Paul Mollière

The ¹² CO/ ¹³ CO ratio was introduced as an indicator for where in the disk a planet has formed. Previously, a lower value compared to that of the host star was suggested to indicate that a planet accreted CO ice beyond the disk’s CO ice line. In this Letter, we aim to determine the ¹² CO/ ¹³ CO value of the directly imaged planet β Pictoris b and whether we can link it to its formation. Its apparent brightness results in an exceptional signal-to-noise ratio of up to ∼60 per wavelength point. We present the first science observations with the upgraded GRAVITY+ instrument at a spectral resolution of R ≈ 4000, which we analysed with petitRADTRANS . Our retrievals robustly indicate ¹³ CO with a ¹² CO/ ¹³ CO ratio of 91 ⁺²⁴ ₋₁₇ , consistent with both a solar to interstellar-medium-like value. Our ¹² CO/ ¹³ CO value corroborates recent interpretations that ¹³ CO may be a less useful tracer of formation location in the disk than previously thought; nonetheless, we discuss theories with which this value is consistent. As our observations span ≈7 hours, this enabled us to search for atmospheric variability in β Pictoris b; we report a tentative constraint on the variability amplitude of about 1.4 ^+0.6 _−0.7 %.

Linking the composition of gas giant planets to their formation paths has long been a goal in exoplanet science. Cold gas giants with temperatures below ~500K have been especially out of reach for detailed atmospheric characterization. With JWST, however, we can reach high signal-to-noise (S∕N) spectra for such cool worlds and can measure not only their main trace gas abundances but even their isotopic content, thus unlocking new possibilities in linking them to their formation paths. In this study, we present the spectrum of one of the coldest planetary-mass companions, COCONUTS-2b (T _eff ≈ 480 K, separation of ~6400 au from its M dwarf host star), obtained with the Mid-InfraRed Instrument Medium Resolution Spectrometer (MIRI/MRS) on board JWST. We received a high S/N spectrum of up to 40 at ~11.8 μm. By combining the MIRI and archival Gemini/FLAMINGOS-2 datasets, we aim to characterize the chemical composition and physical structure of its frigid atmosphere in order set the stage to uncover insights on the formation of COCONUTS-2b. For the first time on a MIRI/MRS dataset, we used the full spectral resolution of MIRI/MRS and performed atmospheric retrievals to enable a search for the faint absorption features of rare molecules and isotopologs. The latter are identified using a leave-one-out analysis and Bayes factor comparison. We robustly detected three isotopologs, namely ¹⁵ NH ₃ , H ₂ ¹⁸ O, and H ₂ ¹⁷ O, in the atmosphere of COCONUTS-2b. We find the first clear evidence of oxygen isotopes in water in a cold companion, complementing previous CO isotope detections. We constrained the ¹⁶ O/ ¹⁸ O and ¹⁶ O/ ¹⁷ O isotope ratios as being significantly enriched compared to the solar and interstellar medium (ISM) values. However, the value for ¹⁸ O/ ¹⁷ O is consistent with the ISM value. We found a nitrogen ratio ¹⁴ N/ ¹⁵ N compatible with the value of the ISM and with previous observations of Y brown dwarfs. The derived effective temperatures as well as the subsolar C/O and subsolar to solar metallicity are in line with previous results. This dataset demonstrates the capability of MIRI/MRS to characterize such cold planetary-mass companion’s atmospheres with respect to their compositional and isotopic content. In the future, the constrained elemental and isotope ratios will provide a unique avenue for comparisons with the host star’s abundances and eventually the tracing of formation scenarios.

