Giant Exoplanets Research Articles

An optimized survey strategy for the ERIS/NIX imager: searching for young giant exoplanets and very low mass brown dwarfs using the K-peak custom photometric filter

ABSTRACT We present optimal survey strategies for the upcoming NIX imager, part of the Enhanced Resolution Imager and Spectrograph instrument to be installed on the Very Large Telescope. We will use a custom 2.2 µm K-peak filter to optimize the efficiency of a future large-scale direct imaging survey, aiming to detect brown dwarfs and giant planets around nearby stars. We use the results of previous large-scale imaging surveys (primarily SPHERE SHINE and Gemini GPIES) to inform our choice of targets, as well as improved planet population distributions. We present four possible approaches to optimize survey target lists for the highest yield of detections: (i) targeting objects with anomalous proper motion trends, (ii) a follow-up survey of dense fields from SPHERE SHINE and Gemini GPIES, (iii) surveying nearby star-forming regions, and (iv) targeting newly discovered members of nearby young moving groups. We also compare the predicted performance of NIX to other state-of-the-art direct imaging instruments.

A Unified Spectroscopic and Photometric Model to Infer Surface Inhomogeneity: Application to Luhman 16B

Extremely large telescopes (ELTs) provide an opportunity to observe surface inhomogeneities for ultracool objects including M dwarfs, brown dwarfs (BDs), and gas giant planets via Doppler imaging and spectrophotometry techniques. These inhomogeneities can be caused by star spots, clouds, and vortices. Star spots and associated stellar flares play a significant role in habitability, either stifling life or catalyzing abiogenesis depending on the emission frequency, magnitude, and orientation. Clouds and vortices may be the source of spectral and photometric variability observed at the L/T transition of BDs and are expected in gas giant exoplanets. We develop a versatile analytical framework to model and infer surface inhomogeneities that can be applied to both spectroscopic and photometric data. This model is validated against a slew of numerical simulations. Using archival spectroscopic and photometric data, we infer starspot parameters (location, size, and contrast) and generate global surface maps for Luhman 16B (an early T dwarf and one of our solar system’s nearest neighbors at a distance of ≈2 pc). We confirm previous findings that Luhman 16B’s atmosphere is inhomogeneous with time-varying features. In addition, we provide tentative evidence of longer timescale atmospheric structures such as dark equatorial and bright midlatitude to polar spots. These findings are discussed in the context of atmospheric circulation and dynamics for ultracool dwarfs. Our analytical model will be valuable in assessing the feasibility of using ELTs to study surface inhomogeneities of gas giant exoplanets and other ultracool objects.

TOI-2119: a transiting brown dwarf orbiting an active M-dwarf from NASA’s TESS mission

ABSTRACT We report the discovery of TOI-2119b, a transiting brown dwarf (BD) that orbits and is completely eclipsed by an active M-dwarf star. Using light-curve data from the Transiting Exoplanet Survey Satellite mission and follow-up high-resolution Doppler spectroscopic observations, we find the BD has a radius of Rb = 1.08 ± 0.03RJ, a mass of Mb = 64.4 ± 2.3MJ, an orbital period of P = 7.200865 ± 0.00002 d, and an eccentricity of e = 0.337 ± 0.002. The host star has a mass of M⋆ = 0.53 ± 0.02M⊙, a radius of R⋆ = 0.50 ± 0.01R⊙, an effective temperature of Teff = 3621 ± 48K, and a metallicity of $\rm [Fe/H]=+0.06\pm 0.08$. TOI-2119b joins an emerging population of transiting BDs around M-dwarf host stars, with TOI-2119 being the ninth such system. These M-dwarf–brown dwarf systems typically occupy mass ratios near q = Mb/M⋆ ≈ 0.1−0.2, which separates them from the typical mass ratios for systems with transiting substellar objects and giant exoplanets that orbit more massive stars. The nature of the secondary eclipse of the BD by the star enables us to estimate the effective temperature of the substellar object to be 2030 ± 84K, which is consistent with predictions by substellar evolutionary models.

