H2O abundances and cloud properties in ten hot giant exoplanets

arXiv (Cornell University), 2018

There has been increasing progress toward detailed characterization of exoplanetary atmospheres, in both observations and theoretical methods. Improvements in observational facilities and data reduction and analysis techniques are enabling increasingly higher quality spectra, especially from ground-based facilities. The high data quality also necessitates concomitant improvements in models required to interpret such data. In particular, the detection of trace species such as metal oxides has been challenging. Extremely irradiated exoplanets (∼3000 K) are expected to show oxides with strong absorption signals in the optical. However, there are only a few hot Jupiters where such signatures have been reported. Here we aim to characterize the atmosphere of the ultra-hot Jupiter WASP-33 b using two primary transits taken 18 orbits apart. Our atmospheric retrieval, performed on the combined data sets, provides initial constraints on the atmospheric composition of WASP-33 b. We report a possible indication of aluminum oxide (AlO) at 3.3-σ significance. The data were obtained with the long slit OSIRIS spectrograph mounted at the 10-m Gran Telescopio Canarias. We cleaned the brightness variations from the light curves produced by stellar pulsations, and we determined the wavelength-dependent variability of the planetary radius caused by the atmospheric absorption of stellar light. A simultaneous fit to the two transit light curves allowed us to refine the transit parameters, and the common wavelength coverage between the two transits served to contrast our results. Future observations with HST as well as other large ground-based facilities will be able to further constrain the atmospheric chemical composition of the planet.

Icarus, 2003

This paper presents the first detailed analysis of acetylene absorption features observed longward of 190.0 nm in a jovian spectrum by the Faint Object Spectrograph on board the Hubble Space Telescope. The presence of two features located near 207.0 nm can only be explained by a substantial abundance of acetylene in the upper troposphere. Using a Rayleigh-Raman radiative transfer model, it was determined that the acetylene vertical profile is characterized by a decrease in the mole fraction with increasing pressure in the upper stratosphere, a minimum around 14 to 29 mbar, followed by an increase to about 1.5 ϫ 10 Ϫ7 in the upper troposphere. Longward of 220 nm, the relatively high contrast of Raman features to the continuum precludes the existence of an optically significant amount of clouds from 150 to 500 mbar unless they are highly reflective. Instead, the reflectivity at these long wavelengths is determined by stratospheric, not tropospheric, scatterers and absorbers. Analysis of the data also suggests that ammonia is extremely undersaturated at pressures below 700 mbar. However, no firm conclusions can be reached because of the uncertainties surrounding its cross section longward of 217.0 nm, which are due to vibrationally excited states.

Proceedings of The International Astronomical Union, 2006

Over 150 extrasolar planets are known to orbit sun-like stars. A growing number of them (9 to date) are transiting "hot Jupiters" whose physical characteristics can be measured. Atmospheres of two of these planets have already been detected. We summarize the atmosphere detections and useful upper limits, focusing on the MOST albedo upper limit and H exosphere detection for HD209458b as the most revelant for photochemical models. We describe our photochemical model for hot Jupiters and present a summary explanation of the main results: a low gas-phase abundance of hydrocarbons; an absence of hydrocarbon hazes; and a large reservoir of H atoms in the upper atmospheres of hot Jupiters. We conclude by relating these model results to the relevant observational data.

Bulletin of the AAS

Currently, the analysis of transmission spectra is the most successful technique to probe the chemical composition of exoplanet atmospheres. However, the accuracy of these measurements is constrained by observational limitations and the diversity of possible atmospheric compositions. Here, we show the UV–VIS–IR transmission spectrum of Jupiter as if it were a transiting exoplanet, obtained by observing one of its satellites, Ganymede, while passing through Jupiter's shadow, i.e., during a solar eclipse from Ganymede. The spectrum shows strong extinction due to the presence of clouds (aerosols) and haze in the atmosphere and strong absorption features from CH 4. More interestingly, the comparison with radiative transfer models reveals a spectral signature, which we attribute here to a Jupiter stratospheric layer of crystalline H 2 O ice. The atomic transitions of Na are also present. These results are relevant for the modeling and interpretation of giant transiting exoplanets. They also open a new technique to explore the atmospheric composition of the upper layers of Jupiter's atmosphere. Key words: eclipses – planets and satellites: atmospheres – planets and satellites: gaseous planets – planets and satellites: individual (Jupiter, Ganymede)