Context. Cold exo-Jovian planets are beginning to be imaged and characterized using the James Webb Space Telescope (JWST) instruments. These observations often reveal new molecular species (CO ₂ , NH ₃ , PH ₃ ), challenge atmospheric models, and raise questions about the formation pathways and evolution of these objects. Aims. We revisited the atmosphere of the cold (T _eff = 483 ₋₅₃ ⁺⁴⁴ K), mature (414 ± 23 Myr), and large-separation (>5000 au) Jovian exoplanet COCONUTS-2 b (WlSEPA J075108.79-763449.6), adding new spectral information beyond 5 μm and combining them with existing spectrophotometry to consolidate the constraints on the object properties and identify disagreements from self-consistent atmospheric models. Methods. We used a high signal-to-noise MIRI-LRS spectrum (5.45-11 μm, R _λ ∼ 100) of COCONUTS-2 b revealing prominent molecular features of H ₂ O, CH ₄ , and NH ₃ . This dataset is combined with spectra from Gemini/FLAMINGOS-2 and JWST/NIRSpec (G395H), as well as photometry from WISE and Spitzer, resulting in almost continuous wavelength coverage from 1 to 15 μm. We analyzed the data using five grids of self-consistent atmospheric models, spanning a wide range of T _eff , log(g), and [M/H]. We also investigated the use of Gaussian processes to account for correlated noise either caused by the spectrograph or by systematic departures of models in the inversion framework. Results. All models manage to fit the overall combined observations, but predict fainter flux in Y- and N-bands. Classical model comparison suggests that the ATMO2020++ synthetic spectra (with and without PH ₃ ) are statistically preferred. However, when accounting for correlated noise using Gaussian processes, Sonora Elf Owl models are favored, although they still provide a comparatively poor fit to the data with bulk properties inconsistent with cooling model predictions. Fitting for the correlated noise of the three spectroscopic instruments, the ATMO2020++ model yields constraints consistent with previous studies and evolutionary model predictions: T _eff = 496 ₋₃ ⁺⁵ K, log(g) = 4.30 _−0.02 ^+0.04 dex, [M/H] = −0.02 _−0.02 ^+0.03 dex, and R = 1.03 _−0.02 ^+0.01 R _Jup . The extended wavelength coverage provided by MIRI (accounting for 41% of the bolometric flux) completes the SED, yielding a precise luminosity estimation of log(L/L _⊙ ) = -6.166 ± 0.002 dex. Combined with a previous estimate of the system age (414 ± 23 Myr), cooling models predict a mass of M = 7.3 ± 0.3 M _Jup . Conclusions. The preferred models suggest a metallicity consistent with that of the primary, potentially supporting a binary-like formation scenario. Remaining discrepancies across spectral bands and between model grids suggest incomplete chemistry modeling and highlight the need for improved treatments of alkali condensation and diabatic processes for models at these low effective temperatures.

Context . Emerging research suggests that elemental and isotopic ratios of exoplanet and brown dwarf atmospheres may serve as potential tracers of their formation pathways. The ESO SupJup Survey aims to shed light on this hypothesis, with a focus on the ¹² CO/ ¹³ CO ratio, by investigating the atmospheric composition of substellar companions and isolated brown dwarfs. Aims . In this work, we aim to characterize the atmospheres and determine the ratios of ¹² CO/ ¹³ CO of the Rho Ophiuchus X-ray source (ROXs) 12 system (∼6 Myr), consisting of an M0 host with an L0 companion, as part of the ESO SupJup survey. This system provides a great opportunity to directly compare the atmospheric compositions of the host star and its companion. Methods . Using high-resolution CRIRES+ K band spectra of these objects, we perform atmospheric retrieval analyses to derive their atmospheric properties, including the ¹² CO/ ¹³ CO ratio. Our retrieval framework is built on the radiative transfer code petitRADTRANS , with which we generate model spectra based on equilibrium chemistry tables computed with FastChem, coupled with the nested sampling algorithm PyMultiNest . Results . We report the presence of H ₂ O, ¹² CO, ¹³ CO, and HF in both the star and companion, with a tentative detection of H ¹⁸ ₂ O in ROXs 12B. The ¹² CO/ ¹³ CO ratios of the two objects show a measurable, though not strongly significant, difference, namely 77 ₋₇ ⁺¹⁰ and 55 ₋₇ ⁺¹⁰ for ROXs 12A and B. Both are consistent with the local present interstellar medium. We measure a C/O ratio of 0.54 ± 0.01 and obtain a lower limit of H ¹⁶ ₂ O/H ¹⁸ ₂ O ⪆ 300 for ROXs 12B, while the C/O ratio of the star is not reliably constrained due to the absence of atomic oxygen lines in the K band. The companion also appears to exhibit a more isothermal temperature structure than expected from models. Furthermore, we retrieve moderate veiling in the host star of r _k = 0.17 _−0.03 ^+0.02 . Conclusions . Systems such as ROXs 12, in which both star and planet can be chemically and isotopically characterized, are crucial for constraining potential formation mechanisms of massive, wide-orbit super-Jupiters. The differing ¹² CO/ ¹³ CO ratios in the ROXs 12 system highlight the need for a broader sample to assess the frequency of isotopic variations and whether they may be linked to formation history.

