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These deviations from the solar elemental composition, whatever their origin see Sect. Molecules with more than two heavy atoms are even more favored, as occurs in the case of CO 2 , whose abundance increases as the square of metallicity Zahnle et al. See more details on this subject in Sects. Whatever their origin see Sect. In moderately warm and hot carbon-rich atmospheres, water is no longer an abundant constituent, the C-bearing molecules C 2 H 2 , CH 4 , and HCN become major constituents, and CO 2 vanishes to a negligible level Madhusudhan ; Kopparapu et al.
The terms cloud and haze are often used interchangeably, but here we follow the spirit of Marley et al. The typical equilibrium condensates expected along the cloud-condensation sequence in a hydrogen-dominated atmosphere e. In the rainout scenario of cloud formation, a condensate forms and gravitationally settles in the atmospheric layer where the temperature equals its condensation temperature, and the atmosphere above the cloud becomes depleted in the elements that take part in that condensate Lewis ; Fegley and Lodders , ; Burrows and Sharp ; Lodders ; Lodders and Fegley ; Sudarsky et al.
Thus, as one moves from the deep and hot atmosphere to upper and cooler regions, elements are progressively removed from the gas phase according to their refractory character. Chemical equilibrium calculations including gas and condensed species are very useful to identify the most plausible cloud-forming species see Fig. In the rainout approach, chemical equilibrium is solved in order of decreasing temperature and, when a given species is found to condense the elements which take part in that condensate are removed in the corresponding stoichiometric proportions before continuing to solve chemical equilibrium at lower temperatures.
Thus, it is not only necessary to know the possible condensates but also the condensation sequence and the main condensates that deplete each element. Unlike the case of Jupiter, whose atmosphere is so cold that water and ammonia condense to form tropospheric clouds, most extrasolar giant planets characterized to date are sufficiently hot to ensure the survival of water vapor and other volatiles in their atmospheres.
In hot Jupiters only the most refractory species can condense to form clouds. Although the exact temperature-pressure profile of the planet in question controls which refractory species will condense first i. Therefore, atmospheres hotter than about K can maintain titanium in the gas phase to form oxides such as TiO, while cooler atmospheres would deplete most of this element in the form of perovskite or other Ca-titanate Burrows and Sharp ; Lodders In fact, it has been proposed that in the atmospheres of very hot giant exoplanets, the survival in the gas phase of species such as TiO and VO can provide a sufficiently high opacity at optical wavelengths as to induce a thermal inversion Hubeny et al.
The detections of TiO and VO in brown dwarf atmospheres at high temperatures Kirkpatrick provide support to this proposition. The presence of such type of clouds has been inferred from transit observations of the hot Jupiter HD b Pont et al. In sufficiently hot atmospheres neutral Na and K atoms can survive in the gas phase and can be readily detected through the Na I doublet at The rainout scenario provides a useful methodology to predict the composition of the clouds that may be present in a given atmosphere and the base level at which each cloud forms.
However, the computation of the size and concentration of particles above this level and the horizontal distribution of the clouds, i. Nevertheless, efforts to this end have been undertaken by various groups Ackerman and Marley ; Helling et al.
The photochemical formation of organic hazes in the atmospheres of hot Jupiters has been addressed in a couple of theoretical studies Liang et al. The general view provided by these studies is that in hot atmospheres, most of the carbon remains locked into CO, while in cooler atmospheres the larger amount of carbon stored into CH 4 can be photochemically driven to larger hydrocarbons, eventually producing hydrocarbon aerosols or soots.
Cooler planets would therefore be expected to have more prevalent photochemical hazes. This scenario agrees with the presence of hazes in the cold atmospheres of Jupiter, Saturn, and Titan e. It is clear that more work is needed in both the theoretical and observational sides to better understand the formation and role of hazes in extrasolar giant planets.
One of the most recent and formidable successes achieved in the field of exoplanets has been the detection by direct imaging of young and self-luminous gas giant planets, opening the way to characterize their atmospheres by direct spectroscopy Janson et al. The few planets that have been characterized by this method are more massive than Jupiter and have effective temperatures in the range — K.
