Ultra-hot rocky super-Earths are thought to be sufficiently irradiated by their host star to melt their surface and allow for long-lasting magma oceans as a result. A number of processes have been ...proposed to explain how such planets may have retained the primordial hydrogen captured during their formation, while moving inward in the planetary system. The new generation of space telescopes such as the James Webb Space Telescope may provide observations that are precise enough to characterize the atmospheres and perhaps the interiors of such exoplanets. We used a vaporization model that calculates the gas-liquid equilibrium between the atmosphere (including hydrogen) and the magma ocean to compute the elemental composition of a variety of atmospheres with different quantities of hydrogen. We then used the elemental composition in a steady-state atmospheric model (ATMO) to compute the atmospheric structure and generate synthetic emission spectra. With this method, we were able to confirm previous results showing that silicate atmospheres exhibit a thermal inversion, with a notable emission peak of SiO at 9 μm. We compared our method to the literature on the inclusion of hydrogen in the atmosphere to show that hydrogen reduces the thermal inversion because of the formation of H 2 O, which has a strong greenhouse potential. However, planets that are significantly irradiated by their host star are sufficiently hot to dissociate H 2 O, thus also allowing them to maintain a thermal inversion. The observational implications are twofold: (1) H 2 O is more likely to be detected in colder atmospheres and (2) detecting a thermal inversion in hotter atmospheres does not a priori exclude the presence of H (in its atomic form). Due to the impact of H on the overall chemistry and atmospheric structure (and, thus, observations), we emphasize the importance of including volatiles in the calculation of the gas-liquid equilibrium. Finally, we provide a criterion to determine potential targets for observation in light of these findings.
The probability distribution function of column density (N-PDF) serves as a powerful tool to characterise the various physical processes that influence the structure of molecular clouds. Studies that ...use extinction maps or H2 column-density maps (N) that are derived from dust show that star-forming clouds can best be characterised by lognormal PDFs for the lower N range and a power-law tail for higher N, which is commonly attributed to turbulence and self-gravity and/or pressure, respectively. The slopes of the power-law tails of the CS, N2H+, and dust PDFs are -1.6, -1.4, and -2.3, respectively, and are thus consistent with free-fall collapse of filaments and clumps. A quasi static configuration of filaments and clumps can also possibly account for the observed N-PDFs, providing they have a sufficiently condensed density structure and external ram pressure by gas accretion is provided. The somehow flatter slopes of N2H+ and CS can reflect an abundance change and/or subthermal excitation at low column densities.
Carbon dioxide (CO
) is a key chemical species that is found in a wide range of planetary atmospheres. In the context of exoplanets, CO
is an indicator of the metal enrichment (that is, elements ...heavier than helium, also called 'metallicity')
, and thus the formation processes of the primary atmospheres of hot gas giants
. It is also one of the most promising species to detect in the secondary atmospheres of terrestrial exoplanets
. Previous photometric measurements of transiting planets with the Spitzer Space Telescope have given hints of the presence of CO
, but have not yielded definitive detections owing to the lack of unambiguous spectroscopic identification
. Here we present the detection of CO
in the atmosphere of the gas giant exoplanet WASP-39b from transmission spectroscopy observations obtained with JWST as part of the Early Release Science programme
. The data used in this study span 3.0-5.5 micrometres in wavelength and show a prominent CO
absorption feature at 4.3 micrometres (26-sigma significance). The overall spectrum is well matched by one-dimensional, ten-times solar metallicity models that assume radiative-convective-thermochemical equilibrium and have moderate cloud opacity. These models predict that the atmosphere should have water, carbon monoxide and hydrogen sulfide in addition to CO
, but little methane. Furthermore, we also tentatively detect a small absorption feature near 4.0 micrometres that is not reproduced by these models.
