Coldest Brown Dwarf Research Articles

Novel constraints on companions to the Helix nebula central star

ABSTRACT The Helix is a visually striking and the nearest planetary nebula, yet any companions responsible for its asymmetric morphology have yet to be identified. In 2020, low-amplitude photometric variations with a periodicity of 2.8 d were reported based on Cycle 1 TESS observations. In this work, with the inclusion of two additional sectors, these periodic light curves are compared with lcurve simulations of irradiated companions in such an orbit. Based on the light-curve modelling, there are two representative solutions: (i) a Jupiter-sized body with 0.102 R$_\odot$ and an arbitrarily small orbital inclination $i=1^{\circ }$, and (ii) a 0.021 R$_\odot$ exoplanet with $i\approx 25^{\circ }$, essentially aligned with the Helix nebular inclination. Irradiated substellar companion models with equilibrium temperature 4970 K are constructed and compared with existing optical spectra and infrared photometry, where Jupiter-sized bodies can be ruled out, but companions modestly larger than Neptune are still allowed. Additionally, any spatially unresolved companions are constrained based on the multiwavelength, photometric spectral energy distribution of the central star. No ultracool dwarf companion earlier than around L5 is permitted within roughly 1200 au, leaving only faint white dwarfs and cold brown dwarfs as possible surviving architects of the nebular asymmetries. While a planetary survivor is a tantalizing possibility, it cannot be ruled out that the light-curve modulation is stellar in nature, where any substellar companion requires confirmation and may be possible with JWST observations.

Microphysical Prescriptions for Parameterized Water Cloud Formation on Ultra-cool Substellar Objects

Water must condense into ice clouds in the coldest brown dwarfs and exoplanets. When they form, these icy clouds change the emergent spectra, temperature structure, and albedo of the substellar atmosphere. The properties of clouds are governed by complex microphysics but these complexities are often not captured by the simpler parameterized cloud models used in climate models or retrieval models. Here, we combine microphysical cloud modeling and 1D climate modeling to incorporate insights from microphysical models into a self-consistent, parameterized cloud model. Using the 1D Community Aerosol and Radiation Model for Atmospheres (CARMA), we generate microphysical water clouds and compare their properties with those from the widely used EddySed cloud model for a grid of Y dwarfs. We find that the mass of water condensate in our CARMA water clouds is significantly limited by available condensation nuclei; in models without additional seed particles for clouds added, the atmosphere becomes supersaturated. We incorporate water latent heat release in the convective and radiative parts of the atmosphere and find no significant impact on water-ice cloud formation for typical gas giant compositions. Our analysis reveals the CARMA cloud profiles have a gradual decrease in opacity of approximately 4% per bar below the cloud base. Incorporating this gradual cloud base falloff and a variable f sed parameter allows spectra generated from the parameterized Eddysed model to better match those of the microphysical CARMA model. This work provides recommendations for efficiently generating microphysically informed water clouds for future models of cold substellar objects with H/He atmospheres.

Gemini Near-infrared Photometry for a Sample of Cold Brown Dwarfs Discovered by Backyard Worlds: Planet 9

We present Gemini Observatory follow-up J-band and K-band photometry for a sample of 16 T/Y dwarf candidates discovered by the Backyard Worlds: Planet 9 citizen science project. The Gemini observations were taken with Gemini-North using the Near-Infrared Imager instrument between 2017 September and 2018 March. Obtaining near-infrared photometry of very cold brown dwarfs is important for enabling and prioritizing future spectroscopic follow-up, particularly in the context of JWST spectroscopy of T and Y dwarfs.

James Webb Space Telescope Spectra of Cold Brown Dwarfs are Well-reproduced by Phosphine-free, Diabatic, ATMO2020++ Models

James Webb Space Telescope (JWST) mid-infrared spectra of cold brown dwarfs (BDs) are becoming available. In Leggett & Tremblin we demonstrated that an ATMO2020++ synthetic spectrum provided an excellent fit to the first published spectrum. Here we show that these models can reproduce the 1–14 μm spectra of BDs with a range of effective temperature: 350 ≤ T eff K ≤ 500. A grid of these models, which include disequilibrium chemistry and non-adiabatic pressure–temperature profiles, is available. In addition, a number of models without phosphine were calculated, as PH3 is absent in the JWST data. Two of the six BDs studied are particularly interesting; one appears to be a young low-mass BD, the other is best fit as a binary. More complete analyses will be possible with an extended model grid, and with additional JWST data. The ATMO2020++ models are recommended for analyses of all BD data.

