Groundbased and spacecraft telescopic observations, combined with an intensive modeling effort, have greatly enhanced our understanding of hot giant planets and brown dwarfs over the past ten years. ...Although these objects are all fluid, hydrogen worlds with stratified atmospheres overlying convective interiors, they exhibit an impressive diversity of atmospheric behavior. Hot Jupiters are strongly irradiated, and a wealth of observations constrain the day-night temperature differences, circulation, and cloudiness. The intense stellar irradiation, presumed tidal locking and modest rotation leads to a novel regime of strong day-night radiative forcing. Circulation models predict large day-night temperature differences, global-scale eddies, patchy clouds, and, in most cases, a fast eastward jet at the equator—equatorial superrotation. The warm Jupiters lie farther from their stars and are not generally tidally locked, so they may exhibit a wide range of rotation rates, obliquities, and orbital eccentricities, which, along with the weaker irradiation, leads to circulation patterns and observable signatures predicted to differ substantially from hot Jupiters. Brown dwarfs are typically isolated, rapidly rotating worlds; they radiate enormous energy fluxes into space and convect vigorously in their interiors. Their atmospheres exhibit patchiness in clouds and temperature on regional to global scales—the result of modulation by large-scale atmospheric circulation. Despite the lack of irradiation, such circulations can be driven by interaction of the interior convection with the overlying atmosphere, as well as self-organization of patchiness due to cloud-dynamical-radiative feedbacks. Finally, irradiated brown dwarfs help to bridge the gap between these classes of objects, experiencing intense external irradiation as well as vigorous interior convection. Collectively, these diverse objects span over six orders of magnitude in intrinsic heat flux and incident stellar flux, and two orders of magnitude in rotation rate—thereby placing strong constraints on how the circulation of giant planets (broadly defined) depend on these parameters. A hierarchy of modeling approaches have yielded major new insights into the dynamics governing these phenomena.
ABSTRACT We model the impact of nonuniform cloud cover on transit transmission spectra. Patchy clouds exist in nearly every solar system atmosphere, brown dwarfs, and transiting exoplanets. Our major ...findings suggest that fractional cloud coverage can exactly mimic high mean molecular weight atmospheres and vice versa over certain wavelength regions, in particular, over the Hubble Space Telescope (HST) Wide Field Camera 3 (WFC3) bandpass (1.1-1.7 m). We also find that patchy cloud coverage exhibits a signature that is different from uniform global clouds. Furthermore, we explain analytically why the "patchy cloud-high mean molecular weight" degeneracy exists. We also explore the degeneracy of nonuniform cloud coverage in atmospheric retrievals on both synthetic and real planets. We find from retrievals on a synthetic solar composition hot Jupiter with patchy clouds and a cloud-free high mean molecular weight warm Neptune that both cloud-free high mean molecular weight atmospheres and partially cloudy atmospheres can explain the data equally well. Another key finding is that the HST WFC3 transit transmission spectra of two well-observed objects, the hot Jupiter HD 189733b and the warm Neptune HAT-P-11b, can be explained well by solar composition atmospheres with patchy clouds without the need to invoke high mean molecular weight or global clouds. The degeneracy between high molecular weight and solar composition partially cloudy atmospheres can be broken by observing the molecular Rayleigh scattering differences between the two. Furthermore, the signature of partially cloudy limbs also appears as a ∼100 ppm residual in the ingress and egress of the transit light curves, provided that the transit timing is known to seconds.
We present the first application of a bin-scheme microphysical and vertical transport model to determine the size distribution of titanium and silicate cloud particles in the atmospheres of hot ...Jupiters. We predict particle size distributions from first principles for a grid of planets at four representative equatorial longitudes, and investigate how observed cloud properties depend on the atmospheric thermal structure and vertical mixing. The predicted size distributions are frequently bimodal and irregular in shape. There is a negative correlation between the total cloud mass and equilibrium temperature as well as a positive correlation between the total cloud mass and atmospheric mixing. The cloud properties on the east and west limbs show distinct differences that increase with increasing equilibrium temperature. Cloud opacities are roughly constant across a broad wavelength range, with the exception of features in the mid-infrared. Forward-scattering is found to be important across the same wavelength range. Using the fully resolved size distribution of cloud particles as opposed to a mean particle size has a distinct impact on the resultant cloud opacities. The particle size that contributes the most to the cloud opacity depends strongly on the cloud particle size distribution. We predict that it is unlikely that silicate or titanium clouds are responsible for the optical Rayleigh scattering slope seen in many hot Jupiters. We suggest that cloud opacities in emission may serve as sensitive tracers of the thermal state of a planet's deep interior through the existence or lack of a cold trap in the deep atmosphere.