Context . The distinct formation pathways proposed for directly imaged exoplanets and isolated brown dwarfs might leave imprints in the inherited chemical composition. Elemental and isotopic tracers could help inform the suspected histories, but this requires a careful characterisation of the sub-stellar atmospheres. In particular, objects at the L-T transition exhibit signs of dynamics that can drive their atmospheres out of chemical equilibrium. Aims . In this work, we studied the nearest L-T brown dwarfs, Luhman 16A and B, to assess the chemical disequilibrium in their atmospheres. We also investigated the elemental and isotopic compositions in the context of their probable formation history within the Oceanus moving group. Methods . As part of the ESO SupJup Survey, we obtained spatially resolved CRIRES ⁺ K-band spectra of the binary. These high-resolution observations were analysed using an atmospheric retrieval framework that couples the radiative transfer code petitRADTRANS to the MultiNest sampling algorithm. Results . We detect and retrieve the abundances of ¹² CO, H ₂ O, CH ₄ , NH ₃ , H ₂ S, HF, and the ¹³ CO isotopologue. We find that both atmospheres are in chemical disequilibrium with somewhat stronger vertical mixing in Luhman 16A compared to B ( K _zz,A ~ 10 ^8.7 vs K _zz,B ~ 10 ^8.2 cm ² s ⁻¹ ). The tested chemical models, free-equilibrium and disequilibrium chemistry, yield consistent mixing ratios and agree with earlier work at shorter wavelengths. The free-chemistry gaseous C/O ratios show evidence of oxygen trapping in silicate-oxide clouds. While the C/O ratios are consistent with the solar composition, the metallicities are modestly enhanced with [C/H] ~ 0.15. The carbon isotope ratios are measured at ¹² C/ ¹³ C _A = 74 ₋₂ ⁺² and ² C/ ¹³ C _B = 74 ₋₃ ⁺³ . Conclusions . The coincident constraints of metallicities and isotopes across the binary provide further evidence in favour of a common formation. The ¹² C/ ¹³ C ratios are aligned with the present-day interstellar medium but lower than the Solar System value. This suggests a recent inheritance and corroborates the relatively young age (~500 Myr) of Luhman 16A and B as members of the Oceanus moving group.

