[en] The transmission spectra of ultra-hot Jupiters observed shortward of 0.5 μm indicate strong absorption. Previous explanations have included scattering, photochemistry, escaping metals, and disequilibrium chemistry. In this Letter, we show that slopes and features shortward of 0.5 μm can be caused by opacity not commonly considered in atmosphere models of exoplanets but guaranteed to be present if conditions are near chemical equilibrium including, but not limited to, atoms and ions of Fe, Ti, Ni, Ca, Cr, Mn, and SiO. Using the PHOENIX atmosphere model, we describe how the short-wavelength transit spectrum varies with equilibrium temperature between 1000 K and 4000 K, as well as the effect that the rainout of condensates has at these wavelengths. We define two spectral indices to quantify the strength of the NUV and blue absorption compared to that in the red-optical, finding that the NUV transit depth will significantly exceed the transit depth from Rayleigh scattering alone for all hot Jupiters down to around 1000 K. In the blue-optical, hot Jupiters warmer than 2000 K will have transit depths larger than that from Rayleigh scattering, but below 2000 K, Rayleigh scattering can dominate, if present. We further show that these spectral indices may be used to trace the effects of rainout. We then compare our simulated transit spectra to existing observations of WASP-12b, WASP-33b, WASP-76b, and WASP-121b. Further observation of exoplanets at these wavelengths should be prioritized in the coming years as the Hubble Space Telescope nears the end of its operational capability.

[en] Water, methane, and carbon monoxide are expected to be among the most abundant molecules besides molecular hydrogen in the hot atmosphere of close-in extrasolar giant planets. Atmospheric models for these planets predict that the strongest spectrophotometric features of those molecules are located at wavelengths ranging from 1 to 10 μm making this region of particular interest. Consequently, transit observations in the mid-infrared (mid-IR) allow the atmospheric content of transiting planets to be determined. We present new primary transit observations of the hot-Jupiter HD 189733b, obtained simultaneously at 4.5 and 8 μm with the Infrared Array Camera onboard the Spitzer Space Telescope. Together with a new refined analysis of previous observations at 3.6 and 5.8 μm using the same instrument, we are able to derive the system parameters, including planet-to-star radius ratio, impact parameter, scale of the system, and central time of the transit from fits of the transit light curves at these four wavelengths. We measure the four planet-to-star radius ratios, to be (R_p /R _*)_3.6μm = 0.1545 ± 0.0003, (R_p /R_*)_4.5μm = 0.1557 ± 0.0003, (R_p /R _*)_5.8μm = 0.1547 ± 0.0005, and (R_p/R_*)_8μm = 0.1544 ± 0.0004. The high accuracy of the planet radii measurement allows the search for atmospheric molecular absorbers. Contrary to a previous analysis of the same data set, our study is robust against systematics and reveals that water vapor absorption at 5.8 μm is not detected in this photometric data set. Furthermore, in the band centered around 4.5 μm we find a hint of excess absorption with an apparent planetary radius ΔR_p /R _* = 0.00128 ± 0.00056 larger (2.3σ) than the one measured simultaneously at 8 μm. This value is 4σ above what would be expected for an atmosphere where water vapor is the only absorbing species in the near-IR. This shows that an additional species absorbing around 4.5 μm could be present in the atmosphere. Carbon monoxide (CO) being a strong absorber at this wavelength is a possible candidate and this may suggest a large CO/H₂O ratio between 5 and 60.

[en] We present a theory for interpreting the sodium lines detected in transmission spectra of exoplanetary atmospheres. Previous analyses employed the isothermal approximation and dealt only with the transit radius. By recognizing the absorption depth and the transit radius as being independent observables, we develop a theory for jointly interpreting both quantities, which allows us to infer the temperatures and number densities associated with the sodium lines. We are able to treat a non-isothermal situation with a constant temperature gradient. Our novel diagnostics take the form of simple-to-use algebraic formulae and require measurements of the transit radii (and their corresponding absorption depths) at line center and in the line wing for both sodium lines. We apply our diagnostics to the HARPS data of HD 189733b, confirm the upper atmospheric heating reported by Huitson et al., derive a temperature gradient of 0.4376 ± 0.0154 K km"−"1, and find densities ∼1–10"4 cm"−"3