A survey of sodium absorption in 10 giant exoplanets with high-resolution transmission spectroscopy

ABSTRACT The alkali metal sodium (Na) is one of the most commonly detected chemical species in the upper atmospheres of giant exoplanets. In this work, we conducted a homogeneous survey of Na in a diverse sample of 10 highly irradiated giant exoplanets using high-resolution transmission spectroscopy. Our sample includes nine planets with previous Na detections and one new detection. We confirm previous detections and assess multiple approaches for deriving Na line properties from high-resolution transmission spectra. The homogeneously measured sodium line depths were used to constrain the atmospheric heights (HNa) with respect to the planetary radii (Rp). We assess an empirical trend describing the relative atmospheric height (HNa/Rp) as a function of planetary equilibrium temperature (Teq) and surface gravity (g), in which HNa/Rp decreases exponentially with ξ ∝ gTeq, approaching a constant at large ξ. We also report the sodium D2/D1 line ratios across our sample and find that seven targets have line ratios that are consistent with unity. Finally, we measured net blueshifted offsets of the sodium absorption lines from their rest frame wavelengths for all 10 planets, corresponding to day–night wind velocities of a few km s−1. This suggests that the broad sample of exoplanets share common underlying processes that govern atmospheric dynamics. Our study highlights a promising avenue for using high-resolution transmission spectroscopy to further our understanding of how atmospheric characteristics vary over a diverse sample of exoplanets.

Dust Accretion onto Exoplanets

Accretion of interplanetary dust onto gas giant exoplanets is considered. Poynting–Robertson drag causes dust particles from distant reservoirs to slowly inspiral toward the star. Orbital simulations for the three-body system of the star, planet, and dust particle show that a significant fraction of the dust may accrete onto massive planets in close orbits. The deceleration of the supersonic dust in the planet’s atmosphere is modeled, including ablation by thermal evaporation and sputtering. The fraction of the accreted dust mass deposited as gas-phase atoms is found to be large for close-in orbits and massive planets. If mass outflow and vertical mixing are sufficiently weak, the accreted dust produces a constant mixing ratio of atoms and remnant dust grains below the stopping layer. When vertical mixing is included along with settling, the solutions interpolate between the mixing ratio due to the meteoric source above the homopause, and that of the well-mixed deeper atmosphere below the homopause. The line opacity from atoms and continuum opacity from remnant dust may be observable in transmission spectra for sufficiently large dust accretion rates, a grain size distribution tilted toward the blowout size, and sufficiently weak vertical mixing. If mixing is strong, the meteoric source may still act to augment heavy elements mixed up from the deep atmosphere as well as provide nucleation sites for the formation of larger particles. The possible role of the Lorentz drag force in limiting the flow speeds and mixing coefficient for pressures ≲1 mbar is discussed.

The Refined Transit Ephemeris of TOI-2180 b

The Transiting Exoplanet Survey Satellite (TESS) observed a single transit of the giant exoplanet TOI-2180 b in Sector 19 of its primary mission. A subsequent effort to characterize this system found that the orbital period of TOI-2180 b is 260.8 ± 0.6 days. This level of precision precludes most efforts to detect future transits and to further understand this exoplanet. We present a new transit of TOI-2180 b observed by TESS in Sector 48 and update the planet’s transit ephemeris with its precisely measured orbital period. The refined ephemeris enables future transit observations to explore this planet’s atmosphere or to search for rings or moons.

