We present the first JWST phase curve of a white dwarf-brown dwarf binary, a NIRSpec PRISM observation of ZTFJ0038+2030. Short-period white dwarf-brown dwarf 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

We present a systematic assessment of two major cloudy atmospheric model grids— SM08 and Sonora Diamondback —when applied to low-resolution, near-infrared (0.8–2.5 μ m) spectroscopy. Our analysis focuses on a uniform sample of 142 age-benchmark brown dwarfs and planetary-mass objects spanning late M, L, and T spectral types, with independently determined ages from 10 Myr to 10 Gyr. We perform forward-model spectral fitting for all benchmarks’ Infrared Telescope Facility/SpeX spectra ( R ∼ 80–250) using both SM08 and Sonora Diamondback atmospheric models to infer effective temperatures, surface gravities, metallicities, radii, and cloud sedimentation efficiencies. The two model grids yield broadly consistent results. Among L4–L9 dwarfs, we identify a statistically significant, population-level age dependence of the cloud parameter f _sed , with young benchmarks (<300 Myr) exhibiting systematically lower f _sed values than older counterparts. This trend is absent across L0–T5 and T0–T5, demonstrating that cloud properties vary with age and surface gravity and offering explanations for the observed gravity-dependent photometric properties at the late L end of the L/T transition. By comparing spectroscopically inferred parameters with predictions from evolution models, we quantify systematic errors in the fitted atmospheric parameters and establish empirical calibrations to anchor future studies using these models. Stacked residuals across the sample reveal wavelength-dependent data–model mismatches associated with key atomic and molecular absorption bands, highlighting the need for improved opacities and rainout chemistry. In particular, persistent residuals in FeH bands likely contribute to the difficulty of robustly constraining $\mathrm{log}(g)$ and mass from spectral fitting for late M and L dwarfs. Finally, we show that including an interstellar-medium-like extinction term significantly improves the spectral fits, confirming and broadening previous findings and suggesting missing opacity sources in current cloudy models.

To better understand the potential habitability of planets orbiting brown dwarfs, this work presents a new set of equilibrium temperature evolution tracks. Unlike most previous work that relied on analytic scaling relationships for brown dwarf luminosity evolution, we use the outputs of modern brown dwarf evolution models that account for the effects of deuterium-burning, cloud formation and dissipation, and the most recent atmospheric opacities. While clouds are present, brown dwarfs cool more slowly than if they did not have clouds, allowing orbiting planets to remain in the habitable zone (HZ) for millions of years longer than previously estimated. Similarly, we find that during the deuterium-burning phase of brown dwarfs, which also slows the evolution, planets at the same orbital radius but orbiting brown dwarfs of different masses can remain in the HZ for the same duration, creating deuterium "sweet spots" for habitability around brown dwarfs near the deuterium-burning limit. For example, at 0.01 au a planet orbiting both a 0.012 and a 0.020 M circle dot brown dwarf stays in the HZ for similar to 170-180 Myr because deuterium burning more strongly affects the cooling of lower-mass brown dwarfs. The size of the effect decreases with decreasing orbital radius, with larger orbital radii having a more pronounced deuterium-burning influence. These effects are absent from the analytic cooling approximations used in prior studies of substellar HZs and are revealed by our application of modern substellar evolution models.

