Abstract
Recently, the first JWST measurement of thermal emission from a rocky exoplanet was reported. The inferred dayside brightness temperature of TRAPPIST-1 b at 15
μ
m is consistent with the ...planet having no atmosphere and therefore no mechanism by which to circulate heat to its nightside. In this Letter, we compare TRAPPIST-1 b's measured secondary eclipse depth to predictions from a suite of self-consistent radiative-convective equilibrium models in order to quantify the maximum atmospheric thickness consistent with the observation. We find that plausible atmospheres (i.e., those that contain at least 100 ppm CO
2
) with surface pressures greater than 0.3 bar are ruled out at 3
σ
, regardless of the choice of background atmosphere, and a Mars-like thin atmosphere with surface pressure 6.5 mbar composed entirely of CO
2
is also ruled out at 3
σ
. Thicker atmospheres of up to 10 bar (100 bar) are consistent with the data at 1
σ
(3
σ
) only if the atmosphere lacks
any
strong absorbers across the mid-IR wavelength range—a scenario that we deem unlikely. We additionally model the emission spectra for bare-rock planets of various compositions. We find that a basaltic, metal-rich, and Fe-oxidized surface best matches the measured eclipse depth to within 1
σ
, and the best-fit gray albedo is 0.02 ± 0.11. We conclude that planned secondary eclipse observations at 12.8
μ
m will serve to validate TRAPPIST-1 b's high observed brightness temperature, but are unlikely to further distinguish among the consistent atmospheric and bare-rock scenarios.
Abstract
The large radii of many hot Jupiters can only be matched by models that have hot interior adiabats, and recent theoretical work has shown that the interior evolution of hot Jupiters has a ...significant impact on their atmospheric structure. Due to its inflated radius, low gravity, and ultrahot equilibrium temperature, WASP-76b is an ideal case study for the impact of internal evolution on observable properties. Hot interiors should most strongly affect the nonirradiated side of the planet, and thus full phase-curve observations are critical to ascertain the effect of the interior on the atmospheres of hot Jupiters. In this work, we present the first Spitzer phase-curve observations of WASP-76b. We find that WASP-76b has an ultrahot dayside and relatively cold nightside with brightness temperatures of 2471 ± 27 K/1518 ± 61 K at 3.6
μ
m and 2699 ± 32 K/1259 ± 44 K at 4.5
μ
m, respectively. These results provide evidence for a dayside thermal inversion. Both channels exhibit small phase offsets of 0.68 ± 0.°48 at 3.6
μ
m and 0.67 ± 0.°2 at 4.5
μ
m. We compare our observations to a suite of general circulation models (GCMs) that consider two endmembers of interior temperature along with a broad range of frictional drag strengths. Strong frictional drag is necessary to match the small phase offsets and cold nightside temperatures observed. From our suite of cloud-free GCMs, we find that only cases with a cold interior can reproduce the cold nightsides and large phase-curve amplitude at 4.5
μ
m, hinting that the hot interior adiabat of WASP-76b does not significantly impact its atmospheric dynamics or that clouds blanket its nightside.
Abstract
Ultra-hot Jupiters with equilibrium temperatures greater than 2000 K are uniquely interesting targets as they provide us crucial insights into how atmospheres behave under extreme ...conditions. This class of giant planets receives intense radiation from their host star and usually has strongly irradiated and highly inflated atmospheres. At such a high temperature, cloud formation is expected to be suppressed and thermal dissociation of water vapor could occur. We observed the ultra-hot Jupiter WASP-76b with seven transits and five eclipses using the Hubble Space Telescope and the Spitzer Space Telescope (Spitzer) for a comprehensive study of its atmospheric chemical and physical processes. We detected TiO and H
2
O absorption in the optical and near-infrared transit spectrum. Additional absorption by a number of neutral and ionized heavy metals like Fe, Ni, Ti, and SiO help explain the short-wavelength transit spectrum. The secondary eclipse spectrum shows muted water feature but a strong CO emission feature in Spitzer’s 4.5
μ
m band indicating an inverted temperature pressure profile. We analyzed both the transit and eclipse spectra with a combination of self-consistent PHOENIX models and atmospheric retrieval. Both spectra were well fitted by the self-consistent PHOENIX forward atmosphere model in chemical and radiative equilibrium at solar metallicity, adding to the growing evidence that both TiO/VO and NUV heavy metals opacity are prominent NUV-optical opacity sources in the stratospheres of ultra-hot Jupiters.
