Jean-Michel Désert

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.

Detecting and characterizing the atmospheres of rocky exoplanets has proven to be challenging for JWST. Transit spectroscopy of the TRAPPIST-1 planets has been impacted by the effects of spots and faculae on the host star. Secondary eclipses have detected hot rocks, but evidence for atmospheres has been difficult to obtain. However, there is a third option that we call color phase curves. This method will apply to synchronously rotating non-transiting planets as well as transiting planets. A color phase curve uses photometry at a long-IR wavelength where the planetary thermal emission is strong (e.g., 21 microns) divided by photometry at a shorter wavelength where the star dominates (e.g., 12 microns). We avoid wavelengths having potentially strong molecular absorption (e.g., 15 microns) to minimize degeneracies in the color phase curve, and we aim to detect and characterize the planetary atmosphere via its longitudinal heat transfer. The ratio of two wavelengths observed nearly simultaneously is designed to isolate thermal emission from the planet, discriminate against the star, and largely cancel instrumental systematic effects. Moreover, we show that invoking mass-radius relations, and using self-consistent physical models, will permit the longitudinal heat transfer to be measured independent of the orbital inclination. Radial velocity surveys are detecting many new exoplanets, including temperate rocky worlds with Earth-like masses. Most of those planets will not transit, but color phase curves have the potential to detect and characterize their atmospheres.

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.

MASCARA-5 b/TOI-1431 b is an ultra-hot Jupiter ( P _orb = 2.650237 ± 0.000003 dayd, T _eq = 2370 ± 70 K, M _p = 3.12 ± 0.18 M _Jup , R _p = 1.49 ± 0.05 R _Jup ) orbiting a bright Am star ( V = 8.0 mag). Recent time-series observations obtained with PEPSI@LBT during the planet’s posteclipse phases have revealed Fe i emission lines indicative of a thermally inverted atmosphere. These observations demonstrate that MASCARA-5 b is well suited to atmospheric characterization via emission spectroscopy, thereby motivating further follow-up observations covering additional orbital phases to constrain the planet’s atmospheric chemistry, thermal structure, and dynamics. Here we present preeclipse time-series observations obtained with the high-resolution optical spectrograph EXPRES@LDT. Our analysis confirms the previous detection of gas-phase Fe i on MASCARA-5 b’s dayside (with a 5.5 σ significance obtained from two nights of observations), and the fact that the thermal profile is inverted with lower and upper temperatures ∼2000 K and ∼4500 K, respectively. A search for Fe ii and Cr i did not yield any plausible detections. We also find that the preeclipse signal exhibits a nonnegligible blueshift of −3.2 ± 1.4 km s ⁻¹ potentially caused by winds.

Detecting and characterizing the atmospheres of rocky exoplanets has proven to be challenging for JWST. Transit spectroscopy of the TRAPPIST-1 planets has been impacted by the effects of spots and faculae on the host star. Secondary eclipses have detected hot rocks, but evidence for atmospheres has been difficult to obtain. However, there is a third option that we call color phase curves. This method will apply to synchronously rotating non-transiting planets as well as transiting planets. A color phase curve uses photometry at a long-IR wavelength near the peak of the planetary thermal emission (e.g., 21 microns) divided by photometry at a shorter wavelength where the star dominates more strongly (e.g., 12 microns). We avoid wavelengths having potentially strong molecular absorption (e.g., 15 microns) to minimize degeneracies in the color phase curve, and we aim to detect and characterize the planetary atmosphere via its longitudinal heat transfer. The ratio of two wavelengths observed nearly simultaneously is designed to isolate thermal emission from the planet, discriminate against the star, and largely cancel instrumental systematic effects. Moreover, we show that invoking mass-radius relations, and using self-consistent physical models, will permit the longitudinal heat transfer to be measured independent of the orbital inclination. Radial velocity surveys are detecting many new exoplanets, including temperate rocky worlds with Earth-like masses. Most of those planets will not transit, but color phase curves have the potential to detect and characterize their atmospheres.

The Galaxy’s most common known planetary systems have several Earth-to-Neptune-size planets in compact orbits ¹ . At small orbital separations, larger planets are less common than their smaller counterparts by an order of magnitude. The young star V1298 Tau hosts one such compact planetary system, albeit with four planets that are uncommonly large (5 to 10 Earth radii) ^2,3 . The planets form a chain of near-resonances that result in transit-timing variations of several hours. Here we present a multi-year campaign to characterize this system with transit-timing variations, a method insensitive to the intense magnetic activity of the star. Through targeted observations, we first resolved the previously unknown orbital period of the outermost planet. The full 9-year baseline from these and archival data then enabled robust determination of the masses and orbital parameters for all four planets. We find the planets have low, sub-Neptune masses and nearly circular orbits, implying a dynamically tranquil history. Their low masses and large radii indicate that the inner planets underwent a period of rapid cooling immediately after dispersal of the protoplanetary disk. Still, they are much less dense than mature planets of comparable size. We predict the planets will contract to 1.5–4.0 Earth radii and join the population of super-Earths and sub-Neptunes that nature produces in abundance.

A handful of enigmatic Jupiter-mass objects have been discovered orbiting pulsars. One such object, PSR J2322–2650b, uniquely resembles a hot-Jupiter exoplanet, due to its minimum density of 1.8 g cm ⁻³ and its ∼1900 K equilibrium temperature. We use JWST to observe PSR J2322–2650b’s emission spectrum across an entire orbit. In stark contrast to every known exoplanet orbiting a main-sequence star, we find an atmosphere rich in molecular carbon (C ₃ , C ₂ ) with strong westward winds. Our observations open up new exoplanetary chemical (ultrahigh C/O and C/N ratios of >100 and >10,000, respectively) and dynamical regimes (ultrafast rotation with external irradiation) to observational study. The extreme carbon enrichment poses a severe challenge to the current understanding of “black-widow” companions, which were expected to consist of a wider range of elements due to their origins as stripped stellar cores.

Highly irradiated giant exoplanets known ‘ultrahot Jupiters’ are anticipated to exhibit large variations of atmospheric temperature and chemistry as a function of longitude, latitude and altitude. Previous observations have hinted at these variations, but the existing data have been fundamentally restricted to probing hemisphere-integrated spectra, thereby providing only coarse information on atmospheric gradients. Here we present a spectroscopic eclipse map of an extrasolar planet, resolving the atmosphere in multiple dimensions simultaneously. We analyse a secondary eclipse of the ultrahot Jupiter WASP-18b observed with the Near Infrared Imager and Slitless Spectrograph instrument on the JWST. The mapping reveals weaker longitudinal temperature gradients than were predicted by theoretical models, indicating the importance of hydrogen dissociation and/or nightside clouds in shaping global thermal emission. In addition, we identify two thermally distinct regions of the planet’s atmosphere: a ‘hotspot’ surrounding the substellar point and a ‘ring’ near the dayside limbs. The hotspot region shows a strongly inverted thermal structure due to the presence of optical absorbers and a water abundance marginally lower than the hemispheric average, in accordance with theoretical predictions. The ring region shows colder temperatures and poorly constrained chemical abundances. Similar future analyses will reveal the three-dimensional thermal, chemical and dynamical properties of a broad range of exoplanet atmospheres.