We present a uniform atmospheric retrieval analysis of 22 late-T and Y-type brown dwarfs within 20 pc, observed with the James Webb Space Telescope NIRSpec PRISM and MIRI low-resolution spectrometer. This dataset provides the first continuous similar to 0.95-12 mu m spectroscopic coverage of late-T and Y-type brown dwarfs, which in turn enables precise constraints on their thermal structures and volume mixing ratios (VMRs) of H2O, CH4, CO, CO2, NH3, H2S, K, Na, and PH3. We find positive correlations between the VMR of H2O and CH4, and CO and CO2, consistent with thermochemical equilibrium chemistry. Using the VMRs, we derive atmospheric metallicity, which is positively correlated with H2O and CH4, showing H2O and CH4 trace oxygen and carbon content, respectively, allowing us to effectively measure (O/H)bulk and (C/H)bulk. We also report tentative PH3 detections in roughly half the sample, suggesting potential vertical mixing or nonequilibrium chemistry. Apart from chemical properties, we retrieve masses and radii spanning similar to 6-77 MJupN and similar to 0.66-1.53 RJupN , respectively. We compare the derived log10(g) (similar to 4-5.5 (cm s-2)) and Teff (similar to 350-1100 K) with Sonora Bobcat evolutionary models and find an age range of 0.4 to 10 Gyr amongst the sample. Comparing our retrieved thermal profiles with the Elf-Owl forward model thermal profiles, we find a systematic difference between the two, likely arising due to the difference in chemistry treatment.

We present the results of a Spitzer Space Telescope Exploration Science Program to search for and characterize variability in Y dwarfs. We observed 14 Y dwarfs over a 24 hr period at [3.6] and [4.5] and then repeated the observations a few months later. We add two Y dwarfs, WD 0806-661B and WISE J085510.83-071442.5, that were also observed with Spitzer so that our sample includes all Y dwarfs observed for variability with Spitzer. We infer variability fractions of 59

Most stars, including our Sun, will one day evolve into red giants and, subsequently, white dwarfs. Several planet candidates have recently been identified orbiting white dwarfs1-4, demonstrating that planets can survive the stellar post-main-sequence stage intact. Little is known about the atmospheric composition of post-main-sequence planets, with the most evolved transiting planets with atmospheric detections so far orbiting subgiants5,6. Here we report an atmospheric detection for the white dwarf planet WD 1856 b, achieved through transmission spectroscopy with the James Webb Space Telescope (JWST) Near-Infrared Spectrograph (NIRSpec) PRISM. Our 0.5-5.0-μm spectrum reveals the presence of hydrocarbons (odds ratio of 167:1-5,377:1, with CH4 preferred at 17:1-30:1), aerosols (2 × 105:1-2 × 106:1) and thermal emission from the planetary nightside (2 × 1063:1-2 × 1073:1). Our spectral analysis constrains the mass of WD 1856 b to 4.3-10.9 MJ, finds a carbon-enriched atmosphere (with a CH4 abundance of approximately 7%) and an effective temperature exceeding the expected planetary equilibrium temperature (390-412 K versus 160 K). On the basis of cooling models, these results indicate that WD 1856 b underwent a migration-related reheating event 3.0-5.5 Gyr into the white dwarf phase, consistent with post-main-sequence tidal evolution to the present-day 0.02-AU circular orbit. Our results provide a window into the ultimate fate of giant planets orbiting stars with masses similar to our Sun.

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.

The Y dwarf WISE 1935 exhibits a thermal inversion in its radiative atmosphere, producing methane emission features in its JWST spectrum, but the physical mechanism responsible for this inversion remains unknown. Using the open-source radiative–convective equilibrium code PICASO, we model atmospheric heating with Chapman energy deposition profiles to reproduce the observed thermal inversion and methane emission feature. Our models require heating rates of approximately 10^5-10^6 erg cm^-2 s^-1. We show that the atmospheric response depends primarily on the integrated heating deposited in the observable atmosphere, revealing a degeneracy between heating magnitude, vertical extent, and emitting surface fraction. Disequilibrium chemistry lowers the required energy input by lowering CH_4 opacity and strengthening the inversion. Comparison with recent electron-beam heating models indicates that reproducing the thermal inversion in W1935 requires substantially greater energy deposition than currently predicted for brown dwarf auroral heating, while the observed methane emission favors energy deposition near 10^-3-10^-2 bar. Our models also predict a prominent methane emission feature near 7.8 microns, along with energy-sensitive ammonia features near 6 microns, implying a bolometric luminosity greater than that yet measured. Finally, we investigate potential sources of the inferred upper-atmospheric heating. We find that Joule heating would require a strong magnetic field and large electron densities, the latter supported by external ionization from an unidentified source. We also consider cometary impacts as a possible source of atmospheric heating.

