With JWST, we are directly imaging cold (similar to 200-300 K) solar-age giant exoplanets for the first time. At these temperatures, many molecular features appear, and water-ice clouds may condense and affect the emission spectrum; early photometric measurements of cold giant planets are already showing some tension with the predictions of cloud-free solar-metallicity atmosphere models. Here, we present new JWST/MIRI coronagraphic observations of the cold giant exoplanet Eps Ind Ab at 11.3 mu m. Together with archival data, we use these new observations to study the atmosphere of this cold exoplanet, and we also refit its orbit, finding an updated mass of 7.6 +/- 0.7MJup and an eccentricity of 0.24-0.08+0.11 . The planet is significantly brighter (by 0.88 +/- 0.08 mag) at 11.3 mu m than at 10.6 mu m, indicating the presence of ammonia. However, this ammonia feature is shallower than expected. This could indicate a low-metallicity or nitrogen-depleted atmosphere, but our preferred explanation is the presence of thick water-ice clouds that suppress the ammonia feature and the near-IR emission of Eps Ind Ab. Photometry of the small but growing sample of cold giant exoplanets demonstrates that they are consistently fainter than expected between 3 and 5 mu m, consistent with the water-ice cloud hypothesis. 10.6 mu m and 11.3 mu m photometry of this cold exoplanet sample would be valuable to determine whether the suppressed ammonia feature is universal, and to frame a new open question about the underlying physical cause.

White dwarfs are among the rarest and faintest stars found in the solar neighbourhood and are therefore persistently challenging to detect. Whereas the Gaia mission has vastly expanded our knowledge of isolated white dwarfs, limited contrast sensitivity means that white dwarfs companions to much brighter solar-type stars are easily missed, leaving a subset of white dwarfs in "Sirius-like systems" that can only be discovered via high-contrast imaging. Here we report the discovery of a white dwarf companion to the nearby K0V star HD 38230 (HIP 27207). The companion, HD 38230 B, was originally detected with Keck/NIRC2 observations taken as part of the TRENDS high-contrast imaging survey, was detected by Gaia without an astrometric solution, and was independently observed using Lick/ShaneAO and Palomar/PHARO in the literature. Combining these multiple imaging detections spanning 6 years, we demonstrate that HD 38230 B is a gravitationally bound companion and has photometry consistent only with a white dwarf. Our analysis of the binary orbit, combining relative astrometry with over 25 years of precise radial velocity observations and Hipparcos-Gaia astrometry, results in strong constraints on the P = 1390^+310_-200 year orbit despite the short observational span, and provides a precise dynamical mass of M_B = 0.71^+0.06_-0.05 M_⊙ for the white dwarf. At 21 pc, this is the 11th-nearest Sirius-like system known to date, highlighting residual incompleteness in the local white dwarf census.

JWST defines a new era for the data-driven approach of retrieval modelling, which has become a cornerstone tool for the statistical inference of exoplanetary and brown dwarf properties. The Early Release Science program #1386 observations of VHS 1256 b represent a huge jump in data quality, data quantity and spectral coverage for such objects. VHS 1256 b is a young, planetary mass and extremely variable companion that populates the enigmatic L/T cohort of substellar atmospheres. In this first retrieval analysis of the full 1 - 18 micron dataset, we apply the Brewster retrieval framework to the NIRSpec and MIRI spectroscopic observations of VHS 1256 b, exploring a variety of cloud species and structures. Using Delta(BIC) we find that the data is best described by a forsterite (Mg_2SiO_4) and enstatite (MgSiO_3) cloud combination. Our analysis shows a strong preference for patchy silicate cloud coverage, which aligns with VHS 1256 b's extensive and well documented spectral variability. Our retrieval is able to place constraints on the abundances of H_2O, CO, CO_2, CH_4 as well as NH_3. We also show that the retrieved parameters are sensitive to the data used and the relative signal-to-noise ratios between data from different instruments. We conclude with the next steps for the wider retrieval community to better understand young and cloudy exoplanetary atmospheres.