Thus, they have some similarities with brown dwarfs and free-floating planets Zapatero Osorio et al. Directly imaged planets share also some characteristics with hot Jupiters, as both are gas giants and hot. However, unlike hot Jupiters, directly imaged planets orbit far from their host star and are young, so that they are heated predominantly from the interior rather than irradiated by the star. This difference leads to qualitative differences in the dynamical and thermal structure of the atmosphere.
Burrows et al. On the other hand, the atmospheres of self-luminous planets and brown dwarfs are largely driven by convection, leading to adiabatic temperature gradients even in the observable atmospheres. The atmospheres of hot Jupiters and directly imaged planets have temperatures of the same order and are expected to have a nearly solar elemental composition, so that one would expect a similar atmospheric chemistry in both types of planets. There are, however, a couple of major differences. First, the atmospheres of directly imaged planets are likely less affected by photochemistry than in the case of hot Jupiters because of their much larger orbital distances; however, directly imaged planets tend to orbit young stars, and young stars tend to have high ultraviolet output, so photochemistry will not be negligible on these planets.
And second, the source of heat being located in the interior of the planet rather than outside imprints differences in the atmospheric thermal profile. According to the recent theoretical study by Zahnle and Marley , the higher temperatures in the deep atmosphere of self-luminous planets favor CO over CH 4 in deep layers, but also in the upper observable atmosphere as a consequence of upward mixing.
Methane is therefore predicted to be a minor atmospheric constituent in most self-luminous planets characterized to date, except for a couple of planets with cool atmospheres whose near-infrared spectra show evidence for CH 4 absorption, such as GJ b Janson et al. Years of study of brown dwarf atmospheres see e. The same chemical processes discussed above for giant planets—thermochemical equilibrium, disequilibrium quenching due to transport, and disequilibrium photochemistry—also affect smaller planets, but the basic ingredients available to the atmospheres of smaller planets differ from those of gas giants.
In the gravitational-instability theory for giant-planet formation e. In the core-accretion theory of giant-planet formation e. Pollack et al. As the solid protoplanetary core grows, it can accrete more and more of the surrounding hydrogen-rich nebular gas Lammer et al. Venturini et al. Planets that reach this runaway gas-accretion stage become hydrogen- and helium-rich gas-giant planets.
Whether a protoplanet can reach this runaway gas-accretion stage or not depends largely on the accretion rate of solids in comparison to the time scale for dissipation of gas from the disk. The formation of giant planets is thus expected to be particularly efficient near the disk snow line. In lower-surface-density regions of the disk farther out from the snow line, the solid accretion rate is slower, and a protoplanet may never reach this runaway gas-accretion phase, leading to less accumulation of the nebular gas. While generally atmospheres of ice-giants are expected to be H 2 -rich, recent theoretical studies have also suggested the possibility of He-dominated atmospheres Hu et al.
These planets will typically contain less hydrogen and helium and thus a higher atmospheric metallicity than giant planets because of the less efficient accretion of nebular gas during their formation and evolution, as well as the higher likelihood of the escape of light gases over time. Outgassing from the planetary interior is expected to contribute additional volatiles—a component of the atmosphere that will become increasingly important for smaller planets—and other evolutionary processes such as atmospheric escape, impact delivery or erosion, atmosphere-surface exchange, weathering, and sequestration of volatiles into the interior can have and major influence on atmospheric composition and chemistry.
The stochastic nature of the different possible evolutionary pathways is expected to lead to highly diverse atmospheric properties for intermediate and small planets e. Pepin ; Dodson-Robinson and Bodenheimer ; Rogers et al. Here, we consider the atmospheric chemistry of exo-Neptunes and super-Earths with widely diverse volatile contents, as well as the chemistry of outgassed atmospheres of smaller, hot, rocky planets.
The atmospheric chemistry of terrestrial planets near the habitable zone is briefly discussed in Sect. Thermochemical equilibrium can be maintained in high-temperature, high-pressure regions of exoplanet atmospheres see Sect. Our own terrestrial planets have demonstrated the importance of secondary outgassing of interior volatiles in shaping the atmospheric properties of solid-surface planets Pepin The theoretical equilibrium composition of super-Earth atmospheres dominated by such an outgassing source has been explored by several investigators.