Aerosols and clouds are expected to be ubiquitous in exoplanet and brown dwarf atmospheres, where they can have a significant impact on transmission and emission spectra. The cloud code Virga is capable of quickly modeling cloud particle sizes as a function of altitude, and has recently been updated to include functionality for aggregates (ranging from very fluffy chains to compact fractals). We analyze the effect that these aggregates have on transmission spectra for typical warm Neptune and hot Jupiter environments, as well as their effect on emission spectra for an L-type brown dwarf, over the wavelength range 0.3–15 μ m. We find significant, measurable differences in spectra when particle shape is changed (particularly the shortest wavelengths, where particle morphology strongly affects the scattering slope). We provide some intuitive rules for how nonabsorbing aggregates impact spectra: When particle sizes are small compared to the wavelength of light, the most elongated and chain-like particles have the highest opacities; when particles are large, the inverse is true (the most compact shapes have the highest opacities). We present an explanation for these effects in terms of the dynamics of how the particles form and move through the atmosphere, as well as in terms of fundamental optics theory. Given the significant impact that particle shape can have on spectra, we strongly encourage the community to include shape as a free parameter in future case studies, atmospheric models, and retrievals.

Temperate sub-Neptune and terrestrial exoplanets could contain large inventories of water in various phases, such as water-dominated atmospheres or even oceans. Observations have shown that many exoplanets, including water worlds, likely contain photochemically generated hazes. Haze particles are a key source of organic matter and may impact the evolution or origin of life; their optical properties are imperative for interpreting observations through theoretical atmospheric modeling. Modelers have thus far assumed haze optical properties that may not represent hazes under sub-Neptune and terrestrial atmospheric conditions. Often orbiting close to M dwarf stars, these planets receive large amounts of radiation, especially during flaring events, which may accelerate atmospheric escape and affect atmospheric compositions. Critically, it remains unknown how stellar flaring affects hazes and the subsequent transmission spectra of sub-Neptune and terrestrial exoplanets. Here, we present optical constants of experimentally generated sub-Neptune haze analogs before and after UV irradiation across a broad wavelength range (0.5-8 mu m). We find that UV irradiation alters haze optical constants, which become generally more absorbing in this wavelength range, which we hypothesize is due to our sample containing more oxygen-rich absorbing bands postirradiation. We use Virga and PICASO to simulate transmission spectra of potentially hazy water-dominated planets GJ 1214b and LHS 1140b, accounting for irradiated haze layers in their atmospheres. For our GJ 1214b CH4-rich haze modeled case, we see a difference in the N-H feature at 2.6 mu m in the resulting transmission spectrum between irradiated and unaltered haze that should be observable within current JWST capabilities. Broadly, we demonstrate the importance of using more representative optical constants, as they have an impact on current and future atmospheric composition interpretations.

Ultracool T (T-eff approximate to 500-1200 K) and Y dwarfs (T-eff (sic) 500 K) have historically been found only a few hundred parsecs from the Sun. The sensitivity and wavelength coverage of the NIRCam instrument on board the James Webb Space Telescope offer a unique method for finding low-temperature brown dwarfs in deep extragalactic datasets out to multiple kiloparsecs. Here we report on the selection of a sample of 41 brown dwarfs and brown dwarf candidates across the JWST Advanced Deep Extragalactic Survey in the GOODS-S and GOODS-N regions. We introduce a new open-source Bayesian tool, Near-Infrared Fitting for T and Y dwarfs (NIFTY), to derive effective temperatures, metallicities, and distances from JWST photometry. We find that 31 candidates have fits consistent with T dwarf temperatures out to 5-6 kpc, and 10 candidates have fits consistent with Y dwarf temperatures out to 1-2 kpc. The majority of the sources are best fit with subsolar metallicity models. We report proper motions for 10 brown dwarf candidates (three are newly presented), and calculate the number density of T and Y dwarfs as a function of temperature and distance above the Milky Way midplane. We further discuss how Y dwarfs can serve as contaminants in the search for ultra-high-redshift galaxies. Together, these results demonstrate the power of deep JWST extragalactic imaging to probe the coldest substellar populations far beyond the solar neighborhood, providing new constraints on the Milky Way's structure and brown dwarf demographics.

