Mapping Atmospheric Features of the Planetary-mass Brown Dwarf SIMP 0136 with JWST NIRISS

Observations of extrasolar planets were not projected to be a substantial part of the Spitzer Space Telescope’s mission when it was conceived and designed. Nevertheless, Spitzer was the first facility to detect thermal emission from a hot Jupiter-sized planet, and the range of its exoplanetary investigations grew to encompass transiting planets, microlensing, brown dwarfs, and direct imaging searches and astrometry. Spitzer used phase curves to measure the longitudinal distribution of heat as well as time-dependent heating on hot Jupiters. Its secondary eclipse observations strongly constrained the dayside thermal emission spectra and corresponding atmospheric compositions of hot Jupiters, and the timings of eclipses were used for studies of orbital dynamics. Spitzer’s sensitivity to carbon-based molecules such as methane and carbon monoxide was key to atmospheric composition studies of transiting exoplanets as well as imaging spectroscopy of brown dwarfs, and complemented Hubble Space Telescope spectroscopy at shorter wavelengths. Its capability for long continuous observing sequences enabled searches for new transiting planets around cool stars and helped to define the architectures of planetary systems such as TRAPPIST-1. Spitzer measured masses for small planets at large orbital distances using microlensing parallax. Spitzer observations of brown dwarfs probed their temperatures, masses and weather patterns. Imaging and astrometry from Spitzer was used to discover new planetary-mass brown dwarfs and to measure distances and space densities of many others. The Spitzer Space Telescope launched when the study of exoplanets was in its infancy, and yet it was remarkably successful in characterizing both exoplanet and brown dwarf systems through their mid-infrared emissions. This Review collates the highlights of Spitzer-based research in these fields.

We present the first brown dwarf spectral binary characterized with JWST: WISE J014656.66+423410.0, the coldest blended-light brown dwarf binary straddling the T/Y transition. We obtained a moderate resolution (R ∼ 2700) G395H spectrum of this unresolved binary with JWST/NIRSpec and we fit it to late T and Y dwarf spectra from JWST/NIRSpec, and model spectra of comparable temperatures, both as individual spectra and pairs mimicking an unresolved binary system. We find that this tightly separated binary is likely composed of two unequal-brightness sources with a magnitude difference of 0.50 ± 0.08 mag in IRAC [4.5] and a secondary 1.01 ± 0.13 mag redder than the primary in [3.6]–[4.5]. Despite the large color difference between the best-fit primary and secondary, their temperature difference is only 92 ± 23 K, a feature reminiscing of the L/T transition. Carbon disequilibrium chemistry strongly shapes the mid-infrared spectra of these sources, as a complex function of the metallicity and surface gravity. While a larger library of JWST/NIRSpec spectra is needed to conclusively examine the peculiarities of blended-light sources, this spectral binary is a crucial pathfinder to both understand the spectral features of planetary-mass atmospheres and detect binarity in unresolved, moderate-resolution spectra of the coldest brown dwarfs.

ABSTRACTObservations of time-varying thermal emission from brown dwarfs suggest that they have large-scale atmospheric circulation. The magnitude of this variability ranges from a few per cent to tens of per cent, implying a range of sizes of atmospheric perturbations. Periodograms of phase curves of the thermal emission reveal a range of peaks with different periods and widths, suggesting different atmospheric flow speeds and directions. This implies a variety of atmospheric circulations in the different brown dwarfs observed to date, but there is no general theoretical understanding of the circulation regimes these objects can support, and the resulting sizes and velocities of their atmospheric features. We therefore use an idealized 2D shallow-water model of a brown dwarf atmosphere to understand their potential large-scale circulation regimes. We non-dimensionalize the model to reduce the number of input parameters to two non-dimensional numbers: the thermal Rossby number and the non-dimensional radiative time-scale. This allows us to define a parameter space that bounds the entire range of brown dwarf behaviour possible in our model. We analyse the resulting height, velocity, and potential vorticity fields in this parameter space, and simulate observed phase curve and periodograms for comparison with real observations. We use our results to qualitatively define four circulation regimes, which we hope will be useful for interpreting observations and for guiding simulations with more detailed physical models.