Schaefer and Fegley have performed similar more detailed calculations, albeit with a focus on the early Earth, to investigate the chemistry of potential steam atmospheres, such as those predicted to be associated with magma oceans e. Both Schaefer and Fegley and Elkins-Tanton and Seager find that the resulting atmospheric composition is a sensitive function of the assumed composition of the meteoritic material being accreted—water-dominated steam atmospheres occur only for CI and CM chondritic starting material. For other assumed meteoritic starting compositions, the atmospheres can be dominated by CO 2 , N 2 , H 2 , CH 4 , or CO, depending on the starting material composition and atmospheric temperatures Schaefer and Fegley Schaefer and Fegley have also pursued the potential atmosphere-surface buffering of hot atmospheres in equilibrium with planetary surfaces.
The possible formation of exotic by solar-system standards silicate atmospheres on hot super-Earths, in which volatile elements such as H, C, N, S, and Cl have already escaped from the planet, is explored by Schaefer and Fegley and Miguel et al. In these calculations, the atmosphere is assumed to be in gas-melt equilibrium with a volatile-free magma ocean or partially molten lithosphere. Ito et al. They find that SiO, in particular, affects the atmospheric opacity, causing thermal inversions and notable infrared emission signatures.
Schaefer et al. Again, these authors emphasize that the equilibrium atmospheric composition of such secondary outgassed atmospheres depends strongly on the source material and temperatures. Outgassing from accreted material with the composition of the bulk silicate Earth or the terrestrial crust would result in a water-dominated atmosphere under conditions relative to GJ b, with CO 2 as an important secondary component see Fig.
The chemistry of possible hot equilibrium atmospheres of Earth-like planets after giant impact events is explored by Lupu et al. Equilibrium atmospheric composition for a GJ b-like exoplanet, assuming the atmosphere results from outgassing from a high-temperature felsic silicate terrestrial crustal composition left or a more mafic bulk-Earth silicate composition right.
Figure from Schaefer et al. The equilibrium atmospheric composition of warm super-Earths like GJ c is investigated by Miller-Ricci et al. For the hydrogen-poor situation, the resulting Venus-like atmosphere has dominant constituents CO 2 and N 2 , with much less H 2 O. The overall hydrogen content therefore has a strong influence on the resulting composition.
Weiss et al.
Hal Leonard Instrumental Fake Books. Moreover, as the planetary signal diminishes with the increased resolution the method has been successfully applied only to planets orbiting the brightest stars. Spain Chick Corea. An atmosphere is in essence a fluid and therefore a variety of processes such as advection, diffusion, and turbulent motions can occur at different scales, having as consequence the transport and mixing of material between different regions. We conclude with a discussion of the future outlook for the field. Near-infrared thermal emission from the hot Jupiter TrES-2b: Ground-based detection of the secondary eclipse. Watercolors Pat Metheny.
Population-synthesis models and other theoretical arguments suggest that the trend of decreasing hydrogen and helium content with decreasing planetary size is a natural consequence of planetary formation and evolution Miller and Fortney ; Rogers et al. The metallicity or bulk mole fraction of H, in general of the atmosphere is therefore an important parameter controlling the chemistry of low-density super-Earths and exo-Neptunes. These investigations reach concensus on several general trends.
First, high-metallicity planets will have higher temperatures at lower pressures than otherwise similar low-metallicity planets, due to greater atmospheric opacity from the heavy i. At sufficiently high metallicities, CO 2 will even replace H 2 as the dominant atmospheric constituent for an otherwise solar-composition atmosphere. Molecular hydrogen remains a major constituent of the atmosphere under the conditions studied in Fig.
Equilibrium mixing ratios for various atmospheric constituents as a function of temperature and metallicity for an assumed typical photospheric pressure of 0. Figure adapted from Moses et al. The sensitivity-to-metallicity calculations in Fig. Of course, that is not likely going to be true for exoplanetary atmospheres, given the different formation scenarios and evolutionary process at work. Moses et al.