2012

Context. The atmosphere of hot Jupiters can be probed by primary transit and secondary eclipse spectroscopy. Owing to the intense UV irradiation, mixing, and circulation, their chemical composition is maintained out of equilibrium and must be modeled with kinetic models. Aims. Our purpose is to release a chemical network and the associated rate coefficients, developed for the temperature and pressure range relevant to hot Jupiters atmospheres. Using this network, we study the vertical atmospheric composition of the two hot Jupiters (HD 209458b and HD 189733b) with a model that includes photolyses and vertical mixing, and we produce synthetic spectra. Methods. The chemical scheme has been derived from applied combustion models that were methodically validated over a range of temperatures and pressures typical of the atmospheric layers influencing the observations of hot Jupiters.We compared the predictions obtained from this scheme with equilibrium calculations, with different schemes available in the literature that contain N-bearing species, and with previously published photochemical models. Results. Compared to other chemical schemes that were not subjected to the same systematic validation, we find significant differences whenever nonequilibrium processes take place (photodissociations or vertical mixing). The deviations from the equilibrium, hence the sensitivity to the network, are larger for HD 189733b, since we assume a cooler atmosphere than for HD 209458b. We found that the abundances of NH3 and HCN can vary by two orders of magnitude depending on the network, demonstrating the importance of comprehensive experimental validation. A spectral feature of NH3 at 10.5 μm is sensitive to these abundance variations and thus to the chemical scheme. Conclusions. Due to the influence of the kinetics, we recommend using a validated scheme to model the chemistry of exoplanet atmospheres. The network we release is robust for temperatures within 300–2500 K and pressures from 10 mbar up to a few hundred bars, for species made of C, H, O, and N. It is validated for species up to 2 carbon atoms and for the main nitrogen species (NH3, HCN, N2, NOx). Although the influence of the kinetic scheme on the hot Jupiters spectra remains within the current observational error bars (with the exception of NH3), it will become more important for atmospheres that are cooler or subjected to higher UV fluxes, because they depart more from equilibrium.

Space Science Reviews, 2005

In seeking to understand the formation of the giant planets and the origin of their atmospheres, the heavy element abundance in well-mixed atmosphere is key. However, clouds come in the way. Thus, composition and condensation are intimately intertwined with the mystery of planetary formation and atmospheric origin. Clouds also provide important clues to dynamical processes in the atmosphere. In this chapter we discuss the thermochemical processes that determine the composition, structure, and characteristics of the Jovian clouds. We also discuss the significance of clouds in the big picture of the formation of giant planets and their atmospheres. We recommend multiprobes at all four giant planets in order to break new ground.

The Astrophysical Journal, 2007

Among the hot Jupiters that transit their parent stars known to date, the two best candidates to be observed with transmission spectroscopy in the mid-infrared (MIR) are HD189733b and HD209458b, due to their combined characteristics of planetary density, orbital parameters and parent star distance and brightness. Here we simulate transmission spectra of these two planets during their primary eclipse in the MIR, and we present sensitivity studies of the spectra to the changes of atmospheric thermal properties, molecular abundances and C/O ratios. Our model predicts that the dominant species absorbing in the MIR on hot Jupiters are water vapor and carbon monoxide, and their relative abundances are determined by the C/O ratio. Since the temperature profile plays a secondary role in the transmission spectra of hot Jupiters compared to molecular abundances, future primary eclipse observations in the MIR of those objects might give an insight on EGP atmospheric chemistry. We find here that the absorption features caused by water vapor and carbon monoxide in a cloud-free atmosphere, are deep enough to be observable by the present and future generation of space-based observatories, such as Spitzer Space Telescope and James Webb Space Telescope. We discuss our results in light of the capabilities of these telescopes.