The atmospheres of warm gas giants can be readily characterized through transmission and emission spectroscopy. WASP-80 b is one such exoplanet, with an unusually low density that is in tension with the metal-rich composition expected for a planet of this mass. The aim of this work to derive precise constraints on WASP-80 b's bulk metal mass fraction, atmospheric composition, and thermal structure. We conducted a suite of retrievals using three approaches: traditional interior-only, atmosphere-only, and joint interior-atmosphere retrievals. We coupled the open-source model GASTLI to describe the planetary structure and thermal evolution and petitRADTRANS to describe the atmospheric chemistry and clouds. Our retrievals combined the mass and age with panchromatic spectra from JWST and HST in both transmission (0.5−4 μm) and emission (1–12 μm) as observational constraints. We identified two fiducial scenarios. In the first, WASP-80 b has an internal temperature consistent with its age in the absence of external heating sources; in addition, its atmosphere is in chemical equilibrium, with an atmospheric metallicity of M/H = 2.75 _−0.56 ^+0.88 × solar, a bulk metal mass fraction Z _planet = 0.12 ± 0.02, and a core mass M _core = 3.49 _−1.59 ^+3.49 M _⊕ . In the second scenario, WASP-80 b would be inflated by an additional heat source, possibly induced by magnetic fields, with an atmospheric metallicity of M/H = 10.00 _−4.75 ^+8.20 × solar, Z _planet = 0.28 ± 0 .11, and M _core = 31.8 _−17.5 ^+21.3 M _⊕ . The super-solar M/H and sub-solar C/O ratios in both scenarios suggest late pebble or planetesimal accretion, while additional heating is required to reconcile the data with the more massive core predicted by the core accretion paradigm. In general, joint retrievals are inherently affected by a degeneracy between atmospheric chemistry and internal structure. Taken together with flexible cloud treatment and an unweighted likelihood, this leads to larger uncertainties in bulk and atmospheric compositions than had previously been claimed.

Silicate clouds have long been known to significantly impact the spectra of late L-type brown dwarfs – with observable absorption features at ~10 μm. The James Webb Space Telescope (JWST) has reopened a window to the mid-infrared with unprecedented sensitivity, bringing the characterization of silicate clouds into focus again. Using JWST, we aim to characterize the planetary-mass brown dwarf PSO J318.5338-22.8603, concentrating on any silicate cloud absorption the object may exhibit. PSO J318’s spectrum is extremely red, and its flux is variable, both of which are thought to be hallmarks of cloud absorption. We present JWST NIRSpec PRISM, G395H, and MIRI MRS observations of PSO J318 from 1 to 18 μm. We introduce a method based on PSO J318’s brightness temperature to generate a list of cloud species that are likely present in its atmosphere. We tested for the species’ presence with petitRADTRANS retrievals. Using retrievals and grids from various climate models, we derived bulk parameters from PSO J318’s spectra, which are mutually compatible. Our retrieval results point to a solar to a slightly super-solar atmospheric C/O, a slightly super-solar metallicity, and a ¹² C/ ¹³ C below ISM values. The atmospheric gravity proves difficult to constrain for both retrievals and grid models. Retrievals describing the flux of PSO J318 by mixing two 1D models (“two-column models”) appear favored over single-column models; this is consistent with PSO J318’s variability. The JWST observations also reveal a pronounced absorption feature at 10 μm. This absorption is best reproduced by introducing a high-altitude cloud layer of small (<0.1 μm) amorphous SiO grains. The retrieved particle size and location of the cloud is consistent with SiO condensing as cloud seeding nuclei. High-altitude clouds comprised of small SiO particles have been suggested in previous studies. Therefore, the SiO nucleation we potentially observe in PSO J318 could be a more widespread phenomenon.