We present a systematic assessment of two major cloudy atmospheric model grids-SM08 and Sonora Diamondback-when applied to low-resolution, near-infrared (0.8-2.5 mu m) spectroscopy. Our analysis focuses on a uniform sample of 142 age-benchmark brown dwarfs and planetary-mass objects spanning late M, L, and T spectral types, with independently determined ages from 10 Myr to 10 Gyr. We perform forward-model spectral fitting for all benchmarks' Infrared Telescope Facility/SpeX spectra (R similar to 80-250) using both SM08 and Sonora Diamondback atmospheric models to infer effective temperatures, surface gravities, metallicities, radii, and cloud sedimentation efficiencies. The two model grids yield broadly consistent results. Among L4-L9 dwarfs, we identify a statistically significant, population-level age dependence of the cloud parameter f(sed), with young benchmarks (<300 Myr) exhibiting systematically lower f(sed)values than older counterparts. This trend is absent across L0-T5 and T0-T5, demonstrating that cloud properties vary with age and surface gravity and offering explanations for the observed gravity-dependent photometric properties at the late L end of the L/T transition. By comparing spectroscopically inferred parameters with predictions from evolution models, we quantify systematic errors in the fitted atmospheric parameters and establish empirical calibrations to anchor future studies using these models. Stacked residuals across the sample reveal wavelength-dependent data-model mismatches associated with key atomic and molecular absorption bands, highlighting the need for improved opacities and rainout chemistry. In particular, persistent residuals in FeH bands likely contribute to the difficulty of robustly constraining log(g) and mass from spectral fitting for late M and L dwarfs. Finally, we show that including an interstellar-medium-like extinction term significantly improves the spectral fits, confirming and broadening previous findings and suggesting missing opacity sources in current cloudy models.

Here we present an open-source cloud model for substellar atmospheres, called Virga . The Virga-v0 series has already been widely adopted in the literature. It is written in Python and has heritage from the A. S. Ackerman & M. S. Marley model (often referred to as eddysed ), used to study clouds on both exoplanets and brown dwarfs. In the development of the official Virga-v1 we have retained all the original functionality of eddysed and updated/expanded several components, including the back-end optical constant data, calculations of the Mie properties, available condensate species, saturation vapor pressure curves, and formalism for fall speed calculations. Here we benchmark Virga by reproducing key results in the literature, including the SiO _2 cloud detection in WASP-17 b and the brown dwarf Sonora Diamondback model series. Development of Virga is ongoing, with future versions already planned and ready for release. We encourage community feedback and collaborations within the GitHub code repository.

JWST has provided unprecedented access to ultra-cool brown dwarfs and has pushed the boundaries of directly imaging temperate giant planets. As we continue to push toward detecting sub-Saturn and Neptune-like planets, it is crucial to develop atmospheric and evolutionary models that better capture the complexity and diversity of planetary atmospheres similar to the gas and ice giants in our Solar System. We present Sonora Flame Skimmer, the next suite of cloud-free 1D atmospheric and evolutionary models in chemical equilibrium and disequilibrium probing colder temperatures (down to 50 K), smaller objects (down to log(g) = 2), and a wide range of metallicities (10x sub-solar to 100x super-solar) and C/O ratios (solar to 2.5x solar). Beyond expanding the physical parameter space of previous Sonora models, we update the opacities and evolutionary model framework from Sonora Bobcat, as well as the chemical treatment of volatiles (H_2O, CH_4, NH_3) and carbon species such as CO_2 from Sonora Elf Owl. For the evolution of these substellar objects, we find that high-metallicity atmospheres lead to slower cooling compared to solar metallicity, while the strength of vertical mixing (K_ zz) has a negligible impact on the evolutionary tracks. At the highest metallicity explored here (100× solar), the deuterium-burning and hydrogen-burning minimum masses fall to 5.39 and 45.03 M_ J, respectively. All the models presented here, including the atmospheric structure, chemical profiles, spectra, synthetic photometry, and evolutionary models, are publicly available.