Epsilon Indi A b (hereafter Eps Ind A b) is a directly imaged similar to 6 M-Jup exoplanet orbiting a nearby (3.6 pc) K dwarf at similar to 30 au. We analyze archival JWST/MIRI 15 mu m coronagraphic imaging of this planet to search for directly imaged satellites orbiting Eps Ind A b. Within the planet's Hill sphere (radius R-H approximate to 2.3 au or 1.3 lambda/D), we compare single and double point-spread-function (PSF) models using Bayesian evidence. We find that a double-PSF (binary planet) fit is preferred. This apparent preference can most plausibly be explained by systematics, although follow-up observations would be required to fully rule out a binary planet interpretation. We construct a contrast curve of the exoplanet after removing this feature, demonstrating sensitivity to companions as faint as 0.03 & times; the F1550C flux of Eps Ind A b (equivalent to T = 130 K, 1.3 M-Jup) at large separations (> 2 au). We also demonstrate sensitivity to brighter companions 0.2 & times; the F1550C flux of Eps Ind A b (equivalent to T = 180 K, 2.5 M-Jup) down to separations of 0.52 au (1.3 pixels; 0.29 lambda/D; 144 mas). This study demonstrates that JWST/MIRI can directly detect exomoons or binary planets inside the Hill sphere of directly imaged exoplanets orbiting neighboring stars.

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 (CO2, NH3, PH3), challenge atmospheric models, and raise questions about the formation pathways and evolution of these objects. Aims. We revisited the atmosphere of the cold (Teff = 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, Rλ ∼ 100) of COCONUTS-2 b revealing prominent molecular features of H2O, CH4, and NH3. 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 Teff, 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 PH3) 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: Teff = 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 RJup. 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 MJup. 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.

We use 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_2O, 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 analyze the data using five grids of self-consistent atmospheric models, spanning a wide range of T_eff, log(g), and [M/H]. We also investigate 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. 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 specra (with and without PH_3) are statistically preferred. Fitting for the correlated noise of the three spectroscopic instruments, ATMO2020++ models yields constraints consistent with previous studies and evolutionary models predictions: T_eff =496^+5_-3 K, log(g) =4.30^+0.04_-0.02 dex, [M/H] =-0.02^+0.03_-0.02 dex, and R =1.03^+0.01_-0.02 R_jup. The extended wavelength coverage provided by MIRI (accounting for 41

Substellar objects occupy a unique place in our universe, bridging the gap between the smallest stars and the largest planets, and serving as powerful laboratories for understanding extrasolar atmospheric physics without the contaminating glare of a host star. Previous studies into the atmospheric structure of these objects have revealed clouds, disequilibrium chemistry, thermal inversions, and auroral processes which each contribute to wavelength-dependent brightness variations. HST remains uniquely positioned to address key open questions in the field, such as resolving the vertical atmospheric structure, long term evolution of the atmosphere, and detection of UV aurora in the upper atmosphere, primarily in conjunction with other facilities that probe wavelength regimes that cannot be reached with instruments on HST. We advocate for three large scale initiatives and argue that the study of the atmospheres of substellar worlds directly prepares the community for atmospheric characterization with the Habitable Worlds Observatory.

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 mu 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 similar to 25) and H2O (S/N similar to 76), with tentative detections of NH3 and CH4 (S/N similar to 3). We test cross-correlation to measure trace-species abundances and isotopic ratios. We measure a volume mixing ratio (VMR) of [NH3]=-5.73-0.14+0.15 and an isotopic ratio 12C/13C=77.8-10+13 , both consistent with free-chemistry retrievals. The derived NH3 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.

We report the direct imaging discovery of a third exoplanet in the β Pictoris system. We detect β Pictoris d in non-coronagraphic observations obtained with VLT/ERIS as well as multi-epoch archival datasets from JWST/NIRCam and VLT/SPHERE. Astrometric measurements over an 11-year baseline demonstrate that it is consistent with a gravitationally-bound source with orbital motion. Joint multi-planet orbit fits of all three planets in the system yield a semi-major axis of 26.0^+2.2_-6.1 au and inclination 89.0^+0.7_-0.6 deg for planet d. β Pictoris d has a larger orbital semi-major axis than the other known planets in the system, but is coplanar with the inner two planets, and its orbit is consistent with sculpting the inner edge of the debris disk. β Pictoris d has a contrast of ΔL^'=12.11±0.15 mag, with colors and luminosity that closely match those of 51 Eri b, another exoplanet in the β Pictoris moving group. Its VLT/ERIS and JWST/NIRCam colors are distinct from those of free-floating planetary-mass objects of a similar age and temperature. Its red F410M-F444W color indicates strong CO_2 absorption in its atmosphere and suggests significant enhancement in metals compared to free-floating objects. From the ATMO hot-start evolutionary models, we estimate an effective temperature of 600^+45_-60 K and mass of 2.4±0.6 M_ Jup, which also closely matches similar estimates for 51 Eri b. β Pictoris d is among the lowest-mass exoplanets imaged from the ground. This discovery highlights the deep sensitivity achievable with ground-based imaging in the mid-infrared and the discovery potential of future high-contrast observations with the Extremely Large Telescope.