The First Y Dwarf Data from JWST Show that Dynamic and Diabatic Processes Regulate Cold Brown Dwarf Atmospheres

The James Webb Space Telescope (JWST) is now observing Y dwarfs, the coldest known brown dwarfs, with effective temperatures T eff ≲ 475 K. The first published observations provide important information: not only is the atmospheric chemistry out of equilibrium, as previously known, but the pressure–temperature profile is not in the standard adiabatic form. The rapid rotation of these Jupiter-size, isolated, brown dwarfs dominates the atmospheric dynamics, and thermal and compositional changes disrupt convection. These processes produce a colder lower atmosphere, and a warmer upper atmosphere, compared to a standard adiabatic profile. Leggett et al. presented empirical models where the pressure–temperature profile was adjusted so that synthetic spectra reproduced the 1 ≲ λ (μm) ≲ 20 spectral energy distributions of brown dwarfs with 260 ≤ T eff (K) ≤ 540. We show that spectra generated by these models fit the first JWST Y dwarf spectrum better than standard-adiabat models. Unexpectedly, there is no 4.3 μm PH3 feature in the JWST spectrum and atmospheres without phosphorus better reproduce the 4 μm flux peak. Our analysis of new JWST photometry indicates that the recently discovered faint secondary of the WISE J033605.05-014350AB system has T eff ≈ 295 K, making it the first dwarf in the significant luminosity gap between the 260 K WISE J085510.83-071442.5, and all other known Y dwarfs. The adiabat-adjusted disequilibrium-chemistry models are recommended for analyses of all brown dwarfs cooler than 600 K, and a grid is publicly available. Photometric color transformations are provided in an appendix.

Exploring the Extremes: Characterizing a New Population of Old and Cold Brown Dwarfs

Mapping out the populations of thick disk and halo brown dwarfs is important for understanding the metallicity dependence of low-temperature atmospheres and the substellar mass function. Recently, a new population of cold and metal-poor brown dwarfs has been discovered, with T eff ≲ 1400 K and metallicity ≲−1 dex. This population includes what may be the first known “extreme T-type subdwarfs” and possibly the first Y-type subdwarf, WISEA J153429.75−104303.3. We have conducted a Gemini YJHK/Ks photometric follow-up campaign targeting potentially metal-poor T and Y dwarfs, utilizing the GNIRS and Flamingos-2 instruments. We present 14 near-infrared photometric detections of eight unique targets: six T subdwarf candidates, one moderately metal-poor Y dwarf candidate, and one Y subdwarf candidate. We have obtained the first-ever ground-based detection of the highly anomalous object WISEA J153429.75−104303.3. The F110W − J color of WISEA J153429.75−104303.3 is significantly bluer than that of other late T and Y dwarfs, indicating that WISEA J153429.75−104303.3 has an unusual spectrum in the 0.9–1.4 μm wavelength range which encompasses the J-band peak. Our J-band detection of WISEA J153429.75−104303.3 and corresponding model comparisons suggest a subsolar metallicity and temperature of 400–550 K for this object. JWST spectroscopic follow-up at near-infrared and mid-infrared wavelengths would allow us to better understand the spectral peculiarities of WISEA J153429.75−104303.3, assess its physical properties, and conclusively determine whether or not it is the first Y-type subdwarf.

Early Results from GLASS-JWST. XIII. A Faint, Distant, and Cold Brown Dwarf* *Based on data collected with JWST.

We present the serendipitous discovery of a late T-type brown dwarf candidate in JWST NIRCam observations of the Early Release Science Abell 2744 parallel field. The discovery was enabled by the sensitivity of JWST at 4 μm wavelengths and the panchromatic 0.9–4.5 μm coverage of the spectral energy distribution. The unresolved point source has magnitudes F115W = 27.95 ± 0.15 and F444W = 25.84 ± 0.01 (AB), and its F115W−F444W and F356W−F444W colors match those expected for other known T dwarfs. We can exclude it as a reddened background star, high redshift quasar, or a very high redshift galaxy. Comparison with stellar atmospheric models indicates a temperature of T eff ≈ 650 K and surface gravity , implying a mass of 0.03 M ⊙ and age of 5 Gyr. We estimate the distance of this candidate to be 570–720 pc in a direction perpendicular to the Galactic plane, making it a likely thick disk or halo brown dwarf. These observations underscore the power of JWST to probe the very low-mass end of the substellar mass function in the Galactic thick disk and halo.