Proceedings of the International Astronomical Union, 2008

The two prototype hot-Jupiter exoplanets HD209458b and HD189733b are currently offering an unprecedented view of their atmospheres. As discussed here, primary transit transmission spectra provide the opportunity to identify specific atomic and molecular species, determine their abundances, and recover temperature-pressure-altitude information. We present a reanalysis of existing HST/STIS data on HD209458b, providing a complete optical transmission spectrum. Analysis of this spectrum have revealed: (1) the planetary abundance of sodium which is ~2X solar (2) a depletion of sodium at high altitudes due to condensation or ionization (3) Rayleigh scattering by H2 (3) a high temperature at pressures of 10's mbar consistent with the dayside inversion (4) a separate high-altitude hot temperature from the…

Cornell University - arXiv, 2016

Advancements in our understanding of exoplanetary atmospheres, from massive gas giants down to rocky worlds, depend on the constructive challenges that occur between observations and models. Both from the ground and from space, we are now on a clear trajectory for improvements in exoplanet observations that will revolutionize our ability to characterize the atmospheric structure, composition, and circulation of these worlds. These improvements stem from significant investments by numerous funding agencies in new missions and facilities, such as the James Webb Space Telescope (JWST) and the several planned ground-based extremely large telescopes. However, while exoplanet science currently has a wide range of sophisticated models that can be applied to the tide of forthcoming observations, the trajectory for preparing these models for the upcoming observational challenges is unclear. Thus, our ability to maximize the insights gained from the next generation of ground-and space-based observatories is not certain. In many cases, uncertainties in a path towards model advancement stems from insufficiencies in the laboratory data that serve as critical inputs to atmospheric physical and chemical tools. This white paper was initiated within NASA's Nexus for Exoplanet System Science (NExSS), in collaboration with the larger exoplanet science community, to review the state of this science. We outline a number of areas where laboratory or ab initio investigations could fill critical gaps in our ability to model exoplanet atmospheric opacities, clouds, and chemistry. Specifically highlighted are needs for: (1) molecular opacity linelists with parameters for a diversity of broadening gases, (2) extended databases for collision-induced absorption and dimer opacities, (3) high spectral resolution opacity data for a variety of relevant molecular species, (4) laboratory studies of haze and condensate formation and optical properties, (5) significantly expanded databases of chemical reaction rates, and (6) measurements of gas photoabsorption cross sections at high temperatures. We hope that by meeting these needs, we can make the next two decades of exoplanet science as productive and insightful as the previous two decades.

The Astrophysical Journal, 2014

The relative abundances of carbon and oxygen have long been recognized as fundamental diagnostics of stellar chemical evolution. Now, the growing number of exoplanet observations enable estimation of these elements in exoplanetary atmospheres. In hot Jupiters, the C/O ratio affects the partitioning of carbon in the major observable molecules, making these elements diagnostic of temperature structure and composition.

The Astrophysical Journal, 2013

Secondary eclipse spectroscopy provides invaluable insights into the temperatures and compositions of exoplanetary atmospheres. We carry out a systematic temperature and abundance retrieval analysis of nine exoplanets (HD189733b, HD209458b, HD149026b, GJ436b, WASP-12b, WASP-19b, WASP-43b, TrES-2b, and TrES-3b) observed in secondary eclipse using a combination of space-and ground-based facilities. Our goal with this analysis is to provide a consistent set of temperatures and compositions from which self-consistent models can be compared and to probe the underlying processes that shape these atmospheres. This paper is the second in a three part series of papers exploring the retrievability of temperatures and abundances from secondary eclipse spectra and the implications of these results for the chemistry of exoplanet atmospheres. In this investigation we present a catalogue of temperatures and abundances for H 2 O, CH 4 , CO, and CO 2 . We find that our temperatures and abundances are generally consistent with those of previous studies, although we do not find any statistically convincing evidence for super-solar C to O ratios (e.g., solar C/O falls in the 1-sigma confidence intervals in eight of the nine planets in our sample). Furthermore, within our sample we find little evidence for thermal inversions over a wide range of effective temperatures (with the exception of HD209458b), consistent with previous investigations. The lack of evidence for inversions for most planets in our sample over such a wide range of effective temperatures provides additional support for the hypothesis that TiO is unlikely to be the absorber responsible for the formation of these inversions.