We present an updated characterization of the planetary system orbiting the nearby M2 dwarf GJ 3090 (TOI-177; d=22 pc), based on new high-precision radial velocity (RV) observations from NIRPS and HARPS. With an orbital period of 2.85 d, the transiting sub-Neptune GJ 3090 b has a mass we refine to 4.52 ± 0.47 M⊕, which, combined with our derived radius of 2.18 ± 0.06 R⊕, yields a density of 2.40−0.30+0.33 g∉cm−3. The combined interior structure and atmospheric constraints indicate that GJ 3090 b is a compelling water-world candidate, with a volatile-rich envelope in which water likely represents a significant fraction. We also confirm the presence of a second planet, GJ 3090 c, a sub-Neptune with a 15.9 d orbit and a minimum mass of 10.0 ± 1.3 M⊕, which does not transit. Despite its proximity to the star’s 18 d rotation period, our joint analysis using a multidimensional Gaussian process (GP) model that incorporates TESS photometry and differential stellar temperature measurements distinguishes this planetary signal from activity-induced variability. In addition, we place new constraints on a non-transiting planet candidate with a period of 12.7 d, suggested in earlier RV analyses. This candidate remains a compelling target for future monitoring. These results highlight the crucial role of multidimensional GP modelling in disentangling planetary signals from stellar activity, enabling the detection of a planet near the stellar rotation period that could have remained undetected with traditional approaches.

Measuring a single elemental ratio (e.g., carbon-to-oxygen) provides insufficient information for understanding the formation mechanisms and evolution that affect our observations of gas giant planet atmospheres. Although the fields of planet formation, protoplanetary disks, and exoplanets are well established and interconnected, our understanding of how to self-consistently and accurately link the theoretical and observational aspects of these fields together is lacking. To foster interdisciplinary conversations, the Max-Planck Institut für Astronomie (MPIA) hosted a week-long workshop called, "Challenge Accepted: Linking Planet Formation with Present-Day Atmospheres." Here, we summarize the latest theories and results in planet formation modeling, protoplanetary disk observations, and atmospheric observations of gas giant atmospheres to address one of the challenges of hosting interdisciplinary conferences: ensuring everyone is aware of the state-of-the-art results and technical language from each discipline represented. Additionally, we highlight key discussions held at the workshop. Our main conclusion is that it is unclear what the ideal observable is to make this link between formation scenarios and exoplanet atmospheres, whether it be multiple elemental abundance ratios, measuring refractory budgets, or something else. Based on discussions held throughout the workshop, we provide several key takeaways of what the workshop attendees feel need the most improvement and exploration within each discipline.

We used multiple epochs of high-contrast imaging spectrophotometric observations to determine the atmospheric characteristics and thermal evolution of two previously detected benchmark L dwarf companions, HD 112863 B and HD 206505 B. We analyzed IRDIS and IFS data from VLT/SPHERE of each companion, both of which have dynamical masses near the stellar-substellar boundary. We compared each companion with empirical spectral standards, and constrained their physical properties through atmospheric model fits. From these analyses, we estimate that HD 112863 B is spectral type L3 +/- 1 and that HD 206505 B is spectral type L2 +/- 1. Using the BT-Settl atmospheric model grids, we found a bimodal solution for the atmospheric model fit of HD 112863 B where T-eff = 1757(-36)(+37) K or 2002(-24)(+23) K and log g = 4.973(-0.063)(+0.057) or 5.253(-0.033)(+0.037), while for HD 206505 B, T-eff = 1754(-13)(+13) K and log g = 4.919(-0.029)(+0.031). The results of a comparison of the bolometric luminosities of the companions with evolutionary models imply that both companions are likely above the hydrogen burning limit.