The origin of the high metallicity of close-in giant exoplanets

Context. The composition of gas giant planets reflects their formation and evolution history. Revealing the origin of the high heavy-element masses in giant exoplanets is an objective of planet formation theories. Planetesimal accretion during the phase of planetary migration could lead to the delivery of heavy elements into gas giant planets. In our previous paper, we used dynamical simulations and showed that planetesimal accretion during planetary migration occurs in a rather narrow region of the protoplanetary disk, which we refer to as the “sweet spot” for accretion. Aims. Our understanding of the sweet spot, however, is still limited. The location of the sweet spot within the disk and how it changes as the disk evolves were not investigated in detail. The goal of this paper is to reveal the nature of the sweet spot using analytical calculations and to investigate the role of the sweet spot in determining the composition of gas giant planets. Methods. We analytically derived the required conditions for the sweet spot. Then, using the numerical integration of the orbits of planetesimals around a migrating planet, we compared the derived equations with the numerical results. Results. We find that the conditions required for the sweet spot can be expressed by the ratio of the aerodynamic gas damping timescale of the planetesimal orbits to the planetary migration timescale. If the planetary migration timescale depends on the surface density of disk gas inversely, the location of the sweet spot does not change with the disk evolution. We expect that the planets observed inner to the sweet spot include a much greater amount of heavy elements than the planets outer to the sweet spot. The mass of planetesimals accreted by the protoplanet in the sweet spot depends on the amount of planetesimals that are shepherded by mean motion resonances. Our analysis suggests that tens Earth-masses of planetesimals can be shepherded into the sweet spot without planetesimal collisions. However, as more planetesimals are trapped into mean motion resonances, collisional cascade can lead to fragmentation and the production of smaller planetesimals. This could affect the location of the sweet spot and the population of small objects in planetary systems. Conclusions. We conclude that the composition of gas giant planets depends on whether the planets crossed the sweet spot during their formation. Constraining the metallicity of cold giant planets, which are expected to be beyond the sweet spot, with future observations would reveal key information for understanding the origin of heavy elements in giant planets.

Polarization of Rotationally Oblate Self-luminous Exoplanets with Anisotropic Atmospheres

Abstract The young self-luminous giant exoplanets are expected to be oblate in shape, owing to the high rotational speeds observed for some objects. Similar to the case of brown dwarfs, the thermal emission from these planets should be polarized by scatterings of molecules and condensate cloud particles, and the rotation-induced asymmetry of the planet’s disk would yield to net nonzero detectable polarization. Considering an anisotropic atmosphere, we present here a three-dimensional approach to estimating the disk-averaged polarization that arises due to the oblateness of the planets. We solve the multiple-scattering vector radiative transfer equations at each location on the planet’s disk and calculate the local Stokes vectors, and then calculate the disk-integrated flux and linear polarization. For a cloud-free atmosphere, the polarization signal is observable only in the visible wavelength region. However, the presence of clouds in the planetary atmospheres leads to a detectable amount of polarization in the infrared wavelength region where the planetary thermal emission peaks. Considering the different broadband filters of the SPHERE-IRDIS instrument of the Very Large Telescope, we present generic models for the polarization at different wavelength bands as a function of their rotation period. We also present polarization models for the exoplanets β Pic b and ROXs 42B b, as two representative cases that can guide future observations. Our insights into the polarization of young giant planets presented here would be useful for the upcoming polarimetric observations of the directly imaged planets.

Diurnal variations in the stratosphere of the ultrahot giant exoplanet WASP-121b

The temperature profile of a planetary atmosphere is a key diagnostic of radiative and dynamical processes governing the absorption, redistribution and emission of energy. Observations have revealed dayside stratospheres that either cool1,2 or warm3,4 with altitude for a small number of gas giant exoplanets, whereas other dayside stratospheres are consistent with constant temperatures5–7. Here we report spectroscopic phase curve measurements for the gas giant WASP-121b (ref. 8) that constrain stratospheric temperatures throughout the diurnal cycle. Variations measured for a water vapour spectral feature reveal a temperature profile that transitions from warming with altitude on the dayside hemisphere to cooling with altitude on the nightside hemisphere. The data are well explained by models assuming chemical equilibrium, with water molecules thermally dissociating at low pressures on the dayside and recombining on the nightside9,10. Nightside temperatures are low enough for perovskite (CaTiO3) to condense, which could deplete titanium from the gas phase11,12 and explain recent non-detections at the day–night terminator13–16. Nightside temperatures are also consistent with the condensation of refractory species such as magnesium, iron and vanadium. Detections15–18 of these metals at the day–night terminator suggest, however, that if they do form nightside clouds, cold trapping does not efficiently remove them from the upper atmosphere. Horizontal winds and vertical mixing could keep these refractory condensates aloft in the upper atmosphere of the nightside hemisphere until they are recirculated to the hotter dayside hemisphere and vaporized.