Abstract It remains to be ascertained whether sub-Neptune exoplanets primarily possess hydrogen-rich atmospheres or whether a population of H 2 O-rich water worlds lurks in their midst. Addressing ...this question requires improved modeling of water-rich exoplanetary atmospheres, both to predict and interpret spectroscopic observations and to serve as upper boundary conditions on interior structure calculations. Here, we present new models of hydrogen-helium-water atmospheres with water abundances ranging from solar to 100% water vapor. We improve upon previous models of high-water-content atmospheres by incorporating updated prescriptions for water self-broadening and a nonideal gas equation of state. Our model grid ( https://umd.box.com/v/water-worlds ) includes temperature–pressure profiles in radiative-convective equilibrium, along with their associated transmission and thermal emission spectra. We find that our model updates primarily act at high pressures, significantly impacting bottom-of-atmosphere temperatures, with implications for the accuracy of interior structure calculations. Upper-atmosphere conditions and spectroscopic observables are less impacted by our model updates, and we find that, under most conditions, retrieval codes built for hot Jupiters should also perform well on water-rich planets. We additionally quantify the observational degeneracies among both thermal emission and transmission spectra. We recover standard degeneracies with clouds and mean molecular weight for transmission spectra, and we find thermal emission spectra to be more readily distinguishable from one another in the water-poor (i.e., near-solar) regime.
Abstract
Retrieval of exoplanetary atmospheric properties from their transmission spectra commonly assumes that the errors in the data are Gaussian and independent. However, non-Gaussian noise can ...occur due to instrumental or stellar systematics and the merging of discrete data sets. We investigate the effect of correlated noise and constrain the potential biases incurred in the retrieved posteriors. We simulate multiple noise instances of synthetic data and perform retrievals to obtain statistics of the goodness of retrieval for varying noise models. We find that correlated noise allows for overfitting the spectrum, thereby yielding a better goodness of fit on average but degrading the overall accuracy of retrievals. In particular, correlated noise can manifest as an apparent non-Rayleigh slope in the optical range, leading to an incorrect estimate of cloud/haze parameters. We also find that higher precision causes correlated results to be further off from the input values in terms of estimated errors. Finally, we show that while correlated noise cannot be reliably distinguished with Hubble Space Telescope observations, inferring its presence and strength may be possible with James Webb Space Telescope observations. As such, we emphasize that caution must be taken in analyzing retrieved posteriors and that estimated parameter uncertainties are best understood as lower limits.
Atmospheric metal enrichment (that is, elements heavier than helium, also called 'metallicity') is a key diagnostic of the formation of giant planets
. The giant planets of the Solar System show an ...inverse relationship between mass and both their bulk metallicities and atmospheric metallicities. Extrasolar giant planets also display an inverse relationship between mass and bulk metallicity
. However, there is significant scatter in the relationship and it is not known how atmospheric metallicity correlates with either planet mass or bulk metallicity. Here we show that the Saturn-mass exoplanet HD 149026b (refs.
) has an atmospheric metallicity 59-276 times solar (at 1σ), which is greater than Saturn's atmospheric metallicity of roughly 7.5 times solar
at more than 4σ confidence. This result is based on modelling CO
and H
O absorption features in the thermal emission spectrum of the planet measured by the James Webb Space Telescope. HD 149026b is the most metal-rich giant planet known, with an estimated bulk heavy element abundance of 66 ± 2% by mass
. We find that the atmospheric metallicities of both HD 149026b and the Solar System giant planets are more correlated with bulk metallicity than planet mass.
There are no planets intermediate in size between Earth and Neptune in our Solar System, yet these objects are found around a substantial fraction of other stars
. Population statistics show that ...close-in planets in this size range bifurcate into two classes on the basis of their radii
. It is proposed that the group with larger radii (referred to as 'sub-Neptunes') is distinguished by having hydrogen-dominated atmospheres that are a few percent of the total mass of the planets
. GJ 1214b is an archetype sub-Neptune that has been observed extensively using transmission spectroscopy to test this hypothesis
. However, the measured spectra are featureless, and thus inconclusive, due to the presence of high-altitude aerosols in the planet's atmosphere. Here we report a spectroscopic thermal phase curve of GJ 1214b obtained with the James Webb Space Telescope (JWST) in the mid-infrared. The dayside and nightside spectra (average brightness temperatures of 553 ± 9 and 437 ± 19 K, respectively) each show more than 3σ evidence of absorption features, with H
O as the most likely cause in both. The measured global thermal emission implies that GJ 1214b's Bond albedo is 0.51 ± 0.06. Comparison between the spectroscopic phase curve data and three-dimensional models of GJ 1214b reveal a planet with a high metallicity atmosphere blanketed by a thick and highly reflective layer of clouds or haze.