[en] A primary goal of exoplanet characterization is to use a planet’s current composition to understand how that planet formed. For example, the C/O ratio has long been recognized as carrying important information on the chemistry of volatile species. Refractory elements, like Fe, Mg, and Si, are usually not considered in this conversation because they condense into solids like Fe(s) or MgSiO₃ and would be removed from the observable, gaseous atmosphere in exoplanets cooler than about 2000 K. However, planets hotter than about 2000 K, called ultra-hot Jupiters (UHJs), are warm enough to largely avoid the condensation of refractory species. In this paper, we explore the insight that the measurement of refractory abundances can provide into a planet’s origins. Through refractory-to-volatile elemental abundance ratios, we can estimate a planet’s atmospheric rock-to-ice fraction and constrain planet formation and migration scenarios. We first relate a planet’s present-day refractory-to-volatile ratio to its rock-to-ice ratio from formation using various compositional models for the rocky and icy components of the protoplanetary disk. We discuss potential confounding factors like the sequestration of heavy metals in the core and condensation. We then show such a measurement using atmospheric retrievals of the low-resolution UV-IR transmission spectrum of WASP-121b with PETRA, from which we estimate a refractory-to-volatile ratio of 5.0${}_{-<!--\; ???\; -->2.7}^{+6.0}\times <!--\; ??\; -->$ solar and a rock-to-ice ratio greater than 2/3. This result is consistent with significant atmospheric enrichment by rocky planetismals. Lastly, we discuss the rich future potential for measuring refractory-to-volatile ratios in UHJs with the arrival of the James Webb Space Telescope and by combining observations at low and high resolution.

[en] We present a primary transit observation for the ultra-hot ( T _e_q ∼ 2400 K) gas giant expolanet WASP-121b, made using the Hubble Space Telescope Wide Field Camera 3 in spectroscopic mode across the 1.12–1.64 μ m wavelength range. The 1.4 μ m water absorption band is detected at high confidence (5.4 σ ) in the planetary atmosphere. We also reanalyze ground-based photometric light curves taken in the B , r ′, and z ′ filters. Significantly deeper transits are measured in these optical bandpasses relative to the near-infrared wavelengths. We conclude that scattering by high-altitude haze alone is unlikely to account for this difference and instead interpret it as evidence for titanium oxide and vanadium oxide absorption. Enhanced opacity is also inferred across the 1.12–1.3 μ m wavelength range, possibly due to iron hydride absorption. If confirmed, WASP-121b will be the first exoplanet with titanium oxide, vanadium oxide, and iron hydride detected in transmission. The latter are important species in M/L dwarfs and their presence is likely to have a significant effect on the overall physics and chemistry of the atmosphere, including the production of a strong thermal inversion.

[en] We report the detection in Ks-band of the secondary eclipse of the hot Jupiter CoRoT-1b from time series photometry with the ARC 3.5 m telescope at Apache Point Observatory. The eclipse shows a depth of 0.336 ± 0.042% and is centered at phase 0.5022^+0.0023_-0.0027, consistent with a zero eccentricity orbit (e cos ω = 0.0035^+0.0036_-0.0042). We perform the first optical to near-infrared multi-band photometric analysis of an exoplanet's atmosphere and constrain the reflected and thermal emissions by combining our result with the recent 0.6, 0.71, and 2.09 μm secondary eclipse detections by Snellen et al., Gillon et al., and Alonso et al. Comparing the multi-wavelength detections to state-of-the-art radiative-convective chemical-equilibrium atmosphere models, we find the near-infrared fluxes difficult to reproduce. The closest blackbody-based and physical models provide the following atmosphere parameters: a temperature T = 2460⁺⁸⁰_-160 K; a very low Bond albedo A_B = 0.000^+0.081_-0.000; and an energy redistribution parameter P_n = 0.1, indicating a small but nonzero amount of heat transfer from the day to nightside. The best physical model suggests a thermal inversion layer with an extra optical absorber of opacity κ_e = 0.05 cm² g^-1, placed near the 0.1 bar atmospheric pressure level. This inversion layer is located 10 times deeper in the atmosphere than the absorbers used in models to fit mid-infrared Spitzer detections of other irradiated hot Jupiters.