The current direct observations of brown dwarfs and exoplanets have been obtained using instruments not specifically designed for overcoming the large contrast ratio between the host star and any wide-separation faint companions. However, we are about to witness the birth of several new dedicated observing platforms specifically geared towards high contrast imaging of these objects. The Gemini Planet Imager, VLT-SPHERE, Subaru HiCIAO, and Project 1640 at the Palomar 5m telescope will return images of numerous exoplanets and brown dwarfs over hundreds of observing nights in the next five years. Along with diffraction-limited coronagraphs and high-order adaptive optics, these instruments also will return spectral and polarimetric information on any discovered targets, giving clues to their atmospheric compositions and characteristics. Such spectral characterization will be key to forming a detailed theory of comparative exoplanetary science which will be widely applicable to both exoplanets and brown dwarfs. Further, the prevalence of aperture masking interferometry in the field of high contrast imaging is also allowing observers to sense massive, young planets at solar system scales (~3–30 AU)— separations out of reach to conventional direct imaging techniques. Such observations can provide snapshots at the earliest phases of planet formation—information essential for constraining formation mechanisms as well as evolutionary models of planetary mass companions. As a demonstration of the power of this technique, I briefly review recent aperture masking observations of the HR 8799 system. Moreover, all of the aforementioned techniques are already extremely adept at detecting low-mass stellar companions to their target stars, and I present some recent highlights.

Brown dwarfs and planetary-mass companions display rotationally modulated photometric variability, especially those near the L/T transition. This variability is commonly attributed to top-of-atmosphere (TOA) inhomogeneities, with proposed models including patchy thick and thin clouds, planetary-scale jets, or chemical disequilibrium. Surface mapping techniques are powerful tools to probe their atmospheric structures and distinguish between models. One of the most successful methods for stellar surface mapping is Doppler imaging, where the existence of TOA inhomogeneities can be inferred from their varying Doppler shifts across the face of a rotating star. We applied Doppler imaging to the nearest brown dwarf binary WISE 1049AB (also known as Luhman 16AB) using time-resolved, high-resolution spectroscopic observations from Gemini IGRINS, and obtained for the first time H- and K-band simultaneous global weather map for brown dwarfs. Compared to the only previous Doppler map for a brown dwarf in 2014 featuring a predominant mid-latitude cold spot on WISE 1049B and no feature on WISE 1049A, our observations detected persistent spot-like structures on WISE 1049B in the equatorial to mid-latitude regions on two nights, and revealed new polar spots on WISE 1049A. Our results suggest stability of atmospheric features over time-scale of days and possible long-term stable or recurring structures. H- and K-band maps displayed similar structures in and out of CO bands, indicating the cold spots not solely due to chemical hotspots but must involve clouds. Upcoming 30-m extremely large telescopes will enable more sensitive Doppler imaging of dozens of brown dwarfs and even a small number of directly imaged exoplanets.

A planetary atmosphere is the outer gas layer of a planet. Besides its scientific significance among the first and most accessible planetary layers observed from space, it is closely connected with planetary formation and evolution, surface and interior processes, and habitability of planets. Current theories of planetary atmospheres were primarily obtained through the studies of eight large planets, Pluto and three large moons (Io, Titan, and Triton) in the Solar System. Outside the Solar System, more than four thousand extrasolar planets (exoplanets) and two thousand brown dwarfs have been confirmed in our Galaxy, and their population is rapidly growing. The rich information from these exotic bodies offers a database to test, in a statistical sense, the fundamental theories of planetary climates. Here we review the current knowledge on atmospheres of exoplanets and brown dwarfs from recent observations and theories. This review highlights important regimes and statistical trends in an ensemble of atmospheres as an initial step towards fully characterizing diverse substellar atmospheres, that illustrates the underlying principles and critical problems. Insights are obtained through analysis of the dependence of atmospheric characteristics on basic planetary parameters. Dominant processes that influence atmospheric stability, energy transport, temperature, composition and flow pattern are discussed and elaborated with simple scaling laws. We dedicate this review to Dr. Adam P. Showman (1968–2020) in recognition of his fundamental contribution to the understanding of atmospheric dynamics on giant planets, exoplanets and brown dwarfs.

