Polychronis Patapis

Kinematic perturbations observed with the Atacama Large Millimeter/submillimeter Array in CO line emission provide evidence for a population of embedded giant protoplanets shaping the structure of protoplanetary disks. We present JWST/MIRI F1140C ( λ = 11.3 μ m) coronagraphic observations of five protoplanetary disks—HD 163296, RXJ 1615.3–3255, RXJ 1842.9–3532, SY Cha, and LkCa 15—with the goal of directly detecting candidate protoplanets orbiting at ≳70 au that were previously inferred from gas kinematics. The data were analyzed using a bespoke methodology that combines subtraction of a reference point-spread function with forward modeling of partially resolved inner disk emission, which otherwise dominates the diffraction pattern in the images. This approach improves the sensitivity to young companions at small separations. No point source consistent with an embedded protoplanet is detected in any of the systems. Instead, in three systems we detect extended emission at 11.3 μ m tracing the outer disk out to radii comparable to those probed by CO. Injection tests indicate upper mass limits of roughly 3–20 M _J at separations of a few hundred astronomical units, assuming no additional thermal contribution from the circumplanetary environment. Even with space-based observations, these limits remain mostly above the ∼1–5 M _J masses inferred from disk kinematics, largely due to the limitations imposed by emission (and/or scattered light) contributions from both the inner disk and outer disk. These observations highlight the challenges of observing protoplanets embedded in their forming environment at large separation with JWST/MIRI. Lessons learned can inform future studies with the Extremely Large Telescope, which will probe separations where the occurrence rate of gas giants is expected to be higher.

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.

VHS 1256 b was the first planetary-mass companion to be observed with the James Webb Space Telescope’s Mid-Infrared Instrument (JWST/MIRI) using the Medium-Resolution Spectrometer (MRS). The MRS provides high-quality integral-field spectral data in the mid-infrared (IR) wavelengths from 4.9–18 μ m. This data set serves as a testbed for applying cross-correlation techniques to characterize exoplanet atmospheres. We implement the so-called molecular mapping approach, which consists of performing a spectral cross-correlation between each spectral pixel and atmospheric model templates. We compare these results with those obtained from cross-correlation of the extracted spectrum. Using a self-consistent Exo-REM atmospheric model grid, we constrain the temperature, surface gravity, C/O ratio, and metallicity, finding values consistent with those obtained from other analysis methods. We detect CO (S/N ∼ 25) and H ₂ O (S/N ∼ 76), with tentative detections of NH ₃ and CH ₄ (S/N ∼ 3). We test cross-correlation to measure trace-species abundances and isotopic ratios. We measure a volume mixing ratio (VMR) of [ NH 3 ] = − 5.7 3 − 0.14 + 0.15 and an isotopic ratio 12 C / 13 C = 77 . 8 − 10 + 13 , both consistent with free-chemistry retrievals. The derived NH ₃ VMR, combined with the measured temperature and radius, is consistent with VHS 1256 b having a mass above the deuterium-burning limit. These results demonstrate the diagnostic power of mid-IR spectroscopy and highlight cross-correlation as a robust method for characterizing directly imaged exoplanets, even in future higher-contrast regimes where spectral extraction becomes challenging. Future MIRI MRS observations across a wider range of temperatures and masses will further expand our understanding of planetary atmospheric chemistry.

Context . The Mid-Infrared Instrument (MIRI) on board the James Webb Space Telescope (JWST) enables the characterisation of young self-luminous gas giants in a previously inaccessible wavelength range, revealing insights into physical processes of the gas, dust, and clouds. Aims . We aim to characterise the young planetary system TWA 27 (2M1207) in the mid-infrared, revealing the atmosphere and disk spectra of the M9 brown dwarf TWA 27A and its L6 planetary-mass companion TWA 27b. Methods . We obtained data from the MIRI Medium Resolution Spectrometer (MRS) from 4.9 to 20 μm, and MIRI Imaging in the F1000W and F1500W filters. We applied high-contrast imaging data processing methods in order to extract the companion spectral energy distribution up to 15 μm at a separation of 0.″78 and contrast of 60. Using published spectra from JWST/NIRSpec, we analysed the 1-20 μm spectra with self-consistent atmosphere grids, and the molecular disk emission from TWA 27A with 0D slab models. Results . We find that the atmosphere of TWA 27A is well fitted with the BT-SETTL model of effective temperature T _eff ~ 2780 K, log g ~ 4.3, and a blackbody component of ∼740 K for the circumstellar disk inner rim. The disk consists of at least 11 organic molecules, and neither water nor silicate dust emission are detected. The atmosphere of the planet TWA 27b matches with a T _eff ~ 1400 K low-gravity model when adding extinction, with the ExoREM grid fitting the best. MIRI spectra and photometry for TWA 27b reveal a silicate cloud absorption feature between 8-10 μm, and evidence (>5σ) of infrared excess at 15 μm that is consistent with predictions from circumplanetary disk emission. Conclusions . The MIRI observations present novel insights into the young planetary system TWA 27, showing a diversity of features that can be studied to understand the formation and evolution of circumplanetary disks and young dusty atmospheres.

