abundance ratios, and bulk metal fraction are all strikingly similar to those of Jupiter, Saturn, and other gas giant exoplanets, despite orbiting a very low-mass star.

Hot and ultra-hot Jupiters offer a unique laboratory for studying atmospheric chemistry at the population level using ground-based high-resolution spectroscopy. Iron (Fe) and titanium (Ti) are key tracers of thermal and chemical structures, yet they exhibit markedly different observational trends across the population. We present a homogeneous re-analysis of high-resolution transmission spectra for ten hot and ultra-hot Jupiters observed with ESPRESSO on ESO’s Very Large Telescope and MAROON-X on Gemini-North. We searched for neutral Fe and Ti absorption and performed injection–recovery tests using forward models spanning a range of Ti-depletion levels and temperature–pressure profiles. To enable direct comparison across planets observed with different instruments and signal-to-noise ratios, we introduced the relative cross-correlation metric, Δ _Ti-Fe . We detect Fe in seven planets and Ti in four planets at significances above 5 σ . Across the population, Δ _Ti-Fe decreases sharply towards lower equilibrium temperatures. Under the assumption of equal Ti depletion across planets, isothermal models fail to reproduce this trend, instead requiring a temperature-dependent depletion of Ti that increases towards cooler planets, consistent with cold-trapping processes in cooler atmospheres. Models with inverted temperature–pressure profiles naturally reproduce the decline without invoking such temperature-dependent depletion. In these atmospheres, Ti is converted into TiO in deeper, cooler layers and subsequently removed from the gas phase through condensation into Ti-bearing species, leading to a strong suppression of the observable atomic Ti signal while Fe remains largely atomic. Nevertheless, even in these gradient models, an additional overall depletion of Ti relative to Fe is required to match the hottest planets. Our results demonstrate that the observable refractory chemistry is governed by the interplay of molecular partitioning, ionisation, condensation, and cold-trapping processes, as well as the vertical structure of ultra-hot Jupiter atmospheres. Although these results reveal clear population-level trends, additional observations will be necessary to distinguish between temperature-dependent cold-trapping and overall depletion scenarios. Expanding homogeneous high-resolution surveys to larger and more diverse samples, targeting both emission and transmission observations, will refine these constraints and provide critical insights into the chemistry of strongly irradiated giant planets.

Hot-Jupiters, gas giant planets with equilibrium temperatures above 1000 K, host large temperature gradients between their permanent day- and nightsides, resulting in circulation regimes that have no solar system analogs. Past Kepler and Spitzer measurements have shown that the phase curves of these objects typically peak westward of the substellar point at optical wavelengths and eastward in the infrared, indicative of a possible interaction between circulation and cloud coverage. However, few hot-Jupiters have joint reflected light and thermal emission measurements, preventing a definitive statement as to the link between these phenomena. Here, we present the first phase curve of a hot-Jupiter that separates the contributions of thermal emission and reflected light through JWST observations of NGTS-10 Ab with the NIRSpec PRISM instrument ( λ = 0.5–5.5 μ m). Using the spectrally resolved phase curve, we jointly retrieve the planet’s thermal and reflected light maps. We show that the temperature and reflectance distributions are anti-correlated, which is best explained by clouds evaporating from the eastern substellar region ( φ ≈ −17° to 48°) where temperatures are highest. This is further confirmed by comparisons of the phase-resolved spectra with three-dimensional circulation models, which show that NGTS-10 Ab’s atmosphere hosts inhomogeneous cloud coverage, likely made-up of μ m-sized silicate particles, and weak atmospheric drag ( τ _drag > 10 ⁶ s). Finally, by measuring the spectral variation of the thermal phase curve offsets, we infer a slope of 7.1 ± 1.9 degrees per pressure dex, indicative of heat transport that becomes more efficient at depth. Future optical and infrared hot-Jupiter phase curve measurements over a wide range of equilibrium temperatures will enable a complete mapping of the interplay between heat transport and cloud formation in highly irradiated exoplanet atmospheres.