The main challenge of exoplanet high-contrast imaging (HCI) is to separate the signal of exoplanets from their host stars, which are many orders of magnitude brighter. HCI for ground-based observations is further exacerbated by speckle noise originating from perturbations in Earth’s atmosphere and imperfections in the telescope optics. Various data postprocessing techniques are used to remove this speckle noise and reveal the faint planet signal. Often, however, a significant part of the planet signal is accidentally subtracted together with the noise. In the present work, we use explainable machine learning to investigate the reason for the loss of the planet signal for one of the most used postprocessing methods: principal component analysis (PCA). We find that PCA learns the shape of the telescope point-spread function for high numbers of PCA components. This representation of the noise captures not only the speckle noise but also the characteristic shape of the planet signal. Building on these insights, we develop a new postprocessing method (4S) that constrains the noise model to minimize this signal loss. We apply our model to 11 archival HCI data sets from the Very Large Telescope NACO instrument in the L ’ band and find that our model consistently outperforms PCA. The improvement is largest at close separations to the star (≤4 λ / D ), providing up to 1.5 mag deeper contrast. This enhancement enables us to detect the exoplanet AF Lep b in data from 2011, 11 yr before its subsequent discovery. We present updated orbital parameters for this object.

T-type brown dwarfs present an opportunity to explore atmospheres teeming with molecules such as H _2 O, CH _4 , and NH _3 , which exhibit a wealth of absorption features in the mid-infrared. With JWST, we can finally explore this chemistry in detail, including for the coldest brown dwarfs that were not yet discovered in the Spitzer era. This allows precise derivations of the molecular abundances, which in turn inform our understanding of vertical transport in these atmospheres and can provide clues about the formation of cold brown dwarfs and exoplanets. This study presents the first JWST/MRS mid-IR spectrum ( R ∼ 1500–3000) of a T dwarf: the T8.5+T9 brown dwarf binary WISE J045853.90+643451.9. We fit the spectrum using a parameterized P – T profile and free molecular abundances (i.e., a retrieval analysis), treating the binary as unresolved. We find a good fit with a cloud-free atmosphere and identify H _2 O, CH _4 , and NH _3 features. Moreover, we make the first detections of HCN and C _2 H _2 (at 13.4 σ and 9.5 σ respectively) in any brown dwarf atmosphere. The detection of HCN suggests intense vertical mixing ( K _zz ∼ 10 ^11 cm ^2 s ^−1 ), challenging previous literature derivations of K _zz values for T-type brown dwarfs. Even more surprising is the C _2 H _2 detection, which cannot be explained with existing atmospheric models for isolated objects. This result challenges model assumptions about vertical mixing and/or our understanding of the C _2 H _2 chemical network, or might hint towards more complex atmospheric processes such as magnetic fields driving aurorae or lightning driving ionization. These findings open a new frontier in studying carbon chemistry within brown dwarf atmospheres.

We present the detection and characterisation of the TOI-1438 multi-planet system discovered by the Transiting Exoplanet Survey Satellite (TESS). To confirm the planetary nature of the candidates and determine their masses, we collected a series of followup observations including high-spectral resolution observations with HARPS-N and HIRES over a period of 5 years. Our combined modelling shows that the K0V star hosts two transiting sub-Neptunes with R-b = 3.04 +/- 0.19 R-circle plus, R-c = 2.75 +/- 0.14 R-circle plus, M-b = 9.4 +/- 1.8 M-circle plus, and M-c =10.6 +/- 2.1 M-circle plus. The orbital periods of planets b and c are 5.1 and 9.4 days, respectively, corresponding to instellations of 145 +/- 10 F-circle plus and 65 +/- 4 F-circle plus. The bulk densities are 1.8 +/- 0.5 g cm(-3) and 2.9 +/- 0.7 g cm(-3), respectively, suggesting a volatile-rich interior composition. By combining the planet and stellar parameters, we were able to compute a set of planet interior structure models. Planet b presents a high-metallicity envelope that can accommodate up to 2.5% in H/He in mass, while planet c cannot have more than 0.2% as H/He in mass. For any composition of the core considered (Fe-rock or ice-rock), both planets would require a volatile-rich envelope. In addition to the two planets, the radial velocity (RV) data clearly reveal a third signal, likely coming from a non-transiting planet, with an orbital period of 7.6(-2.4)(+1.6) years and an RV semi-amplitude of 35(-5)(+3) m s(-1). Our best-fit model finds a minimum mass of 2.1 +/- 0.3 M-J and an eccentricity of 0.25(-0.11)(+0.08). However, several RV activity indicators also show strong signals at similar periods, suggesting this signal might (partly) originate from stellar activity. More data over a longer period of time are needed to conclusively determine the nature of this signal. If it is confirmed as a triple-planet system, TOI-1438 would be one of the few detected systems to date characterised by an architecture with two small, short-period planets and one massive, long-period planet, where the inner and outer systems are separated by an orbital period ratio of the order of a few hundred.