Context . Cold brown dwarf atmospheres provide a good training ground for the analysis of atmospheres of temperate giant planets. WISEP J173835.52+273258.9 (WISE 1738) is an isolated cold brown dwarf and a Y0 spectral standard with a temperature between 350-400 K, lying at the boundary of the T-Y transition. Although its atmosphere has been extensively studied in the near-infrared, its bulk physical parameters and atmospheric chemistry and dynamics are not well understood. Aims . Using a Mid-Infrared Instrument (MIRI) medium-resolution spectrum (5-18 μm), combined with near-infrared spectra (0.982.2 μm) from Hubble Space Telescope’s (HST) Wide Field Camera 3 (WFC3) and Gemini Observatory’s Near-Infrared Spectrograph (GNIRS), we aim to accurately characterize the atmospheric chemistry and bulk physical parameters of WISE 1738. Methods . We perform a combined atmospheric retrieval on the MIRI, GNIRS, and WFC3 spectra using a machine learning algorithm called Neural Posterior Estimation (NPE) assuming a cloud-free model implemented using petitRADTRANS . We demonstrate how this combined retrieval approach ensures robust constraints on the abundances of major atmospheric species, the pressure-temperature ( P - T ) profile, bulk C/O, and metallicity [M/H], along with bulk physical properties such as effective temperature, radius, surface gravity, mass, and luminosity. We estimate 1D and 2D marginal posterior distributions for the constrained parameters and evaluate our results using several qualitative and quantitative Bayesian diagnostics, including Local Classifier 2-Sample Test (L-C2ST), coverage, and posterior predictive checks. Results . The combined atmospheric retrieval confirms previous constraints on H ₂ O, CH ₄ , NH ₃ , and for the first time provides constraints on CO, CO ₂ , and ¹⁵ NH ₃ . It also gives better constraints on the physical parameters and the P - T profile while also revealing potential biases in characterizing objects using data from limited wavelength ranges. The retrievals further suggest the presence of disequilibrium chemistry, as evidenced by the constrained abundances of CO and CO ₂ , which are otherwise expected to be depleted and hence not visible beyond the near-infrared wavelengths under equilibrium conditions. We estimate the physical parameters of the object as follows: an effective temperature of 402 ₋₉ ⁺¹² K, surface gravity (log g ) of 4.43 _−0.34 ^+0.26 cm s ⁻² , mass of 13 ₋₇ ⁺¹¹ M _Jup , radius of 1.14 _−0.03 ^+0.03 R _Jup , and a bolometric luminosity of −6.52 _−0.04 ^+0.05 log L/L _⊙ . Based on these values, the evolutionary models suggest an age between 1 and 4 Gyr, which is consistent with a high rotation rate of 6 h of the brown dwarf. We further obtain an upper bound on the ¹⁵ NH ₃ abundance, enabling a 3σ lower bound calculation of the ¹⁴ N/ ¹⁵ N ratio = 275, unable to interpret the formation pathway as core collapse. Additionally, we calculate a C/O ratio of 1.35 _−0.31 ^+0.39 and a metallicity of 0.34 _−0.11 ^+0.12 without considering any oxygen sequestration effects.

Characterizing exoplanets’ spectra is a crucial step in understanding the chemical and physical processes shaping their atmospheres and constraining their formation and evolutionary history. The X-SHYNE library is a homogeneous sample of 43 medium-resolution (R_λ ~ 8000) infrared (0.3–2.5 μm) spectra of young (Jup), and cold (T_eff ~600–2000 K) isolated brown dwarfs and wide-separation companions observed with the VLT/X-Shooter instrument. To characterize our targets, we performed a global comparative analysis. We first applied a semiempirical approach. By refining their age and bolometric luminosity, we derived key atmospheric and physical properties, such as T_eff, mass, surface gravity (g), and radius, using the evolutionary model COND03. These results were then compared with the results from a synthetic analysis based on three self-consistent atmospheric models: the cloudy models Exo-REM and Sonora Diamondback, and the cloudless model ATMO. To compare our spectra with these grids we used the Bayesian inference code ForMoSA. We found similar L_bol estimates between both approaches, but an underestimated T_eff from the cloudy models, likely due to a lack of absorbers that could dominate the J and H bands of early L. We also observed a discrepancy in the log(g) estimates, which are dispersed between 3.5 and 5.5 dex for mid-L objects. We interpret this as a bias caused by a range of rotational velocities leading to cloud migration toward equatorial latitudes, combined with a variety of viewing angles that result in different observed atmospheric properties (cloud column densities, atmospheric pressures, etc.). This interpretation is supported by the correlation of the color anomaly Δ(J–K) of each object with log(g) and the parameter f_sed that drives the sedimentation of the clouds. Finally, while providing robust estimates of [M/H] and C/O for individual objects remains challenging, the X-SHYNE library globally suggests solar values that are consistent with a formation via stellar formation mechanisms. This study highlights the strength of homogeneous datasets in performing comparative analyses, reducing the impact of systematics, and ensuring robust conclusions while avoiding overinterpretation.