Keck NIRES Spectral Standards for L, T, and Y Dwarfs

We present medium-resolution (λ/Δλ = 2700), near-infrared spectral standards for field L0–L2, L4, and L7–Y0 dwarfs obtained with the Near-Infrared Echellette Spectrometer on the Keck II 10 m telescope. These standards allow for detailed spectral comparative analysis of cold brown dwarfs discovered through ongoing ground-based projects such as Backyard Worlds: Planet 9, and forthcoming space-based spectral surveys such as the James Webb Space Telescope, SPHEREx, Euclid, and the Nancy Grace Roman Space Telescope.

A 1.46–2.48 μm spectroscopic atlas of a T6 dwarf (1060 K) atmosphere with IGRINS: first detections of H2S and H2, and verification of H2O, CH4, and NH3 line lists

ABSTRACT We present Gemini South/IGRINS observations of the 1060 K T6 dwarf 2MASS J08173001−6155158 with unprecedented resolution ($R\equiv \lambda /\Delta \lambda =45\, 000$) and signal-to-noise ratio (S/N > 200) for a late-type T dwarf. We use this benchmark observation to test the reliability of molecular line lists used up-to-date atmospheric models. We determine which spectroscopic regions should be used to estimate the parameters of cold brown dwarfs and, by extension, exoplanets. We present a detailed spectroscopic atlas with molecular identifications across the H and K bands of the near-infrared. We find that water (H2O) line lists are overall reliable. We find the most discrepancies amongst older methane (CH4) line lists, and that the most up-to-date CH4 line lists correct many of these issues. We identify individual ammonia (NH3) lines, a hydrogen sulfide (H2S) feature at 1.5900 $\mu$m, and a molecular hydrogen (H2) feature at 2.1218 $\mu$m. These are the first unambiguous detections of H2S and H2 absorption features in an extra-solar atmosphere. With the H2 detection, we place an upper limit on the atmospheric dust concentration of this T6 dwarf: at least 500 times less than the interstellar value, implying that the atmosphere is effectively dust-free. We additionally identify several features that do not appear in the model spectra. Our assessment of the line lists is valuable for atmospheric model applications to high-dispersion, low-S/N, high-background spectra, such as an exoplanet around a star. We demonstrate a significant enhancement in the detection of the CH4 absorption signal in this T6 dwarf with the most up-to-date line lists.

Measuring and Replicating the 1–20 μm Energy Distributions of the Coldest Brown Dwarfs: Rotating, Turbulent, and Nonadiabatic Atmospheres

Abstract Cold, low-mass, field brown dwarfs are important for constraining the terminus of the stellar mass function, and also for optimizing atmospheric studies of exoplanets. In 2020 new model grids for such objects were made available: Sonora-Bobcat and ATMO 2020. Also, new candidate cold brown dwarfs were announced, and new spectroscopic observations at λ ≈ 4.8 μm were published. In this paper we present new infrared photometry for some of the coldest brown dwarfs, and put the new data and models together to explore the properties of these objects. We reconfirm the importance of mixing in these atmospheres, which leads to CO and NH3 abundances that differ by orders of magnitude from chemical equilibrium values. We also demonstrate that the new models retain the known factor ≳3 discrepancy with observations at 2 ≲ λ μm ≲ 4, for brown dwarfs cooler than 600 K. We show that the entire 1 ≲ λ μm ≲ 20 energy distribution of six brown dwarfs with 260 ≤ T eff K ≤ 475 can be well reproduced, for the first time, by model atmospheres which include disequilibrium chemistry as well as a photospheric temperature gradient which deviates from the standard radiative/convective equilibrium value. This change to the pressure–temperature profile is not unexpected for rotating and turbulent atmospheres that are subject to diabatic processes. A limited grid of modified-adiabat model colors is generated, and used to estimate temperatures and metallicities for the currently known Y dwarfs. A compilation of the photometric data used here is given in Appendix C.