Ultrahot Jupiters (UHJs; T _eq ≳ 2000 K) enable simultaneous detection of volatile (ice-forming) and refractory (rock-forming) elements in planetary atmospheres, providing a powerful diagnostic of planet formation and atmospheric processing. We present a comprehensive high-resolution cross-correlation spectroscopy analysis of the UHJ MASCARA-1 b ( T _eq ≈ 2600 K) using the IGRINS and IGRINS-2 spectrographs. We detect robust (signal-to-noise ratio > 4) signals from H ₂ O, CO, OH, Fe i , Mg i , Ca i , and Ti i , marking the most complete atmospheric inventory of MASCARA-1 b to date. Using a chemically consistent atmospheric inference framework, we constrain elemental abundances to a typical precision of ≈0.2 dex, retrieving a solar atmospheric metallicity ([M/H] _⊙ = 0.0 7 − 0.13 + 0.17 ≈1.2 × solar), a C/O ratio (C/O = 0.6 5 − 0.08 + 0.08 ) consistent with solar value (C/O = 0.59), an enhanced refractory abundance ([ R /H] _⊙ = 0.4 0 − 0.17 + 0.23 ≈ 2.5 × solar; ≈3.8 × stellar), and a moderately supersolar refractory-to-volatile ratio ([ R / V ] _⊙ = 0.3 6 − 0.09 + 0.11 ≈ 2.3× solar). Comparison with formation models suggests that MASCARA-1 b most likely accreted material between the soot–H ₂ O or H ₂ O–CO snowlines (at 68% confidence). We additionally find stellar values for atmospheric Ti/Fe and Ca/Fe ratios (at 68% confidence). The Mg/Fe is also found to be consistent with stellar value at 95% confidence. Therefore, we do not find strong indication of nightside cold trapping in MASCARA-1 b. As homogeneous refractory-to-volatile measurements expand across the UHJ population, particularly with upcoming Extremely Large Telescopes, these diagnostics will enable statistically robust tests of emerging trends in giant planet formation and atmospheric evolution.

Discovered deep within the “Neptunian desert,” LTT9779b remains the only known ultrahot Neptune, prompting significant speculation regarding its unique formation and evolutionary history. Its exceptionally high geometric albedo has previously been attributed either to the presence of clouds or to an extremely metal-rich atmosphere. Here, we present a comprehensive panchromatic analysis of its dayside atmosphere using JWST NIRISS and NIRSpec/G395H observations to characterize its atmospheric structure and composition. Leveraging the exceptional signal-to-noise ratio (SNR) in the observed spectra, we report a 3 σ -to-5 σ detection of dayside clouds, with strong evidence for Mg ₂ SiO ₄ (s) (silicate) condensation. This constitutes the first statistically significant detection of clouds on the dayside of a Neptunian-mass exoplanet. We demonstrate that a highly reflective cloud deck, rather than an extremely high-metallicity atmosphere, is the most likely explanation for the planet’s anomalously high optical albedo. Furthermore, our atmospheric retrievals yield robust detections of both CO (∼4.88 σ ) and CO ₂ (∼8.76 σ ), while providing tentative constraints on the H ₂ O abundance and upper limits on SiO, TiO, and VO. Finally, our analysis places a robust constraint on the C/O ratio of 0.984 ± 0.019. This aligns LTT9779b with other known ultrahot Jupiters exhibiting supersolar C/O ratios, suggesting a broader trend driven by the sequestration of oxygen-bearing condensates in ultrahot atmospheres.

Close-in exoplanets are tidally locked to their host star and thus exhibit extreme atmospheric temperature gradients. It has been theorized that the fraction of star light absorbed by such planets during transit changes as a function of orbital phase as progressively hotter or colder atmospheric gas rotates into view, but this effect has not been observed so far. Here we show that two transits of the ultrahot Jupiter WASP-121 b, acquired with JWST/NIRSpec and NIRISS, exhibit asymmetric light curves caused by the planet’s rotation during transit. We observe increasing CO absorption and slightly decreasing H ₂ O absorption in the transmission spectrum as the planet rotates. These results are indicative of a stronger longitudinal temperature gradient across the evening than across the morning terminator, consistent with higher temperatures in the eastern half than in the western half of the dayside. The observed changes of the transmission spectrum with orbital phase are in line with the temperature increase causing thermal dissociation of H ₂ O, while CO remains abundant. The observation of longitudinal gradients in atmospheric temperature and chemistry from the planet’s rotational transit provides a new probe for constraining atmospheric heterogeneity using JWST beyond differences between morning and evening terminators from limb asymmetries.