Abstract
The spectroscopic characterization of terrestrial exoplanets over a wide spectral range from the near- to the mid-infrared will be made possible for the first time with the JWST. One ...challenge is that it is not known a priori whether such planets possess optically thick atmospheres or even any atmospheres altogether. However, this challenge also presents an opportunity, the potential to detect the surface of an extrasolar world. This study explores the feasibility of characterizing with the JWST the atmosphere and surface of LHS 3844b, the highest signal-to-noise rocky thermal emission target among planets that are cool enough to have nonmolten surfaces. We model the planetary emission, including the spectral signal of both the atmosphere and surface, and we explore all scenarios that are consistent with the existing Spitzer 4.5
μ
m measurement of LHS 3844b from Kreidberg et al. In summary, we find a range of plausible surfaces and atmospheres that are within 3
σ
of the observationless reflective metal-rich, iron-oxidized, and basaltic compositions are allowed, and atmospheres are restricted to a maximum thickness of 1 bar, if near-infrared absorbers at ≳100 ppm are included. We further make predictions on the observability of surfaces and atmospheres and find that a small number (∼3) of eclipse observations should suffice to differentiate between surface and atmospheric features. We also perform a Bayesian retrieval analysis on simulated JWST data and find that the surface signal may make it harder to precisely constrain the abundance of atmospheric species and may falsely induce a weak H
2
O detection.
Know thy star, know thy planetary atmosphere. Every exoplanet with atmospheric measurements orbits around a star, and the stellar environment directly affects the planetary atmosphere. Here we ...present the emission spectrum of ultra-hot Jupiter KELT-20b which provides an observational link between host-star properties and planet atmospheric thermal structure. It is currently the only planet with thermal emission measurements in the T eq ∼ 2200 K range that orbits around an early A-type star. By comparing it with other similar ultra-hot Jupiters around FGK stars, we can better understand how different host-star types influence planetary atmospheres. The emission spectrum covers 0.6–4.5 μm with data from TESS, HST WFC3/G141, and Spitzer 4.5 μm channel. KELT-20b has a 1.4 μm water feature strength metric of \({{\rm{S}}}_{{{\rm{H}}}_{2}{\rm{O}}}\) = −0.097 ± 0.02 and a blackbody brightness temperature difference of 528 K between WFC3/G141 (T b = 2402 ± 14 K) and Spitzer 4.5 μm channel (T b = 2930 ± 59 K). These very large H2O and CO emission features combined with the A-type host star make KELT-20b a unique planet among other similar hot Jupiters. The abundant FUV, NUV, and optical radiation from its host star (Teff = 8720 ± 250 K) is expected to be the key that drives its strong thermal inversion and prominent emission features based on previous PHOENIX model calculations.
Abstract
We present a comprehensive analysis of the 0.3–5
μ
m transit spectrum for the inflated hot Jupiter HAT-P-41b. The planet was observed in transit with Hubble STIS and WFC3 as part of the ...Hubble Panchromatic Comparative Exoplanet Treasury (PanCET) program, and we combine those data with warm Spitzer transit observations. We extract transit depths from each of the data sets, presenting the STIS transit spectrum (0.29–0.93
μ
m) for the first time. We retrieve the transit spectrum both with a free-chemistry retrieval suite (AURA) and a complementary chemical equilibrium retrieval suite (PLATON) to constrain the atmospheric properties at the day–night terminator. Both methods provide an excellent fit to the observed spectrum. Both AURA and PLATON retrieve a metal-rich atmosphere for almost all model assumptions (most likely O/H ratio of
and
, respectively); this is driven by a 4.9
σ
detection of H
2
O as well as evidence of gas absorption in the optical (>2.7
σ
detection) due to Na, AlO, and/or VO/TiO, though no individual species is strongly detected. Both retrievals determine the transit spectrum to be consistent with a clear atmosphere, with no evidence of haze or high-altitude clouds. Interior modeling constraints on the maximum atmospheric metallicity (
) favor the AURA results. The inferred elemental oxygen abundance suggests that HAT-P-41b has one of the most metal-rich atmospheres of any hot Jupiters known to date. Overall, the inferred high metallicity and high inflation make HAT-P-41b an interesting test case for planet formation theories.
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