We have performed a search for planetary-mass brown dwarfs in the Chamaeleon I star-forming region using proper motions and photometry measured from optical and infrared images from the Spitzer Space Telescope, the Hubble Space Telescope, and ground-based facilities. Through near-IR spectroscopy at Gemini Observatory, we have confirmed six of the candidates as new late-type members of Chamaeleon I (≥M8). One of these objects, Cha J11110675−7636030, has the faintest extinction-corrected M K among known members, which corresponds to a mass of 3–6 according to evolutionary models. That object and two other new members have redder mid-IR colors than young photospheres at ≤M9.5, which may indicate the presence of disks. However, since those objects may be later than M9.5 and the mid-IR colors of young photospheres are ill-defined at those types, we cannot determine conclusively whether color excesses from disks are present. If Cha J11110675−7636030 does have a disk, it would be a contender for the least-massive known brown dwarf with a disk. Since the new brown dwarfs that we have found extend below our completeness limit of 6–10 M , deeper observations are needed to measure the minimum mass of the initial mass function in Chamaeleon I.

Using the Hubble Space Telescope, the 4 m Blanco Telescope at the Cerro Tololo Inter-American Observatory, and the Spitzer Space Telescope, we have performed deep imaging from 0.8 to 8 μm of the southern subcluster in the Chamaeleon I star-forming region. In these data, we have discovered an object, Cha 110913-773444, whose colors and magnitudes are indicative of a very low mass brown dwarf with a circumstellar disk. In a near-infrared spectrum of this source obtained with the Gemini Near-Infrared Spectrograph, the presence of strong steam absorption confirms its late-type nature (M9.5) while the shapes of the H- and K-band continua and the strengths of the Na I and K I lines demonstrate that it is a young, pre-main-sequence object rather than a field dwarf. A comparison of the bolometric luminosity of Cha 110913-773444 to the luminosities predicted by the evolutionary models of Chabrier & Baraffe and Burrows and coworkers indicates a mass of 8MJ, placing it fully within the mass range observed for extrasolar planetary companions (M 15MJ). The spectral energy distribution of this object exhibits mid-infrared excess emission at λ > 5 μm, which we have successfully modeled in terms of an irradiated viscous accretion disk with 10-12 M☉ yr-1. Cha 110913-773444 is now the least massive brown dwarf observed to have a circumstellar disk, and indeed is one of the least massive free-floating objects found to date. These results demonstrate that the raw materials for planet formation exist around free-floating planetary-mass bodies.

We present first results from a major program of methane filter photometry for low-mass stars and brown dwarfs. The definition of a new methane filter photometric system is described. A recipe is provided for the differential calibration of methane imaging data using existing 2MASS photometry. We show that these filters are effective in discriminating T dwarfs from other types of stars, and demonstrate this with Anglo-Australian Telescope observations using the IRIS2 imager. Methane imaging data and proper motions are presented for ten T dwarfs identified as part of the 2MASS "Wide Field T Dwarf Search" -- seven of them initially identified as T dwarfs using methane imaging. We also present near-infrared moderate resolution spectra for five T dwarfs, newly discovered by this technique. Spectral types obtained from these spectra are compared to those derived from both our methane filter observations, and spectral types derived by other observers. Finally, we suggest a range of future programs to which these filters are clearly well suited: the winnowing of T dwarf and Y dwarf candidate objects coming from the next generation of near-infrared sky surveys; the robust detection of candidate planetary-mass brown dwarfs in clusters; the detection of T dwarf companions to known L and T dwarfs via deep methane imaging; and the search for rotationally-modulated time-variable surface features on cool brown dwarfs.