We present JWST NIRSpec prism spectra for three ultracool brown dwarfs discovered in extragalactic survey data, two from the JWST Advanced Deep Extragalactic Survey (JADES), and one from Public Release IMaging for Extragalactic Research (PRIMER) survey observed as part of the Mirage or Miracle (MoM) program. The spectra for these sources indicate that one is a T6 dwarf (JADES-GS-BD-11, T_eff = ∼ 700 K) and two are Y0-Y1 dwarfs (JADES-GS-BD-5, T_eff = ∼ 400 K, and MoM-239450, T_eff = ∼ 500 K). Model atmospheric fits with to the spectra are consistent with this classification, and indicate that JADES-GS-BD-5 is only ∼ 150 pc from the Sun, MoM-239450 is ∼ 700 - 800 pc from the Sun, and JADES-GS-BD-11 is ∼ 1 kpc from the Sun, with these latter two more distant sources being best fit at sub-solar metallicities. JADES-GS-BD-5 has an observed spectrum with significantly weaker J and H band emission than Y dwarf atmospheric models, potentially indicating the presence of water ice clouds in the brown dwarf. The spectrum for JADES-GS-BD-11 has a feature at 4.3μm consistent with absorption from the rarely seen phosphine molecule at 2.6σ confidence. Given the low metallicity for this source ([M/H] = -0.7), our finding supports the theory that detecting phosphine in brown dwarf atmospheres is tied to atmospheric metallicity. JWST/NIRSpec spectroscopy continues to be a powerful tool for understanding the properties of these distant, and very cold brown dwarfs.

WD 0806 b is a rare exoplanet companion orbiting a white dwarf, currently with a projected orbital distance of 2500 au. The Spitzer mid-IR photometry suggests that the temperature is as cold as 350 K, making it one of the coldest directly imaged exoplanets. In this paper, we present the Near-infrared Camera (NIRCam) F150W2, F200W, F356W, and F444W broadband photometry and a 3-5 mu m Near-Infrared spectroscopy (NIRSpec) G395M spectrum obtained with the James Webb Space Telescope. We develop a new retrieval framework based on the open-source PICASO software that includes additive and multiplicative systematic parameters. Our retrieval results reveal bounded abundances of H2S, CO2, CO, NH3, H2O, and CH4. We present a new chemical analysis framework that utilizes retrieved abundances to measure altitude-dependent eddy diffusion coefficients (Kzz) at multiple quenched pressures. We find that the eddy diffusion coefficients decrease from around 104-102 cm2 s-1 as the atmospheric pressure decreases from from 50 to 20 bar. To our knowledge, this is the first study to report altitude-dependent vertical mixing (or, equivalently, quenched-species-dependent vertical mixing) based on the measured molecular abundances of CO, CH4, and CO2. With the 1-21 mu m NIRCam, NIRSpec, and the previously published MIRI data, we measure the bolometric luminosity to be log(L/L circle dot) = -6.75 +/- 0.01 and derive the mass to be 8 +/- 1MJ. The retrieval results suggest that WD 0806 b has an elevated C/O ratio of 0.76, or 1.3 & times; solar, subsolar metallicity ([M/H ]= -0.25), and a nearly solar C/S ratio (1.17x solar).