ABSTRACT Over a large range of equilibrium temperatures, clouds shape the transmission spectrum of hot Jupiter atmospheres, yet their composition remains unknown. Recent observations show that the ...Kepler light curves of some hot Jupiters are asymmetric: for the hottest planets, the light curve peaks before secondary eclipse, whereas for planets cooler than ∼1900 K, it peaks after secondary eclipse. We use the thermal structure from 3D global circulation models to determine the expected cloud distribution and Kepler light curves of hot Jupiters. We demonstrate that the change from an optical light curve dominated by thermal emission to one dominated by scattering (reflection) naturally explains the observed trend from negative to positive offset. For the cool planets the presence of an asymmetry in the Kepler light curve is a telltale sign of the cloud composition, because each cloud species can produce an offset only over a narrow range of effective temperatures. By comparing our models and the observations, we show that the cloud composition of hot Jupiters likely varies with equilibrium temperature. We suggest that a transition occurs between silicate and manganese sulfide clouds at a temperature near 1600 K, analogous to the L/T transition on brown dwarfs. The cold trapping of cloud species below the photosphere naturally produces such a transition and predicts similar transitions for other condensates, including TiO. We predict that most hot Jupiters should have cloudy nightsides, that partial cloudiness should be common at the limb, and that the dayside hot spot should often be cloud-free.
Aerosols appear to be ubiquitous in close-in gas giant atmospheres, and disequilibrium chemistry likely impacts the emergent spectra of these planets. Lofted aerosols and disequilibrium chemistry are ...caused by vigorous vertical transport in these heavily irradiated atmospheres. Here we numerically and analytically investigate how vertical transport should change over the parameter space of spin-synchronized gas giants. In order to understand how tracer transport depends on planetary parameters, we develop an analytic theory to predict vertical velocities and mixing rates (Kzz) and compare the results to our numerical experiments. We find that both our theory and numerical simulations predict that if the vertical mixing rate is described by an eddy diffusivity, then this eddy diffusivity Kzz should increase with increasing equilibrium temperature, decreasing frictional drag strength, and increasing chemical loss timescales. We find that the transition in our numerical simulations between circulation dominated by a superrotating jet and that with solely day-to-night flow causes a marked change in the vertical velocity structure and tracer distribution. The mixing ratio of passive tracers is greatest for intermediate drag strengths that correspond to this transition between a superrotating jet with columnar vertical velocity structure and day-to-night flow with upwelling on the dayside and downwelling on the nightside. Finally, we present analytic solutions for Kzz as a function of planetary effective temperature, chemical loss timescales, and other parameters, for use as input to 1D chemistry models of spin-synchronized gas giant atmospheres.
ABSTRACT
Transmission spectra contain a wealth of information about the atmospheres of transiting exoplanets. However, large thermal and chemical gradients along the line of sight can lead to biased ...inferences in atmospheric retrievals. In order to determine how far from the limb plane the atmosphere still impacts the transmission spectrum, we derive a new formula to estimate the opening angle of a planet. This is the angle subtended by the atmospheric region that contributes to the observation along the line of sight, as seen from the planet centre. We benchmark our formula with a 3D Monte Carlo radiative transfer code and we define an opening angle suitable for the interpretation of JWST observations, assuming a 10-ppm noise floor. We find that the opening angle is only a few degrees for planets cooler than ca. 500 Kelvins, while it can be as large as 25 degrees for (ultra-)hot Jupiters and 50 degrees for hot Neptunes. Compared to previous works, our more robust approach leads to smaller estimates for the opening angle across a wide range scale heights and planetary radii. Finally, we show that ultra-hot Jupiters have an opening angle that is smaller than the angle over which the planet rotates during the transit. This allows for time-resolved transmission spectroscopy observations that probe independent parts of the planetary limb during the first and second half of the transit.
GJ 1214b stands out among the detected low-mass exoplanets, because it is, so far, the only one amenable to transmission spectroscopy. Up to date there is no consensus about the composition of its ...envelope although most studies suggest a high molecular weight atmosphere. In particular, it is unclear if hydrogen and helium are present or if the atmosphere is water dominated. Here, we present results on the composition of the envelope obtained by using an internal structure and evolutionary model to fit the mass and radius data. By examining all possible mixtures of water and H/He, with the corresponding opacities, we find that the bulk amount of H/He of GJ 1214b is at most 7% by mass. In general, we find the radius of warm sub-Neptunes to be most sensitive to the amount of H/He. We note that all (Kepler-11b,c,d,f, Kepler-18b, Kepler-20b, 55Cnc-e, Kepler-36c, and Kepler-68b) but two (Kepler-11e and Kepler-30b) of the discovered low-mass planets so far have less than 10% H/He. In fact, Kepler-11e and Kepler-30b have 10%-18% and 5%-15% bulk H/He. Conversely, little can be determined about the H sub(2)O or rocky content of sub-Neptune planets. We find that although a 100% water composition fits the data for GJ 1214b, based on formation constraints the presence of heavier refractory material on this planet is expected, and hence, so is a component lighter than water required. The same is true for Kepler-11f. A robust determination by transmission spectroscopy of the composition of the upper atmosphere of GJ 1214b will help determine the extent of compositional segregation between the atmosphere and the envelope.