We present a comprehensive analysis of a Transiting Exoplanet Survey Satellite (TESS) high-quality light curve for a young brown dwarf, MHO 4 having spectral type M7.0, in the Taurus star-forming region. We investigate the rotation periods and characterize the brown dwarf’s dynamic atmosphere and surface features. Our light curve analysis of MHO 4 reveals a rotation period of approximately 2.224 days. Remarkably, MHO 4 exhibits two significant flaring events. Furthermore, we estimate the bolometric flare energies to be within the energy range of 1034 to 1035 erg, classifying them as superflares.

We present a comprehensive analysis of TESS high-quality light curves from sectors 43 and 44 of a few samples of young (∼2–3 Myr) brown dwarfs in the Taurus molecular cloud. They are well characterized and bona fide members of Taurus. We aim to search for the fast rotations of brown dwarfs and to picturize their dynamic atmosphere and surface features. Out of 11 young BDs, we found that 72% are periodic, in the period range of 1–7 days; among them, three BDs have periods <1.5 day and the period of one object is estimated for the first time. The sinusoidal periodic variations are related to a large spot or group of small spots corotating with the objects. Interestingly, we have detected four flare events in three young BDs, with one object, MHO 4, showing two flares in two different sectors. From the flared light curves, we have estimated the total bolometric flared energy in a range of 1035–1036 erg, which is close to the superflare energy range (≥1034 erg). To produce such kinds of superflare events, we have calculated the required magnetic field strength, which comes out at the order of a few kilogauss. Such superflares have a strong effect on the habitability of planets around M dwarfs.

We present near-infrared photometry (J band) and low-resolution optical spectroscopy (600-1000 nm) for one of the faintest substellar member candidates in the young σ Orionis cluster, S Ori 47 (I = 20.53; Bejar, Zapatero Osorio, & Rebolo). Its very red (I-J) = 3.3 ± 0.1 color and its optical spectrum allow us to classify S Ori 47 as an L1.5-type object that fits the low-luminosity end of the cluster photometric and spectroscopic sequences. It also displays atmospheric features indicative of low gravity, such as weak alkaline lines and hydride and oxide bands, consistent with the expectation for a very young object still undergoing gravitational collapse. Our data lead us to conclude that S Ori 47 is a true substellar member of the σ Orionis cluster. Additionally, we present the detection of Li I in its atmosphere, which provides an independent confirmation of youth and substellarity. Using current theoretical evolutionary tracks and adopting an age interval of 1-5 Myr for the σ Orionis cluster, we estimate the mass of S Ori 47 to be 0.015 ± 0.005 M☉, i.e., at the minimum mass for deuterium burning, which has been proposed as a definition for the boundary between brown dwarfs and giant planets. S Ori 47 could well be the result of a natural extension of the process of cloud fragmentation down to the deuterium-burning mass limit; a less likely alternative is that it has originated from a protoplanetary disk around a more massive cluster member and was later ejected from its orbit because of interacting effects within this rather sparse (~12 objects pc-3) young cluster. The study of this object serves as a guide for future deep searches for free-floating objects with planetary masses.

Context. Young brown dwarfs exhibit atmospheric characteristics similar to those of super-Jupiters, providing a unique opportunity to study planetary atmospheres. Atmospheric retrievals of high-resolution spectra reveal detailed properties of these objects, with elemental and isotopic ratios offering insights into their formation history. The ESO SupJup Survey, utilising CRIRES+ on the Very Large Telescope, aims to assess the role of 12C/13C as a formation tracer. Aims. We present observations of three young brown dwarfs: 2MASS J12003792-7845082, TWA 28, and 2MASS J08561384-1342242. Our goal is to constrain their chemical compositions, thermal profiles, surface gravities, spin rotations, and 12C/13C. Methods. We conducted atmospheric retrievals of CRIRES+ K-band spectra, coupling the radiative transfer code petitRADTRANS with the Bayesian inference algorithm MultiNest. Results. The retrievals provide a detailed characterisation of the atmospheres of the three objects. We report the volume mixing ratios of the main molecular and atomic species: H216O,12CO, HF, Na, Ca, and Ti, including the novel detection of hydrogen fluoride (HF) in the atmosphere of a brown dwarf. We determine 12C/13C values of 81−19+28 and 79−14+20 in the atmospheres of TWA 28 and J0856, respectively, with strong significance (&gt;3σ). We also report tentative evidence (~2σ) of 13CO in J1200, at 12C/13C = 114−33+69. Additionally, we detect H218O at moderate significance in J0856 (3.3σ) and TWA 28 (2.1σ). The retrieved thermal profiles are consistent with hot atmospheres (2300–2600 K) with low surface gravities and slow spins, as expected for young objects. Conclusions. The measured carbon isotope ratios are consistent among the three objects and show no significant deviation from that of the local interstellar medium, suggesting a fragmentation-based formation mechanism similar to star formation. The tentative detection of H218O in two objects of our sample highlights the potential of high-resolution spectroscopy to probe additional isotope ratios, such as 16O/18O, in the atmospheres of brown dwarfs and super-Jupiters.