Spitzer Follow-up of Extremely Cold Brown Dwarfs Discovered by the Backyard Worlds: Planet 9 Citizen Science Project

Abstract We present Spitzer follow-up imaging of 95 candidate extremely cold brown dwarfs discovered by the Backyard Worlds: Planet 9 citizen science project, which uses visually perceived motion in multiepoch Wide-field Infrared Survey Explorer (WISE) images to identify previously unrecognized substellar neighbors to the Sun. We measure Spitzer [3.6]–[4.5] color to phototype our brown dwarf candidates, with an emphasis on pinpointing the coldest and closest Y dwarfs within our sample. The combination of WISE and Spitzer astrometry provides quantitative confirmation of the transverse motion of 75 of our discoveries. Nine of our motion-confirmed objects have best-fit linear motions larger than 1″ yr−1; our fastest-moving discovery is WISEA J155349.96+693355.2 (μ ≈ 2.″15 yr−1), a possible T-type subdwarf. We also report a newly discovered wide-separation (∼400 au) T8 comoving companion to the white dwarf LSPM J0055+5948 (the fourth such system to be found), plus a candidate late T companion to the white dwarf LSR J0002+6357 at 5.′5 projected separation (∼8700 au if associated). Among our motion-confirmed targets, five have Spitzer colors most consistent with spectral type Y. Four of these five have exceptionally red Spitzer colors suggesting types of Y1 or later, adding considerably to the small sample of known objects in this especially valuable low-temperature regime. Our Y dwarf candidates begin bridging the gap between the bulk of the Y dwarf population and the coldest known brown dwarf.

Improved Infrared Photometry and a Preliminary Parallax Measurement for the Extremely Cold Brown Dwarf CWISEP J144606.62-231717.8

Abstract We present follow-up Spitzer observations at 3.6 μm (ch1) and 4.5 μm (ch2) of CWISEP J144606.62–231717.8, one of the coldest known brown dwarfs in the solar neighborhood. This object was found by mining the Wide-field Infrared Survey Explorer (WISE) and NEOWISE data via the CatWISE Preliminary Catalog by Meisner et al., where an initial Spitzer color of ch1–ch2 = 3.71 ± 0.44 mag was reported, implying it could be one of the reddest, and hence coldest, known brown dwarfs. Additional Spitzer data presented here allows us to revise its color to ch1–ch2 = 2.986 ± 0.048 mag, which makes CWISEP J144606.62–231717.8 the fifth reddest brown dwarf ever observed. A preliminary trigonometric parallax measurement, based on a combination of WISE and Spitzer astrometry, places this object at a distance of pc. Based on our improved Spitzer color and preliminary parallax, CWISEP J144606.62–231717.8 has a T eff in the 310–360 K range. Assuming an age of 0.5–13 Gyr, this corresponds to a mass between 2 and 20 M Jup.

WISE J072003.20-084651.2B is a Massive T Dwarf∗ †

Abstract We present individual dynamical masses for the nearby M9.5+T5.5 binary WISE J072003.20−084651.2AB, a.k.a. Scholz’s star. Combining high-precision Canada–France–Hawaii Telescope/WIRCam photocenter astrometry and Keck adaptive optics resolved imaging, we measure the first high-quality parallactic distance ( pc) and orbit ( yr period) for this system composed of a low-mass star and brown dwarf. We find a moderately eccentric orbit ( ), incompatible with previous work based on less data, and dynamical masses of 99 ± 6 and 66 ± 4 for the two components. The primary mass is marginally inconsistent (2.1σ) with the empirical mass–magnitude–metallicity relation and models of main-sequence stars. The relatively high mass of the cold ( K) brown dwarf companion indicates an age older than a few gigayears, in accord with age estimates for the primary star, and is consistent with our recent estimate of ≈70 for the stellar/substellar boundary among the field population. Our improved parallax and proper motion, as well as an orbit-corrected system velocity, improve the accuracy of the system’s close encounter with the solar system by an order of magnitude. WISE J0720−0846AB passed within 68.7 ± 2.0 kau of the Sun 80.5 ± 0.7 kyr ago, passing through the outer Oort cloud where comets can have stable orbits.