The Saturn-mass exoplanet WASP-39b has been the subject of extensive efforts to determine its atmospheric properties using transmission spectroscopy
. However, these efforts have been hampered by ...modelling degeneracies between composition and cloud properties that are caused by limited data quality
. Here we present the transmission spectrum of WASP-39b obtained using the Single-Object Slitless Spectroscopy (SOSS) mode of the Near Infrared Imager and Slitless Spectrograph (NIRISS) instrument on the JWST. This spectrum spans 0.6-2.8 μm in wavelength and shows several water-absorption bands, the potassium resonance doublet and signatures of clouds. The precision and broad wavelength coverage of NIRISS/SOSS allows us to break model degeneracies between cloud properties and the atmospheric composition of WASP-39b, favouring a heavy-element enhancement ('metallicity') of about 10-30 times the solar value, a sub-solar carbon-to-oxygen (C/O) ratio and a solar-to-super-solar potassium-to-oxygen (K/O) ratio. The observations are also best explained by wavelength-dependent, non-grey clouds with inhomogeneous coverageof the planet's terminator.
Transmission spectroscopy
of exoplanets has revealed signatures of water vapour, aerosols and alkali metals in a few dozen exoplanet atmospheres
. However, these previous inferences with the Hubble ...and Spitzer Space Telescopes were hindered by the observations' relatively narrow wavelength range and spectral resolving power, which precluded the unambiguous identification of other chemical species-in particular the primary carbon-bearing molecules
. Here we report a broad-wavelength 0.5-5.5 µm atmospheric transmission spectrum of WASP-39b
, a 1,200 K, roughly Saturn-mass, Jupiter-radius exoplanet, measured with the JWST NIRSpec's PRISM mode
as part of the JWST Transiting Exoplanet Community Early Release Science Team Program
. We robustly detect several chemical species at high significance, including Na (19σ), H
O (33σ), CO
(28σ) and CO (7σ). The non-detection of CH
, combined with a strong CO
feature, favours atmospheric models with a super-solar atmospheric metallicity. An unanticipated absorption feature at 4 µm is best explained by SO
(2.7σ), which could be a tracer of atmospheric photochemistry. These observations demonstrate JWST's sensitivity to a rich diversity of exoplanet compositions and chemical processes.
Context.
Ultrahot (>1500 K) rocky exoplanets may be covered by a magma ocean, resulting in the formation of a vapor rich in rocky components (e.g., Mg, Si, Fe) with a low total pressure and high ...molecular mass. However, exoplanets may have also captured a significant amount of hydrogen from the nebular gas during their formation. Ultrahot rocky exoplanets around the Fulton gap (~1.8
R
⊕
) are sufficiently large to have retained some fraction of their primordial hydrogen atmosphere.
Aims.
Here, we investigate how small amounts of hydrogen (much smaller than the mass of the planet) above a magma ocean may modify the atmospheric chemistry and its tendency to thermally escape.
Methods.
We use a chemical model of a magma ocean coupled to a gas equilibrium code (that includes hydrogen) to compute the atmospheric composition at thermodynamical equilibrium for various H contents and temperatures. An energy-limited model is used to compute atmospheric escape and is scaled to consider H-rich and H-poor atmospheres.
Results.
The composition of the vapor above a magma ocean is drastically modified by hydrogen, even for very modest amounts of H (≪10
−6
planetary mass). Hydrogen consumes much of the O
2
(g), which, in turn, promotes the evaporation of metals and metal oxides (SiO, Mg, Na, K, Fe) from the magma ocean. Vast amounts of H
2
O are produced by the same process. At high hydrogen pressures, new hydrogenated species such as SiH
4
form in the atmosphere. In all cases, H, H
2
, and H
2
O are the dominant nonmetal-bearing volatile species. Sodium is the dominant atmospheric metal-bearing species at
T <
2000 K and low H content, whereas Fe is dominant at high H content and low temperature, while SiO predominates at
T
> 3000 K. We find that the atmospheric Mg/Fe, Mg/Si, and Na/Si ratios deviate from those in the underlying planet and from the stellar composition. As such, their determination may constrain the planet’s mantle composition and H content. As the presence of hydrogen promotes the evaporation of silicate mantles, it is conceivable that some high-density, irradiated exoplanets may have started life as hydrogen-bearing planets and that part of their silicate mantle evaporated (up to a few 10% of Si, O, and Fe) and was subsequently lost owing to the reducing role of H.
Conclusions.
Even very small amounts of H can alter the atmospheric composition and promote the evaporation to space of heavy species derived from the molten silicate mantle of rocky planets. Through transit spectroscopy, the measurement of certain elemental ratios, along with the detection of atmospheric water or hydrogen, may help to determine the nature of a surface magma ocean.