Context . The chemical composition of warm gas giant exoplanet atmospheres (with T _eq < 1000 K) is not well known due to the lack of observational constraints. Aims . HAT-P-12 b is a warm, sub-Saturn-mass transiting exoplanet that is ideal for transmission spectroscopy. We aim to characterise its atmosphere and probe the presence of carbonaceous species using near-infrared observations. Methods . One transit of HAT-P-12 b was observed in spectroscopy with JWST NIRSpec in the 2.87–5.10 µm range with a resolving power of ~1000. The JWST data are combined with archival observations from HST WFC3 covering the 1.1–1.7 µm range. The data were analysed using two data reduction pipelines and two atmospheric retrieval tools. Atmospheric simulations using chemical forward models were performed to interpret the spectra. Results . CO ₂ , CO, and H ₂ O are detected at 12.2, 4.1, and 6.0 σ confidence, respectively. Their volume mixing ratios are consistent with an atmosphere of ~10× solar metallicity and production of CO ₂ by photochemistry. CH ₄ is not detected and seems to be lacking, which could be due to a high intrinsic temperature with strong vertical mixing or other phenomena. SO ₂ is also not detected and its production seems limited by low upper atmosphere temperatures (~500 K at P ≲ 10 ⁻³ bar derived from one-dimensional retrievals), insufficient to produce it in detectable quantities (≳ 800 K required according to photochemical models). H ₂ S is marginally detected using one data analysis method, but not by the other. Retrievals indicate the presence of clouds between 2 and 11 mbar using one data analysis method, and between 5 and 269 mbar using the other. The derived C/O ratio is below unity, but is not well constrained. Conclusions . This study points towards an atmosphere for HAT-P-12 b that could be enriched in carbon and oxygen with respect to its host star, a possibly cold upper atmosphere that may explain the non-detection of SO ₂ , and a CH ₄ depletion that is yet to be fully understood. When including the production of CO ₂ via photochemistry, an atmospheric metallicity that is close to Saturn’s can explain the observations. Metallicities inferred for other gas giant exoplanets based on their CO ₂ mixing ratios may need to account for its photochemical production pathways. This may impact studies on mass-metallicity trends and links between exoplanet atmospheres, interiors, and formation history.

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.

The Mid-Infrared Instrument (MIRI) on the James Webb Space Telescope (JWST) enables the characterisation of young self-luminous gas giants at previously inaccessible wavelengths, revealing physical processes in gas, dust, and clouds. We characterise the young planetary system TWA 27 (2M1207) in the mid-infrared (MIR), studying the atmosphere and disk spectra of the M9 brown dwarf TWA 27A and its L6 planetary-mass companion TWA 27b. We obtained data with the MIRI Medium Resolution Spectrometer (MRS) from 4.9 to 20 um, and MIRI Imaging in the F1000W and F1500W filters. We applied high-contrast imaging methods to extract the companion's spectral energy distribution up to 15 um at 0.78 arcsec separation and a contrast of 60. Combining these with published JWST/NIRSpec spectra, we analysed the 1-20 um range using self-consistent atmospheric grids and 0D slab models for molecular disk emission. The atmosphere of TWA 27A is well fitted by a BT-SETTL model with Teff 2780 K, log g 4.3, plus a 740 K blackbody for the inner disk rim. The disk shows at least 11 organic molecules, with no water or silicate dust emission detected. The atmosphere of TWA 27b is matched by a Teff 1400 K low-gravity model with extinction, best fit by the ExoREM grid. MIRI spectra and photometry for TWA 27b reveal a silicate cloud absorption feature between 8-10 um and significant (>5 sigma) infrared excess at 15 um consistent with circumplanetary disk emission. These MIRI observations provide new insights into TWA 27, revealing diverse features to study the formation and evolution of circumplanetary disks and young dusty atmospheres.

Context. The Enhanced Resolution Imager and Spectrograph (ERIS) is the new adaptive optics (AO) assisted infrared instrument at the Very Large Telescope (VLT). Its refurbished integral field spectrograph (IFS) SPIFFIER leverages a new AO module, enabling high-contrast imaging applications and giving access to the orbital and atmospheric characterisation of super-Jovian exoplanets. Aims. We tested the detection limits of ERIS, and demonstrate its scientific potential by exploring the atmospheric composition of the young super-Jovian AF Lep b. Additionally, we improved its orbital solution by measuring its radial velocity relative to its host star. Methods. We present new spectroscopic observations of AF Lep b in K-band at R ∼ 11 000 obtained with ERIS/SPIFFIER at the VLT. We reduced the data using the standard pipeline together with a custom wavelength calibration routine, and removed the stellar point spread function using principal component analysis along the spectral axis. We computed molecular maps by cross-correlating the residuals with molecular spectral templates and measured the radial velocity of the planet relative to the star. Furthermore, we computed contrast grids for molecular mapping by injecting fake planets. Results. We detect a strong signal from H₂O and CO but not from CH₄ or CO₂. This result corroborates the hypothesis of chemical disequilibrium in the atmosphere of AF Lep b. Our measurement of the RV of the planet yields Δ v_{R, P ⋆} = 7.8 ± 1.7 km s⁻¹. This enables us to disentangle the degeneracy of the orbital solution; specifically, the correct longitude of the ascending node is Ω = 248_−0.7^+0.4 deg and the argument of periapsis is ω = 109₋₂₁⁺¹³ deg. Our detection limits reach a contrast of ΔK = 11.5 mag at 0′′.12 for the spectral templates of H₂O and CO, significantly extending the parameter space available to moderately high spectral resolution towards small angular separation. Conclusions. Our results demonstrate the competitiveness of the new ERIS/SPIFFIER instrument for the orbital and atmospheric characterisation of exoplanets at high contrast and small angular separation.