Understanding the interior structure of gaseous giant exoplanets with machine learning techniques

Context. Characterizing the interiors of gaseous giant exoplanets is currently one of the main objectives in exoplanetary sciences. In particular, the planetary heavy-element mass provides a critical constraint on planet formation from exoplanetary systems. However, gas giant exoplanets show large diversities in thermal states and their interior properties vary across a wide magnitude range. Forward modeling of their interiors exhibits a larger degeneracy with respect to rocky exoplanets. Aims. We applied machine learning techniques based on mixture density networks (MDNs) to investigate the interiors of gaseous giant exoplanets. We aim to provide a well-trained MDN for quick and efficient predictions. Methods. Based on our current knowledge of gas giants in the Solar System, we discussed an effect of model uncertainties on planetary interiors and presented a data set for gas giants with masses between 0.1 and 10 Jupiter masses using two-layer interior models. Then, MDNs were constructed to train the generated data set and their performance was evaluated in order to achieve a well-trained one. Results. The MDN using planetary mass and radius as inputs exhibits the well-known degeneracy of interior models. The surface temperature of a planet bears constraints on the thermal state of planetary interiors, and adding it as additional input considerably breaks the degeneracy of possible interior structures. The MDN with inputs of mass, radius, and surface temperature is found to show excellent performance in predicting the interior properties of gaseous giant exoplanets, although these interior properties span over a very wide range. We also applied the well-trained MDN to four gas giants in the Solar System and beyond. The MDN predictions are in good agreement with the interior model solutions within the observational and systematic uncertainties. Conclusions. We offer a convenient and powerful tool available online providing knowledge of the interiors of gaseous giant exoplanets in addition to rocky exoplanets, which could be helpful for our understanding of planet formation in diverse protoplanetary environments.

Direct emission spectroscopy of exoplanets with the medium resolution imaging spectrometer on board JWST MIRI

Context. The Medium Resolution Spectrometer (MRS) of the Mid-Infrared Instrument (MIRI) on board the James Webb Space Telescope (JWST) will give access to mid-infrared (mid-IR) spectra (5–28 microns) while retaining spatial information. With the unparalleled sensitivity of JWST and the MIRI detectors, the MRS has the potential to revolutionise our understanding of giant exoplanet atmospheres. Aims. Molecular mapping is a promising detection and characterisation technique used to study the spectra of directly imaged exoplanets. We aim to examine the feasibility and application of this technique to MRS observations. Methods. We used the instrument simulator MIRISIM to create mock observations of resolved star and exoplanet systems. As an input for the simulator, we used stellar and planet parameters from literature, with the planet spectrum being modelled with the radiative transfer code petitRADTRANS. After processing the raw data with the JWST pipeline, we high pass filter the data to account for the stellar point spread function, and used a forward modelling approach to detect the companions and constrain the chemical composition of their atmospheres through their molecular signatures. Results. We identified limiting factors in spectroscopic characterisation of directly imaged exoplanets with the MRS and simulated observations of two representative systems, HR8799 and GJ504. In both systems, we could detect the presence of multiple molecules that were present in the input model of their atmospheres. We used two different approaches with single molecule forward models, used in literature, that are sensitive to detecting mainly H2O, CO, CH4, and NH3, and a log-likelihood ratio test that uses full atmosphere forward models and is sensitive to a larger number of less dominant molecular species. Conclusions. We show that the MIRI MRS can be used to characterise widely separated giant exoplanets in the mid-IR using molecular mapping. Such observations would provide invaluable information for the chemical composition of the atmosphere, complementing other JWST observing modes, as well as ground-based observations.