We present an analysis of Spitzer/Infrared Array Camera primary transit and secondary eclipse light curves measured for HD 209458b, using Gaussian process models to marginalize over the intrapixel ...sensitivity variations in the 3.6 and 4.5 μm channels and the ramp effect in the 5.8 and 8.0 μm channels. The main advantage of this approach is that we can account for a broad range of degeneracies between the planet signal and systematics without actually having to specify a deterministic functional form for the latter. Our results do not confirm a previous claim of water absorption in transmission. Instead, our results are more consistent with a featureless transmission spectrum, possibly due to a cloud deck obscuring molecular absorption bands. For the emission data, our values are not consistent with the thermal inversion in the dayside atmosphere that was originally inferred from these data. Instead, we agree with another re-analysis of these same data, which concluded a non-inverted atmosphere provides a better fit. We find that a solar-abundance clear-atmosphere model without a thermal inversion underpredicts the measured emission in the 4.5 μm channel, which may suggest the atmosphere is depleted in carbon monoxide. An acceptable fit to the emission data can be achieved by assuming that the planet radiates as an isothermal blackbody with a temperature of 1484 ± 18 K.
Abstract
Photochemistry is a fundamental process of planetary atmospheres that regulates the atmospheric composition and stability
1
. However, no unambiguous photochemical products have been ...detected in exoplanet atmospheres so far. Recent observations from the JWST Transiting Exoplanet Community Early Release Science Program
2,3
found a spectral absorption feature at 4.05 μm arising from sulfur dioxide (SO
2
) in the atmosphere of WASP-39b. WASP-39b is a 1.27-Jupiter-radii, Saturn-mass (0.28
M
J
) gas giant exoplanet orbiting a Sun-like star with an equilibrium temperature of around 1,100 K (ref.
4
). The most plausible way of generating SO
2
in such an atmosphere is through photochemical processes
5,6
. Here we show that the SO
2
distribution computed by a suite of photochemical models robustly explains the 4.05-μm spectral feature identified by JWST transmission observations
7
with NIRSpec PRISM (2.7
σ
)
8
and G395H (4.5
σ
)
9
. SO
2
is produced by successive oxidation of sulfur radicals freed when hydrogen sulfide (H
2
S) is destroyed. The sensitivity of the SO
2
feature to the enrichment of the atmosphere by heavy elements (metallicity) suggests that it can be used as a tracer of atmospheric properties, with WASP-39b exhibiting an inferred metallicity of about 10× solar. We further point out that SO
2
also shows observable features at ultraviolet and thermal infrared wavelengths not available from the existing observations.
We present simultaneous 0.65–2.5 μm medium resolution (3300 ≤
R
λ
≤ 8100) VLT/X-shooter spectra of the relatively young (150–300 Myr) low-mass (19 ± 5
M
Jup
) L–T transition object VHS 1256−1257 b, ...a known spectroscopic analog of HR8799d. The companion is a prime target for the JWST Early Release Science (ERS) and one of the highest-amplitude variable brown dwarfs known to date. We compare the spectrum to the custom grids of cloudless ATMO models, exploring the atmospheric composition with the Bayesian inference tool
ForMoSA
. We also reanalyze low-resolution HST/WFC3 1.10–1.67 μm spectra at minimum and maximum variability to contextualize the X-shooter data interpretation. The models reproduce the slope and most molecular absorption from 1.10 to 2.48 μm self-consistently, but they fail to provide a radius and a surface gravity consistent with evolutionary model predictions. They do not reproduce the optical spectrum and the depth of the K I doublets in the J band consistently. We derived
T
eff
= 1380±54 K, log(
g
) = 3.97±0.48 dex, M/H = 0.21±0.29, and C/O > 0.63. Our inversion of the HST/WFC3 spectra suggests a relative change of $ 27^{+6}_{-5} $ K of the disk-integrated
T
eff
correlated with the near-infrared brightness. Our data anchor the characterization of that object in the near-infrared and could be used jointly to the ERS mid-infrared data to provide the most detailed characterization of an ultracool dwarf to date.
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