We present aperture masking interferometry (AMI) observations of the star HIP 65426 at 3.8 μ m, as part of the JWST Direct Imaging Early Release Science program, obtained using the Near Infrared Imager and Slitless Spectrograph instrument. This mode provides access to very small inner working angles (even separations slightly below the Michelson limit of 0.5 λ / D for an interferometer), which are inaccessible with the classical inner working angles of the JWST coronagraphs. When combined with JWST’s unprecedented infrared sensitivity, this mode has the potential to probe a new portion of parameter space across a wide array of astronomical observations. Using this mode, we are able to achieve a 5 σ contrast of Δ m F380M ∼ 7.62 ± 0.13 mag relative to the host star at separations ​​​​​≳0 . ″ 07 , and the contrast deteriorates steeply at separations ≲0 . ″ 07. However, we detect no additional companions interior to the known companion HIP 65426b (at separation ∼0 . ″ 82 or 8 7 − 31 + 108 au ). Our observations thus rule out companions more massive than 10–12 M Jup at separations ∼10–20 au from HIP 65426, a region out of reach of ground- or space-based coronagraphic imaging. These observations confirm that the AMI mode on JWST is sensitive to planetary mass companions at close-in separations (≳0 . ″ 07), even for thousands of more distant stars at ∼100 pc, in addition to the stars in the nearby young moving groups and associations, as stated in previous works. This result will allow the planning and successful execution of future observations to probe the inner regions of nearby stellar systems, opening an essentially unexplored parameter space.

M dwarfs have become increasingly important in the detection of exoplanets and the study of Earth-sized planets and their habitability. However, 20%–30% of M dwarfs have companions that can impact the formation and evolution of planetary systems. We use high-resolution imaging and Gaia astrometry to detect stellar companions around M dwarf exoplanet hosts discovered by TESS and determine the projected separation and estimated stellar masses for each system. We find 47 companions around 216 M dwarfs and a multiplicity rate of 19.4% ± 2.7% that is consistent with field M dwarfs. The binary projected separation distribution is shifted to larger separations, confirming the lack of close binaries hosting transiting exoplanets seen in previous studies. We correct the radii of planets with nearby companions and examine the properties of planets in M dwarf multistar systems. We also note three multiplanet systems that occur in close binaries (≲50 au) where planet formation is expected to be suppressed.

Most observed multiplanet systems are coplanar, in a dynamically “cold” configuration of concentric orbits like our own solar system. With the James Webb Space Telescope, we have detected 14 Her c, the first mature and cold exoplanet directly imaged in a dynamically “hot” multiplanet system. With large eccentricities and a nonzero mutual inclination, the present-day architecture of this system points to a turbulent past and ongoing angular momentum exchange between the planetary orbits of 14 Her b and c. The temperature of 14 Her c rivals both the coldest imaged exoplanet and the coldest known brown dwarf. Moreover, its photometry at 4.4 μ m is consistent with the presence of carbon disequilibrium chemistry and water-ice clouds in its atmosphere. 14 Her c presents a unique laboratory to study giant planet formation, the dynamical evolution of multiplanet system architectures, and atmospheric composition and dynamics in extremely cold worlds.