Migration of Jupiter mass planets in discs with laminar accretion flows

Context. Migration of giant planets in discs with low viscosity has been studied recently. Results have shown that the proportionality between migration speed and the disc’s viscosity is broken by the presence of vortices that appear at the edges of the planet-induced gap. Under some conditions, this ‘vortex-driven’ migration can be very slow and eventually stops. However, this result has been obtained for discs whose radial mass transport is too low (due to the small viscosity) to be compatible with the mass accretion rates that are typically observed for young stars. Aims. Our goal is to investigate vortex-driven migration in low-viscosity discs in the presence of radial advection of gas, as expected from angular momentum removal by magnetised disc winds. Methods. We performed three dimensional simulations using the grid-based code FARGOCA. We mimicked the effects of a disc wind by applying a synthetic torque on a surface layer of the disc characterised by a prescribed column density ΣA so that it results in a disc accretion rate of ṀA = 10−8 M⊙ yr−1. We have considered values of ΣA typical of the penetration depths of different ionising processes. Discs with this structure are called ‘layered’ and the layer where the torque is applied is denoted as ‘active’. We also consider the case of accretion focussed near the disc midplane to mimic transport properties induced by a large Hall effect or by weak Ohmic diffusion. Results. We observe two migration phases: in the first phase, which is exhibited by all simulations, the migration of the planet is driven by the vortex and is directed inwards. This phase ends when the vortex disappears after having opened a secondary gap, as is typically observed in vortex-driven migration. Migration in the second phase depends on the ability of the torque from the planet to block the accretion flow. When the flow is fast and unimpeded, corresponding to small ΣA, migration is very slow, similar to when there is no accreting layer in the disc. When the accretion flow is completely blocked, migration is faster (typically ṙp ~ 12 AU Myr−1 at 5 au) and the speed is controlled by the rate at which the accretion flow refills the gap behind the migrating planet. The transition between the two regimes, occurs at ΣA ~ 0.2 g cm−2 and 0.65 g cm−2 for Jupiter or Saturn mass planets at 5.2 au, respectively. Conclusions. The migration speed of a giant planet in a layered protoplanetary disc depends on the thickness of the accreting layer. The lack of large-scale migration apparently experienced by the majority of giant exoplanets can be explained if the accreting layer is sufficiently thin to allow unimpeded accretion through the disc.

A New Sedimentation Model for Greater Cloud Diversity in Giant Exoplanets and Brown Dwarfs

Abstract The observed atmospheric spectrum of exoplanets and brown dwarfs depends critically on the presence and distribution of atmospheric condensates. The Ackerman and Marley methodology for predicting the vertical distribution of condensate particles is widely used to study cloudy atmospheres and has recently been implemented in an open-source python package, Virga. The model relies upon input parameter f sed, the sedimentation efficiency, which until now has been held constant. The relative simplicity of this model renders it useful for retrieval studies due to its rapidly attainable solutions. However, comparisons with more complex microphysical models such as CARMA have highlighted inconsistencies between the two approaches, namely that the cloud parameters needed for radiative transfer produced by Virga are dissimilar to those produced by CARMA. To address these discrepancies, we have extended the original Ackerman and Marley methodology in Virga to allow for non-constant f sed values, in particular, those that vary with altitude. We discuss one such parameterization and compare the cloud mass mixing ratio produced by Virga with constant and variable f sed profiles to that produced by CARMA. We find that the variable f sed formulation better captures the profile produced by CARMA with heterogeneous nucleation, yet performs comparatively to constant f sed for homogeneous nucleation. In general, Virga has the capacity to handle any f sed with an explicit anti-derivative, permitting a plethora of alternative cloud profiles that are otherwise unattainable by constant f sed values. The ensuing flexibility has the potential to better agree with increasingly complex models and observed data.

A rich population of free-floating planets in the Upper Scorpius young stellar association

The nature and origin of free-floating planets (FFPs) are still largely unconstrained because of a lack of large homogeneous samples to enable a statistical analysis of their properties. So far, most FFPs have been discovered using indirect methods; microlensing surveys have proved particularly successful to detect these objects down to a few Earth masses. However, the ephemeral nature of microlensing events prevents any follow-up observations and individual characterization. Several studies have identified FFPs in young stellar clusters and the Galactic field but their samples are small or heterogeneous in age and origin. Here we report the discovery of between 70 and 170 FFPs (depending on the assumed age) in the region encompassing Upper Scorpius and Ophiuchus, the closest young OB association to the Sun. We found an excess of FFPs by a factor of up to seven compared with core-collapse model predictions, demonstrating that other formation mechanisms may be at work. We estimate that ejection from planetary systems might have a contribution comparable to that of core-collapse in the formation of FFPs. Therefore, ejections due to dynamical instabilities in giant exoplanet systems must be frequent within the first 10 Myr of a system's life.