Precise Dynamical Masses of Directly Imaged Companions from Relative Astrometry, Radial Velocities, and Hipparcos–Gaia DR2 Accelerations

We measure dynamical masses for five objects--three ultracool dwarfs, one low-mass star, and one white dwarf--by fitting orbits to a combination of the Hipparcos-Gaia Catalog of Accelerations, literature radial velocities, and relative astrometry. Our approach provides precise masses without any assumptions about the primary star, even though the observations typically cover only a small fraction of an orbit. We also perform a uniform re-analysis of the host stars' ages. Two of our objects, HD 4747B and HR 7672B, already have precise dynamical masses near the stellar/substellar boundary and are used to validate our approach. For Gl 758B, we obtain a mass of $m=38.1_{-1.5}^{+1.7}$ $M_{Jup}$, the most precise mass measurement of this companion to date. Gl 758B is the coldest brown dwarf with a dynamical mass, and the combination of our low mass and slightly older host-star age resolves its previously noted discrepancy with substellar evolutionary models. HD 68017B, a late-M dwarf, has a mass of $m=0.147\pm 0.003$ $M_\odot$, consistent with stellar theory and previous empirical estimates based on its absolute magnitude. The progenitor of the white dwarf Gl 86B has been debated in the literature, and our dynamical measurement of $m=0.595 \pm 0.010$ $M_\odot$ is consistent with a higher progenitor mass and younger age for this planet-hosting binary system. Overall, these case studies represent only five of the thousands of accelerating systems identified by combining Hipparcos and Gaia. Our analysis could be repeated for many of them to build a large sample of companions with dynamical masses.

CWISEP J193518.59–154620.3: An Extremely Cold Brown Dwarf in the Solar Neighborhood Discovered with CatWISE

Abstract We present the discovery of an extremely cold, nearby brown dwarf in the solar neighborhood, found in the CatWISE catalog. Photometric follow-up with Spitzer reveals that the object, CWISEP J193518.59–154620.3, has ch1–ch2 = 3.24 ± 0.31 mag, making it one of the reddest brown dwarfs known. Using the Spitzer photometry and the polynomial relations from Kirkpatrick et al. we estimate an effective temperature in the ∼270–360 K range, and a distance estimate in the 5.6–10.9 pc range. We combined the WISE, NEOWISE, and Spitzer data to measure a proper motion of mas yr−1, μ δ = −50 ± 97 mas yr−1, which implies a relatively low tangential velocity in the range 7–22 km s−1.

Y Dwarf Trigonometric Parallaxes from the Spitzer Space Telescope

Abstract Y dwarfs provide a unique opportunity to study free-floating objects with masses <30 M Jup and atmospheric temperatures approaching those of known Jupiter-like exoplanets. Obtaining distances to these objects is an essential step toward characterizing their absolute physical properties. Using Spitzer’s Infrared Array Camera (IRAC) [4.5] images taken over baselines of ∼2–7 years, we measure astrometric distances for 22 late-T and early Y dwarfs, including updated parallaxes for 18 objects and new parallax measurements for 4 objects. These parallaxes will make it possible to explore the physical parameter space occupied by the coldest brown dwarfs. We also present the discovery of six new late-T dwarfs, updated spectra of two T dwarfs, and the reclassification of a new Y dwarf, WISE J033605.04−014351.0, based on Keck/NIRSPEC J-band spectroscopy. Assuming that effective temperatures are inversely proportional to absolute magnitude, we examine trends in the evolution of the spectral energy distributions of brown dwarfs with decreasing effective temperature. Surprisingly, the Y dwarf class encompasses a large range in absolute magnitude in the near- to mid-infrared photometric bandpasses, demonstrating a larger range of effective temperatures than previously assumed. This sample will be ideal for obtaining mid-infrared spectra with the James Webb Space Telescope because their known distances will make it easier to measure absolute physical properties.

Direct imaging of an ultracool substellar companion to the exoplanet host star HD 4113 A

Using high-contrast imaging with the SPHERE instrument at the Very Large Telescope (VLT), we report the first images of a cold brown dwarf companion to the exoplanet host star HD 4113A. The brown dwarf HD 4113C is part of a complex dynamical system consisting of a giant planet, a stellar host, and a known wide M-dwarf companion. Its separation of 535 ± 3 mas and H-band contrast of 13.35 ± 0.10 mag correspond to a projected separation of 22 AU and an isochronal mass estimate of 36 ± 5 MJ based on COND models. The companion shows strong methane absorption, and through fitting an atmosphere model, we estimate a surface gravity of logg = 5 and an effective temperature of ~500–600 K. A comparison of its spectrum with observed T dwarfs indicates a late-T spectral type, with a T9 object providing the best match. By combining the observed astrometry from the imaging data with 27 years of radial velocities, we use orbital fitting to constrain its orbital and physical parameters, as well as update those of the planet HD 4113A b, discovered by previous radial velocity measurements. The data suggest a dynamical mass of 66−4+5 MJ and moderate eccentricity of 0.44−0.07+0.08 for the brown dwarf. This mass estimate appears to contradict the isochronal estimate and that of objects with similar temperatures, which may be caused by the newly detected object being an unresolved binary brown dwarf system or the presence of an additional object in the system. Through dynamical simulations, we show that the planet may undergo strong Lidov-Kozai cycles, raising the possibility that it formed on a quasi-circular orbit and gained its currently observed high eccentricity (e ~ 0.9) through interactions with the brown dwarf. Follow-up observations combining radial velocities, direct imaging, and Gaia astrometry will be crucial to precisely constrain the dynamical mass of the brown dwarf and allow for an in-depth comparison with evolutionary and atmosphere models.