WD0806 b is a rare exoplanet companion orbiting a white dwarf, currently with a projected orbital distance of 2500 au. The Spitzer mid-IR photometry suggests that the temperature is as cold as 350K, making it one of the coldest directly imaged exoplanets. In this paper, we present the Near-infrared Camera (NIRCam) F150W2, F200W, F356W, and F444W broadband photometry and a 3–5Near-Infrared spectroscopy (NIRSpec) G395M spectrum obtained with the James Webb Space Telescope (JWST). We develop a new retrieval framework based on the open-source PICASO software that includes additive and multiplicative systematic parameters. Our retrieval results reveal bounded abundances of H_2S, CO_2, CO, NH_3, H_2O, and CH_4. We present a new chemical analysis framework that utilizes retrieved abundances to measure altitude-dependent eddy diffusion coefficients (K_zz) at multiple quenched pressures. We find that the eddy diffusion coefficients decrease from around 10^4 to 10^2 cm^2/s as the atmospheric pressure decreases from from 50 to 20 bars. To our knowledge, this is the first study to report altitude-dependent vertical mixing (or, equivalently, quenched-species-dependent vertical mixing) based on the measured molecular abundances of CO, CH_4, and CO_2. With the 1–21NIRCam, NIRSpec and the previously published MIRI data, we measure the bolometric luminosity to be log(L/L_⊙) = -6.75±0.01 and derive the mass to be 8± 1 M_J. The retrieval results suggest that has an elevated C/O ratio of 0.76, or 1.3× solar, sub-solar metallicity ([M/H ]= -0.25), and a nearly solar C/S ratio (1.17x solar).

(Abridged) We present a systematic assessment of two major cloudy atmospheric model grids – SM08 (Saumon Marley 2008) and Sonora Diamondback – when applied to low-resolution near-infrared (0.8-2.5 μm) spectroscopy. Our analysis focuses on a uniform sample of 142 age-benchmark brown dwarfs and planetary-mass objects spanning late-M, L, and T spectral types, with independently determined ages from 10 Myr to 10 Gyr. We perform forward-model spectral fitting for all benchmarks' IRTF/SpeX spectra (R∼80-250) using both SM08 and Sonora Diamondback atmospheric models to infer effective temperatures, surface gravities, metallicities, radii, and cloud sedimentation efficiencies. The two model grids yield broadly consistent results. Among L4-L9 dwarfs, we identify a statistically significant, population-level age dependence of the cloud parameter f_ sed, with young benchmarks (<300 Myr) exhibiting systematically lower f_ sed values than older counterparts. This trend is absent across L0-T5 and T0-T5, demonstrating that cloud properties vary with age and surface gravity and offering explanations for the observed gravity-dependent photometric properties at the late-L end of the L/T transition. By comparing spectroscopically inferred parameters with predictions from evolution models, we quantify systematic errors in the fitted atmospheric parameters and establish empirical calibrations to anchor future studies using these atmospheric models. Stacked residuals of the sample reveal wavelength-dependent data-model mismatches associated with key atomic and molecular absorption bands, highlighting the need for improved opacities and rainout chemistry. Finally, we show that including an interstellar-medium-like extinction term significantly improves the spectral fits, confirming and broadening previous findings and suggesting missing opacity sources in current cloudy models.

We present a major update to the open-source atmospheric modeling package PICASO , designed for simulating the thermal structure and spectra of hydrogen-rich atmospheres of brown dwarfs and exoplanets. This release, PICASO 4.0 , expands upon the existing radiative-convective equilibrium model framework by incorporating several new capabilities. Key additions include the integration of Virga for self-consistent cloud modeling, new flexible treatments for rainout and cold trapping of volatile species, and support for photochemistry. We also introduce a parameterized energy injection scheme to simulate additional external or internal heating processes. These features are motivated by lessons from recent JWST observations that reveal the prevalence of nonequilibrium chemistry and clouds. We benchmark the new functionalities against previously published results in the literature, including the Sonora Diamondback grid, energy injected atmospheres, patchy cloud models, and other photochemical models of WASP-39b. PICASO continues to be actively developed as an open-source package aimed at enabling reproducible, community-driven atmospheric modeling of all substellar objects.