The 12CO/13CO isotopologue ratio of a young, isolated brown dwarf

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.

Flux-limited Diffusion Approximation Models of Giant Planet Formation by Disk Instability. II. Quadrupled Spatial Resolution

Abstract While collisional accumulation is nearly universally accepted as the formation mechanism of rock and ice worlds, the situation regarding gas giant planet formation is more nuanced. Gas accretion by solid cores formed by collisional accumulation is the generally favored mechanism, but observations increasingly suggest that gas disk gravitational instability might explain the formation of at least the massive or wide-orbit gas giant exoplanets. This paper continues a series aimed at refining three-dimensional (3D) hydrodynamical models of disk instabilities, where the handling of the gas thermodynamics is a crucial factor. Boss (2017, 2021) used the β cooling approximation to calculate 3D models of disks with initial masses of 0.091 M ⊙ extending from 4 to 20 au around 1 M ⊙ protostars. Here we employ 3D flux-limited diffusion (FLD) approximation models of the same disks, in order to provide a superior treatment of disk gas thermodynamics. The new models have quadrupled spatial resolution compared to previous 3D FLD models, in both the radial and azimuthal spherical coordinates, resulting in the highest spatial resolution 3D FLD models to date. The new models continue to support the hypothesis that such disks can form self-gravitating, dense clumps capable of contracting to form gas giant protoplanets, and suggest that the FLD models yield similar numbers of clumps as β cooling models with β ∼ 1 to ∼10, including the critical value of β = 3 for fragmentation proposed by Gammie.

Dynamical orbital evolution scenarios of the wide-orbit eccentric planet HR 5183b

ABSTRACT The recently discovered giant exoplanet HR5183b exists on a wide, highly eccentric orbit (a = 18 au, e = 0.84). Its host star possesses a common proper-motion companion which is likely on a bound orbit. In this paper, we explore scenarios for the excitation of the eccentricity of the planet in binary systems such as this, considering planet–planet scattering, Lidov–Kozai cycles from the binary acting on a single-planet system, or Lidov–Kozai cycles acting on a two-planet system that also undergoes scattering. Planet–planet scattering, in the absence of a binary companion, has a $2.8{-}7.2{{\ \rm per\ cent}}$ probability of pumping eccentricities to the observed values in our simulations, depending on the relative masses of the two planets. Lidov–Kozai cycles from the binary acting on an initially circular orbit can excite eccentricities to the observed value but require very specific orbital configurations for the binary and overall there is a low probability of catching the orbit at the high observed high eccentricity ($0.6{{\ \rm per\ cent}}$). The best case is provided by planet–planet scattering in the presence of a binary companion: here, the scattering provides the surviving planet with an initial eccentricity boost that is subsequently further increased by Kozai cycles from the binary. We find a success rate of $14.5{{\ \rm per\ cent}}$ for currently observing e ≥ 0.84 in this set-up. The single-planet plus binary and two-planet plus binary cases are potentially distinguishable if the mutual inclination of the binary and the planet can be measured, as the latter permits a broader range of mutual inclinations. The combination of scattering and Lidov–Kozai forcing may also be at work in other wide-orbit eccentric giant planets, which have a high rate of stellar binary companions.