Here is where the diversity of potential heavy-element-rich super-Earths and exo-Neptunes really stands out. The dominant equilibrium atmospheric constituent on intermediate-sized planets will typically be H 2 at low-enough metallicities e. Between the potential for hot, silicate- and metal-rich outgassed atmospheres and the variety of volatile compositions available from inefficient accretion of nebular gas, the super-Earths and exo-Neptune population can be expected to have a rich diversity of atmospheric compositions from thermochemical equilibrium considerations, and disequilibrium processes below simply augment this possible diversity.
Both photochemistry and transport-induced quenching can affect the atmospheric composition of intermediate-size planets, just as on giant planets see Sects. The first photochemical models specifically applied to intermediate-sized planets were those of Line et al. Thermochemistry, photochemistry, and transport-induced quenching are considered in the Miller-Ricci Kempton et al.
Given that both GJ b and GJ b are expected to be relatively cool transiting planets, the results from both models are qualitatively similar. At the quench point where interconversion between CH 4 and CO shuts down, methane is the main carbon component. However, transport-induced quenching causes CO to be more abundant than it otherwise would have been in equilibrium.
This situation represents the opposite of the case for hotter planets, where quenching in the CO-dominated regime causes CO to be the major carbon component, with methane then being a less-abundant, but still important, quenched component see Sect. Photolysis of methane at high altitudes leads to the production of C 2 H x hydrocarbons and, because of interactions with water photolysis products, the photochemical production of CO and CO 2.
Coupled methane-ammonia photochemistry causes the production of HCN. However, methane is not removed from the photospheric region of either planet due to photochemistry as was suggested as a possibility by Madhusudhan and Seager , which is problematic, given that cloudless H 2 -rich models with equilibrium methane abundances do not reproduce transit and eclipse observations of these planets e. This model-data mismatch led Moses et al.
As discussed above, higher metallicities lead to hotter photospheres at lower pressures and shift the atmosphere toward the CO and CO 2 stability fields and away from the CH 4 stability field. The atmosphere transitions from being hydrogen-dominated, with abundant hydrogen-saturated components like H 2 O, CH 4 , and NH 3 and photochemically produced hydrocarbons and nitriles at low metallicities, to becoming CO 2 -, CO-, H 2 O-, and N 2 -rich, with more oxidized photochemical products like O 2 and NO at high metallicities.
Condensed graphite is a likely cloud component in the very-high-metallicity scenarios. Venot et al. Miguel et al. On the other hand, other strong X-ray and EUV lines could have a similar effect to that described in Miguel et al. Hu and Seager have examined the sensitivity of the disequilibrium atmospheric composition of several intermediate-sized exoplanets e.
Figure 8 shows their results for GJ b. These results are similar to the equilibrium results shown in Fig. Unless graphite formation is somehow kinetically inhibited see Moses et al. In an earlier study, Hu et al. Figure from Hu and Seager Conspicuously absent from the current literature is a study of how ion chemistry affects the gas-phase composition and possible formation of hazes on super-Earths and exo-Neptunes. Given that ion chemistry initiates the formation of high-molecular-weight organics in the N 2 - and CH 4 -rich upper atmosphere of Titan Waite et al. Basic concepts of cloud and haze formation in extrasolar-planet atmospheres are discussed in Sect.
The same physics and chemistry that was described for giant planets is relevant to intermediate-sized planets, but the different starting ingredients can lead to different possible aerosol compositions. Morley et al. For a solar-metallicity atmosphere, the total available mass for some of these clouds e. They find that ZnS and KCl clouds could obscure the transit spectra of a metal-rich GJ b if the particles are lofted to sufficiently high altitudes and have sedimentation times that are sufficiently long e. Graphite can also be stable for a range of other compositions relevant to outgassed atmospheres of intermediate-sized planets Schaefer and Fegley , , as can various other equilibrium condensates, such as alkali salts Schaefer et al.
Formation of sulfuric acid H 2 SO 4 clouds is likely through photochemical processes under a wide variety of conditions for high-metallicity or CO 2 -rich atmospheres that have sufficient SO 2 e. S8 can be photochemically produced e. However, the detailed formation pathways of photochemical hazes within the diverse atmospheres of intermediate-size planets has received relatively little attention and is still poorly understood.