An L Band Spectrum of the Coldest Brown Dwarf

Abstract The coldest brown dwarf, WISE 0855, is the closest known planetary-mass, free-floating object and has a temperature nearly as cold as the solar system gas giants. Like Jupiter, it is predicted to have an atmosphere rich in methane, water, and ammonia, with clouds of volatile ices. WISE 0855 is faint at near-infrared wavelengths and emits almost all its energy in the mid-infrared. Skemer et al. presented a spectrum of WISE 0855 from 4.5–5.1 μm (M band), revealing water vapor features. Here, we present a spectrum of WISE 0855 in the L band, from 3.4–4.14 μm. We present a set of atmosphere models that include a range of compositions (metallicities and C/O ratios) and water ice clouds. Methane absorption is clearly present in the spectrum. The mid-infrared color can be better matched with a methane abundance that is depleted relative to solar abundance. We find that there is evidence for water ice clouds in the M band spectrum, and we find a lack of phosphine spectral features in both the L and M band spectra. We suggest that a deep continuum opacity source may be obscuring the near-infrared flux, possibly a deep phosphorous-bearing cloud, ammonium dihyrogen phosphate. Observations of WISE 0855 provide critical constraints for cold planetary atmospheres, bridging the temperature range between the long-studied solar system planets and accessible exoplanets. The James Webb Space Telescope will soon revolutionize our understanding of cold brown dwarfs with high-precision spectroscopy across the infrared, allowing us to study their compositions and cloud properties, and to infer their atmospheric dynamics and formation processes.

MEASURING HIGH-PRECISION ASTROMETRY WITH THE INFRARED ARRAY CAMERA ON THE SPITZER SPACE TELESCOPE*

ABSTRACT The Infrared Array Camera (IRAC) on the Spitzer Space Telescope currently offers the greatest potential for high-precision astrometry of faint mid-IR sources across arcminute-scale fields, which would be especially valuable for measuring parallaxes of cold brown dwarfs in the solar neighborhood and proper motions of obscured members of nearby star-forming regions. To more fully realize IRAC's astrometric capabilities, we have sought to minimize the largest sources of uncertainty in astrometry with its 3.6 and 4.5 μm bands. By comparing different routines that estimate stellar positions, we have found that Point Response Function (PRF) fitting with the Spitzer Science Center's Astronomical Point Source Extractor produces both the smallest systematic errors from varying intra-pixel sensitivity and the greatest precision in measurements of positions. In addition, self-calibration has been used to derive new 7th and 8th order distortion corrections for the 3.6 and 4.5 μm arrays of IRAC, respectively. These corrections are suitable for data throughout the mission of Spitzer when a time-dependent scale factor is applied to the corrections. To illustrate the astrometric accuracy that can be achieved by combining PRF fitting with our new distortion corrections, we have applied them to archival data for a nearby star-forming region, arriving at total astrometric errors of ∼20 and 70 mas at signal to noise ratios of 100 and 10, respectively.

Distances, Luminosities, and Temperatures of the Coldest Known Substellar Objects

The coolest known brown dwarfs are our best analogs to extrasolar gas-giant planets. The prolific detections of such cold substellar objects in the past 2 years have spurred intensive follow-up, but the lack of accurate distances is a key gap in our understanding. We present a large sample of precise distances based on homogeneous mid-infrared astrometry that robustly establishes absolute fluxes, luminosities, and temperatures. The coolest brown dwarfs have temperatures of 400 to 450 kelvin and masses almost equal to 5 to 20 times that of Jupiter, showing they bridge the gap between hotter brown dwarfs and gas-giant planets. At these extremes, spectral energy distributions no longer follow a simple correspondence with temperature, suggesting an increasing role of other physical parameters, such as surface gravity, vertical mixing, clouds, and metallicity.