[en] We present transmission spectroscopy of the warm Saturn-mass exoplanet WASP-39b made with the Very Large Telescope FOcal Reducer and Spectrograph (FORS2) across the wavelength range 411–810 nm. The transit depth is measured with a typical precision of 240 parts per million (ppm) in wavelength bins of 10 nm on a V  = 12.1 mag star. We detect the sodium absorption feature (3.2 σ ) and find evidence of potassium. The ground-based transmission spectrum is consistent with Hubble Space Telescope ( HST ) optical spectroscopy, supporting the interpretation that WASP-39b has a largely clear atmosphere. Our results demonstrate the great potential of the recently upgraded FORS2 spectrograph for optical transmission spectroscopy, with which we obtained HST -quality light curves from the ground.

[en] We present results from an atmospheric circulation study of nine hot Jupiters that compose a large transmission spectral survey using the Hubble and Spitzer Space Telescopes. These observations exhibit a range of spectral behavior over optical and infrared wavelengths, suggesting diverse cloud and haze properties in their atmospheres. By utilizing the specific system parameters for each planet, we naturally probe a wide phase space in planet radius, gravity, orbital period, and equilibrium temperature. First, we show that our model “grid” recovers trends shown in traditional parametric studies of hot Jupiters, particularly equatorial superrotation and increased day–night temperature contrast with increasing equilibrium temperature. We show how spatial temperature variations, particularly between the dayside and nightside and west and east terminators, can vary by hundreds of kelvin, which could imply large variations in Na, K, CO and ${\mathrm{C}\mathrm{H}}_4$ abundances in those regions. These chemical variations can be large enough to be observed in transmission with high-resolution spectrographs, such as ESPRESSO on VLT, METIS on the E-ELT, or MIRI and NIRSpec aboard JWST. We also compare theoretical emission spectra generated from our models to available Spitzer eclipse depths for each planet and find that the outputs from our solar-metallicity, cloud-free models generally provide a good match to many of the data sets, even without additional model tuning. Although these models are cloud-free, we can use their results to understand the chemistry and dynamics that drive cloud formation in their atmospheres.

[en] We report the detection of the eclipse of the very hot Jupiter WASP-12b via z'-band time-series photometry obtained with the 3.5 m Astrophysical Research Consortium telescope at Apache Point Observatory. We measure a decrease in flux of 0.082% ± 0.015% during the passage of the planet behind the star. That planetary flux is equally well reproduced by atmospheric models with and without extra absorbers, and blackbody models with f ≥ 0.585 ± 0.080. It is therefore necessary to measure the planet at other wavelengths to further constrain its atmospheric properties. The eclipse appears centered at phase φ = 0.5100^+0.0072_-0.0061, consistent with an orbital eccentricity of |ecos ω| = 0.016^+0.011_-0.009 (see note at the end of Section 4). If the orbit of the planet is indeed eccentric, the large radius of WASP-12b can be explained by tidal heating.

[en] We present a secondary eclipse observation for the hot Jupiter HD 189733b across the wavelength range 290-570 nm made using the Space Telescope Imaging Spectrograph on the Hubble Space Telescope. We measure geometric albedos of A_g = 0.40 ± 0.12 across 290-450 nm and A_g < 0.12 across 450-570 nm at 1σ confidence. The albedo decrease toward longer wavelengths is also apparent when using six wavelength bins over the same wavelength range. This can be interpreted as evidence for optically thick reflective clouds on the dayside hemisphere with sodium absorption suppressing the scattered light signal beyond ∼450 nm. Our best-fit albedo values imply that HD 189733b would appear a deep blue color at visible wavelengths