Context. Linking atmospheric characteristics of planets to their formation pathways is a central theme in the study of extrasolar planets. Although the 12C/13C isotope ratio shows little variation in the Solar System, the atmosphere of a super-Jupiter was recently shown to be rich in 13CO, possibly as a result of dominant ice accretion beyond the CO snow line during its formation. Carbon isotope ratios are therefore suggested to be a potential tracer of formation pathways of planets. Aims. In this work, we aim to measure the 12CO/13CO isotopologue ratio of a young, isolated brown dwarf. While the general atmospheric characteristics of young, low-mass brown dwarfs are expected to be very similar to those of super-Jupiters, their formation pathways may be different, leading to distinct isotopologue ratios. In addition, such objects allow high-dispersion spectroscopy at high signal-to-noise ratios. Methods. We analysed archival K-band spectra of the L dwarf 2MASS J03552337+1133437 taken with NIRSPEC at the Keck telescope. A free retrieval analysis was applied to the data using the radiative transfer code petitRADTRANS coupled with the nested sampling tool PyMultiNest to determine the isotopologue ratio 12CO/13CO in its atmosphere. Results. The isotopologue 13CO is detected in the atmosphere through the cross-correlation method at a signal-to-noise of ~8.4. The detection significance is determined to be ~9.5σ using a Bayesian model comparison between two retrieval models (including or excluding 13CO). We retrieve an isotopologue 12CO/13CO ratio of 97−18+25 (90% uncertainty), marginally higher than the local interstellar standard. Its C/O ratio of ~0.56 is consistent with the solar value. Conclusions. Although only one super-Jupiter and one brown dwarf now have a measured 12CO/13CO ratio, it is intriguing that they are different, possibly hinting to distinct formation pathways. Regardless of spectroscopic similarities, isolated brown dwarfs may experience a top-down formation via gravitational collapse, which resembles star formation, while giant exoplanets favourably form through core accretion, which potentially alters isotopologue ratios in their atmospheres depending on the material they accrete from protoplanetary disks. This further emphasises atmospheric carbon isotopologue ratio as a tracer of the formation history of exoplanets. In the future, analyses such as those presented here should be conducted on a wide range of exoplanets using medium-to-high-resolution spectroscopy to further assess planet formation processes.

Searches for very low mass objects in young star clusters have uncovered evidence for free-floating objects with inferred masses possibly as low as 5 to 15 Jupiter masses (MJup), similar to the masses of several extrasolar planets. We show here that the process which forms single and multiple protostars, namely collapse and fragmentation of molecular clouds, might be able to produce self-gravitating objects with initial masses less than ˜ 1MJup. Models are calculated with a three dimensional, finite differences code which solves the equations of hydrodynamics, gravitation, and radiative transfer in the Eddington and diffusion approximations. Magnetic pressure is added to the gas pressure, magnetic tension is approximated by gravity dilution once collapse is well underway, and ambipolar diffusion is treated approximately as well. Initially oblate clouds fragment into multiple protostar systems containing a small number (of order four) of fragments. If such fragments can be ejected from an unstable quadruple protostar system, prior to gaining significantly more mass, protostellar collapse might then be able to explain the formation of free-floating objects with masses below 13MJup. These objects might then be best termed “sub-brown dwarfs”.