The Astrophysical Journal, 2016

We present a comprehensive study of the abundance of carbon dioxide in exoplanetary atmospheres in hot, hydrogen-dominated atmospheres. We construct novel analytical models of systems in chemical equilibrium that include carbon monoxide, carbon dioxide, water, methane and acetylene and relate the equilibrium constants of the chemical reactions to temperature and pressure via the tabulated Gibbs free energies. We prove that such chemical systems may be described by a quintic equation for the mixing ratio of methane. By examining the abundances of these molecules across a broad range of temperatures (spanning equilibrium temperatures from 600 to 2500 K), pressures (via temperature-pressure profiles that explore albedo and opacity variations) and carbon-to-oxygen ratios, we conclude that carbon dioxide is subdominant compared to carbon monoxide and water. Atmospheric mixing does not alter this conclusion if carbon dioxide is subdominant everywhere in the atmosphere. Carbon dioxide and carbon monoxide may attain comparable abundances if the metallicity is greatly enhanced, but this property is negated by temperatures above 1000 K. For hydrogen-dominated atmospheres, our generic result has the implication that retrieval studies may wish to set the subdominance of carbon dioxide as a prior of the calculation and not let its abundance completely roam free as a fitting parameter, because it directly affects the inferred value of the carbon-to-oxygen ratio and may produce unphysical conclusions. We discuss the relevance of these implications for the hot Jupiter WASP-12b and suggest that some of the previous results are chemically impossible. The relative abundance of carbon dioxide to acetylene is potentially a sensitive diagnostic of the carbon-to-oxygen ratio.

arXiv (Cornell University), 2022

The transmission spectra of exoplanet atmospheres observed with the Hubble Space Telescope (HST) in the near-infrared range (1.1-1.65µm) frequently show evidence for some combination of clouds and hazes. Identification of systematic trends in exoplanet clouds and hazes is potentially important for understanding atmospheric composition and temperature structure. Here we report on the analysis of spectral modulation using a large, uniformly processed sample of HST/WFC3 transit spectra from 62 exoplanets. The spectral retrieval includes the capability to detect and represent atmospheres in which the composition departs from thermochemical equilibrium. By using this unique catalog and measuring the dampening of spectral modulations compared to strictly clear atmospheres, we identify two populations. One is completely cloud/haze free spanning a wide temperature range, while the other population, identified as "Partial cloud/hazes", follows a trend from mostly cloudy/hazy around 500 K to mostly clear at ∼1500 K. We also find that a partially transparent aerosol component is frequently present and that it is typically vertically distributed throughout the atmospheric column. Our findings also suggest that while clouds and hazes are common in exoplanet atmospheres, the majority of planets have some level of detectable spectral modulation. Additionally, the empirical trend that clouds and hazes are minimized at 1460.86K +316 −405 revealed in our catalog has predictive utility for modelling the performance of large-scale transiting exoplanets survey, such as planned with the Ariel mission. This trend can also be used for making a probability-based forecast of spectral modulation for a given source in the context of future JWST observations. Future observations including the optical and/or a broader spectral coverage may be useful to further quantify the trend reported here.

The Astrophysical Journal, 2004

The close-in extrasolar giant planets (CEGPs) reside in irradiated environments much more intense than that of the giant planets in our solar system. The high UV irradiance strongly influences their photochemistry and the general current view believed that this high UV flux will greatly enhance photochemical production of hydrocarbon aerosols. In this letter, we investigate hydrocarbon aerosol formation in the atmospheres of CEGPs. We find that the abundances of hydrocarbons in the atmospheres of CEGPs are significantly less than that of Jupiter except for models in which the CH 4 abundance is unreasonably high (as high as CO) for the hot (effective temperatures 1000 K) atmospheres. Moreover, the hydrocarbons will be condensed out to form aerosols only when the temperature-pressure profiles of the species intersect with the saturation profiles-a case almost certainly not realized in the hot CEGPs atmospheres. Hence our models show that photochemical hydrocarbon aerosols are insignificant in the atmospheres of CEGPs. In contrast, Jupiter and Saturn have a much higher abundance of hydrocarbon aerosols in their atmospheres which are responsible for strong absorption shortward of 600 nm. Thus the insignificance of photochemical hydrocarbon aerosols in the atmospheres of CEGPs rules out one class of models with low albedos and featureless spectra shortward of 600 nm.