Context. The Near InfraRed Planet Searcher (NIRPS) joined HARPS on the 3.6-m ESO telescope at La Silla Observatory in April 2023, dedicating part of its Guaranteed Time Observations (GTO) program to the radial velocity follow-up of TESS planet candidates to confirm and characterize transiting planets around M dwarfs. Aims. We present the "Sub-Neptunes" subprogram of the NIRPS-GTO, aimed at investigating the composition and formation of sub-Neptunes orbiting M dwarfs. We report the first results of this program with the characterization of the TOI-756 system, which consists of TOI-756 b, a transiting sub-Neptune candidate detected by TESS, as well as TOI-756 c, an additional non-transiting planet discovered by NIRPS and HARPS. Methods. We analyzed TESS and ground-based photometry, high-resolution imaging, and high-precision radial velocities (RVs) from NIRPS and HARPS to characterize the two newly discovered planets orbiting TOI-756, as well as to derive the fundamental properties of the host star. A dedicated approach was employed for the NIRPS RV extraction to mitigate telluric contamination, particularly when the star's systemic velocity was shown to overlap with the barycentric Earth radial velocity. Results. TOI-756 is a M1V-type star with an effective temperature of T-eff similar to 3657 K and a super-solar metallicity ([Fe/H]) of 0.20 +/- 0.03 dex. TOI-756 b is a 1.24-day period sub-Neptune with a radius of 2.81 +/- 0.10 R-circle plus and a mass of 9.8(-1.6)(+1.8) M-circle plus. TOI-756 c is a cold eccentric (e(c) = 0.45 +/- 0.01) giant planet orbiting with a period of 149.6 days around its star with a minimum mass of 4.05 +/- 0.11 M-Jup. Additionally, a linear trend of 146 m s(-1) yr(-1) is visible in the radial velocities, hinting at a third component, possibly in the planetary or brown dwarf regime. Conclusions. We present the discovery and characterization of the transiting sub-Neptune TOI-756 b and the non-transiting eccentric cold giant TOI-756 c. This system is unique in the exoplanet landscape, standing as the first confirmed example of such a planetary architecture around an M dwarf. With a density of 2.42 +/- 0.49 g cm(-3), the inner planet, TOI-756 b, is a volatile-rich sub-Neptune. Assuming a pure H/He envelope, we inferred an atmospheric mass fraction of 0.023 and a core mass fraction of 0.27, which is well constrained by stellar refractory abundances derived from NIRPS spectra. It falls within the still poorly explored radius cliff and at the lower boundary of the Neptune desert, making it a prime target for a future atmospheric characterization with JWST to improve our understanding of this population.

With a temperature of ∼285 K, WISEJ0855–0714 (hereafter, WISE 0855) is the coldest brown dwarf observed thus far. Studying such cold gas giants allows us to probe the atmospheric physics and chemistry of evolved objects that resemble Solar System gas giants. Using James Webb Space Telescope (JWST), we obtained observations to characterize WISE 0855's atmosphere, focusing on vertical variation in the water steam abundance, measuring trace gas abundances, and obtaining the bulk parameters for this cold object. We observed the ultra-cool dwarf WISE 0855 using the Mid-Infrared Instrument Medium Resolution Spectrometer (MIRI/MRS) on board JWST at a spectral resolution of up to 3'750. We combined the observation with published data from the Near-Infrared Spectrograph (NIRSpec) G395M and PRISM modes, yielding a spectrum ranging from 0.8 to 22 µm. We applied atmospheric retrievals using petitRADTRANS to measure the atmospheric abundances, pressure-temperature structure, radius, and gravity of the brown dwarf. We also employed publicly available clear and cloudy self-consistent grid models to estimate the bulk properties of the atmosphere such as the effective temperature, radius, gravity, and metallicity. Atmospheric retrievals have constrained a variable water abundance profile in the atmosphere, as predicted by equilibrium chemistry. We detected the mathrm NH_3 isotopolog and inferred a ratio of volume fraction of mathrm NH_3 /mathrm NH_3 for the clear retrieval. We measured the bolometric luminosity by integrating the presented spectrum, obtaining a value of mathrm log(L/ L _⊙) = -7.291 ± 0.008. The detected water depletion indicates that water condenses out in the upper atmosphere due to the very low effective temperature of WISE 0855. The height in the atmosphere where this occurs is covered by the MIRI/MRS data, thereby demonstrating the potential of MIRI to characterize the atmospheres of cold gas giants. After comparing the data to retrievals and self-consistent grid models, we did not detect any signs of water ice clouds, although their spectral features have been predicted in previous studies.