Measurements of the atmospheric carbon (C) and oxygen (O) relative to hydrogen (H) in hot Jupiters (relative to their host stars) provide insight into their formation location and subsequent orbital ...migration
. Hot Jupiters that form beyond the major volatile (H
O/CO/CO
) ice lines and subsequently migrate post disk-dissipation are predicted have atmospheric carbon-to-oxygen ratios (C/O) near 1 and subsolar metallicities
, whereas planets that migrate through the disk before dissipation are predicted to be heavily polluted by infalling O-rich icy planetesimals, resulting in C/O < 0.5 and super-solar metallicities
. Previous observations of hot Jupiters have been able to provide bounded constraints on either H
O (refs.
) or CO (refs.
), but not both for the same planet, leaving uncertain
the true elemental C and O inventory and subsequent C/O and metallicity determinations. Here we report spectroscopic observations of a typical transiting hot Jupiter, WASP-77Ab. From these, we determine the atmospheric gas volume mixing ratio constraints on both H
O and CO (9.5 × 10
-1.5 × 10
and 1.2 × 10
-2.6 × 10
, respectively). From these bounded constraints, we are able to derive the atmospheric C/H (Formula: see text × solar) and O/H (Formula: see text × solar) abundances and the corresponding atmospheric carbon-to-oxygen ratio (C/O = 0.59 ± 0.08; the solar value is 0.55). The sub-solar (C+O)/H (Formula: see text × solar) is suggestive of a metal-depleted atmosphere relative to what is expected for Jovian-like planets
while the near solar value of C/O rules out the disk-free migration/C-rich
atmosphere scenario.
Context. A new class of exoplanets has emerged: the ultra hot Jupiters, the hottest close-in gas giants. The majority of them have weaker-than-expected spectral features in the 1.1−1.7 μm bandpass ...probed by HST/WFC3 but stronger spectral features at longer wavelengths probed by Spitzer. This led previous authors to puzzling conclusions about the thermal structures and chemical abundances of these planets. Aims. We investigate how thermal dissociation, ionization, H− opacity, and clouds shape the thermal structures and spectral properties of ultra hot Jupiters. Methods. We use the SPARC/MITgcm to model the atmospheres of four ultra hot Jupiters and discuss more thoroughly the case of WASP-121b. We expand our findings to the whole population of ultra hot Jupiters through analytical quantification of the thermal dissociation and its influence on the strength of spectral features. Results. We predict that most molecules are thermally dissociated and alkalies are ionized in the dayside photospheres of ultra hot Jupiters. This includes H2O, TiO, VO, and H2 but not CO, which has a stronger molecular bond. The vertical molecular gradient created by the dissociation significantly weakens the spectral features from H2O while the 4.5 μm CO feature remains unchanged. The water band in the HST/WFC3 bandpass is further weakened by the continuous opacity of the H− ions. Molecules are expected to recombine before reaching the limb, leading to order of magnitude variations of the chemical composition and cloud coverage between the limb and the dayside. Conclusions. Molecular dissociation provides a qualitative understanding of the lack of strong spectral features of water in the 1−2 μm bandpass observed in most ultra hot Jupiters. Quantitatively, our model does not provide a satisfactory match to the WASP-121b emission spectrum. Together with WASP-33b and Kepler-33Ab, they seem the outliers among the population of ultra hot Jupiters, in need of a more thorough understanding.
We present one of the most precise emission spectra of an exoplanet observed so far. We combine five secondary eclipses of the hot Jupiter WASP-18b (Tday ∼ 2900 K) that we secured between 1.1 and 1.7 ...m with the Wide Field Camera 3 instrument on board the Hubble Space Telescope. Our extracted spectrum (S/N = 50, R ∼ 40) does not exhibit clearly identifiable molecular features but is poorly matched by a blackbody spectrum. We complement this data with previously published Spitzer/Infrared Array Camera observations of this target and interpret the combined spectrum by computing a grid of self-consistent, 1D forward models, varying the composition and energy budget. At these high temperatures, we find there are important contributions to the overall opacity from H− ions, as well as the removal of major molecules by thermal dissociation (including water), and thermal ionization of metals. These effects were omitted in previous spectral retrievals for very hot gas giants, and we argue that they must be included to properly interpret the spectra of these objects. We infer a new metallicity and C/O ratio for WASP-18b, and find them well constrained to be solar (M/H = −0.01 0.35, C/O < 0.85 at 3 confidence level), unlike previous work but in line with expectations for giant planets. The best-fitting self-consistent temperature-pressure profiles are inverted, resulting in an emission feature at 4.5 m seen in the Spitzer photometry. These results further strengthen the evidence that the family of very hot gas giant exoplanets commonly exhibit thermal inversions.
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