A unique hot Jupiter spectral sequence with evidence for compositional diversity

The emergent spectra of close-in, giant exoplanets ("hot Jupiters") are expected to be distinct from those of self-luminous objects with similar effective temperatures because hot Jupiters are primarily heated from above by their host stars rather than internally from the release of energy from their formation. Theoretical models predict a continuum of dayside spectra for hot Jupiters as a function of irradiation level, with the coolest planets having absorption features in their spectra, intermediate-temperature planets having emission features due to thermal inversions, and the hottest planets having blackbody-like spectra due to molecular dissociation and continuum opacity from the H- ion. Absorption and emission features have been detected in the spectra of a number of individual hot Jupiters, and population-level trends have been observed in photometric measurements. However, there has been no unified, population-level study of the thermal emission spectra of hot Jupiters such as has been done for cooler brown dwarfs and transmission spectra of hot Jupiters. Here we show that hot Jupiter secondary eclipse spectra centered around a water absorption band at 1.4 microns follow a common trend in water feature strength with temperature. The observed trend is broadly consistent with model predictions for how the thermal structures of solar-composition planets vary with irradiation level. Nevertheless, the ensemble of planets exhibits some degree of scatter around the mean trend for solar composition planets. The spread can be accounted for if the planets have modest variations in metallicity and/or elemental abundance ratios, which is expected from planet formation models. (abridged abstract)

T Tauri stars: magnetic fields and planets

In this short review we present the results of a study of the large-scale magnetic topologies of T Tauri stars (TTS). A small spectropolarimetric survey of 8 young stars was carried out within two international projects MaPP (Magnetic Protostars and Planets) and MaTYSSE (Magnetic Topologies of Young Stars and the Survival of massive close-in Exoplanets) between 2009 and 2016. For each of our targets we reconstructed the brightness map and the magnetic field topology using Zeeman–Doppler imaging (ZDI). This review contains a brief description of spectropolarimetricdata, the ZDI method, one example of the reconstruction of brightness and magnetic maps, and the properties of magnetic fields of 8 TTS. Our results suggest that AA Tau and LkCa 15 interact with their disks in the propeller mode when their rotation is actively slowed by the star/disk magnetic coupling. We find that magnetic fields of some TTS are variable on a time scale of a few years and are thus intrinsically nonstationary. We report on the detection of a giant exoplanet around V830 Tau and TAP 26. These two new detections suggest that the type II disk migration is efficient at generating newborn hot Jupiters (hJs) around young TTS. The result of our survey is compared to the global picture of magnetic field properties of twenty TTS in the Hertzsprung–Russell diagram. The comparison shows that WTTS exhibit a wider range of field topologies as compared to CTTS, and that magnetic fields of all TTS (CTTS and WTTS as a whole) are mostly poloidal and axisymmetric when they are mostly convective and cooler than 4300 K. This needs to be confirmed with a larger sample of stars.

Revealing the Vertical Cloud Structure of a Young Low-mass Brown Dwarf, an Analog to the β-Pictoris b Directly Imaged Exoplanet, through Keck I/MOSFIRE Spectrophotometric Variability

Abstract Young brown dwarfs are analogs to giant exoplanets, as they share effective temperatures, near-infrared colors, and surface gravities. Thus, the detailed characterization of young brown dwarfs might shed light on the study of giant exoplanets that we are currently unable to observe with a sufficient signal-to-noise to allow a precise characterization of their atmospheres. 2MASS J22081363+2921215 is a young L3 brown dwarf, and a member of the β-Pictoris young moving group (23 ± 3 Myr), which shares its effective temperature and mass with the β Pictoris b giant exoplanet. We performed a ∼2.5 hr spectrophotometric J-band monitoring of 2MASS J22081363+2921215 with the MOSFIRE multi-object spectrograph, installed at the Keck I telescope. We measured a minimum variability amplitude of 3.22 ± 0.42% for its J-band light curve. The ratio between the maximum and the minimum flux spectra of 2MASS J22081363+2921215 shows a weak wavelength dependence, and a potentially enhanced variability amplitude in its alkali lines. Further analysis suggests that the variability amplitudes of the alkali lines are higher than its overall variability amplitude (4.5%–11%, depending on the lines). The variability amplitudes of these lines are lower if we degrade the resolution of the original MOSFIRE spectra to R ∼ 100, which explains why this potentially enhanced variability of the alkali lines had not been found previously in Hubble Space Telescope (HST)/WFC3 light curves. Using radiative-transfer models, we obtained the different cloud layers that might be introducing the spectrophotometric variability we observe for 2MASS J22081363+2921215, which further supports the measured enhanced variability amplitudes of the alkali lines. We provide an artistic recreation of the vertical cloud structure of this β-Pictoris b analog.