The last decade has witnessed substantial progress in observational inferences of chemical species in the atmospheres of giant exoplanets through a variety of methods. The planets for which such detections have been made are mostly hot giant planets, either in close-in orbits i. The methods employed include differential photometry and spectroscopy of transiting hot Jupiters, high-resolution Doppler spectroscopy of transiting and non-transiting hot Jupiters, and high-resolution spectroscopy of directly-imaged planets.
Whereas atomic species have been detected across the ultraviolet and visible, molecular species have been detected primarily in the near infrared. These detections were made thanks to pioneering observations using Spitzer, HST , and ground-based facilities. In what follows, we review the developments in each of these various areas. But, given the limited spectral resolution of the observations of most planets, molecular compositions have been inferred for only a few exoplanets to date. The inferred molecules typically include the most abundant and spectroscopically dominant molecules expected in hot atmospheres e.
The strong line cores and wide pressure broadened wings are observable in optical transmission spectra of hot Jupiters. The Na resonance doublet was also detected from ground in the optical transmission spectrum of the hot Jupiter HD b Redfield et al. However, the ensemble of observations spanning UV to visible showed that the Na line is significantly weaker than that observed for HD b, with a clear lack of broad line wings.
Besides the weak Na I line, the spectrum was found to be largely featureless with a blue-ward slope which was consistent with the presence of strong scattering due to a thick haze of condensate grains Vidal-Madjar et al. In recent years, Na I has been inferred in visible transmission spectra of a few other hot Jupiters, e. XO-2b Sing et al. Visible transmission spectra of hot Jupiters have also revealed other atomic species. Similar to Na, several studies have also detected the K resonance double at nm using transmission spectra of hot Jupiters from both space and ground-based instruments, e.
Several atomic species have also been detected in the exospheres of hot Jupiters using UV transmission spectroscopy. Subsequent observations have revealed a rich population of atomic species in several hot Jupiter exospheres, e. H Lecavelier Des Etangs et al. Most recently, Ehrenreich et al.
Early inferences of molecules in exoplanetary atmospheres were based on few channels of photometry or low-resolution spectra obtained using then available instruments on Spitzer and HST. For example, some early studies used 2—3 near-infrared photometric observations in transmission to infer the presence of H 2 O at the day-night terminator regions of HD b Barman and HD b Tinetti et al. Beaulieu et al. Early attempts were also made to detect molecules in a handful of hot Jupiters using infrared spectroscopy with HST and Spitzer. Similar molecular inferences were also made using observations over a longer spectral baseline using Spitzer photometry and spectroscopy.
Grillmair et al. However, several of the Spitzer photometric observations have also since been revised drastically Knutson et al. Hansen et al. Therefore, early inferences based on such observations are not currently substantiated. Stevenson et al. Given the relatively lower equilibrium temperature of the planet, the presence of CO and absence of CH 4 was suggested to be indicative of strong chemical disequilibrium and high metallicity in the atmosphere.
While these data, and hence the molecular inferences, were originally contested by Beaulieu et al. Another major observational advancement that followed was the possibility of detecting thermal emission from hot Jupiters in the near-infrared from ground e. Again, the Spitzer photometric observations were a subject of intense debate initially Crossfield et al.
However, subsequent multi-epoch Spitzer observations in the same bandpasses have reinstated the original data and the conclusions Stevenson et al. In recent years, near-infrared HST spectroscopy has led to detections of H 2 O in the atmospheres of several transiting hot Jupiters. Since H 2 O is one of the most abundant molecules expected in giant exoplanetary atmospheres, the HST WFC3 instrument provides a unique opportunity to constrain H 2 O abundances in such atmospheres. As shown in Fig. The vertical axis shows absorption transit depth.
The blue circles show the data: HD b from Deming et al. The red curves shows the best-fit model spectra, and the cyan circles show the models binned to the same resolution as the data. The peaks around 1. Top : eclipse spectrum. Bottom : transmission spectrum. In both cases, the black circles with error bars show the data and solid curves show best-fit model spectra.
The filled dark blue circles show the best-fit model binned to the same resolution as the data. Several subsequent studies reported single-event transmission spectra of transiting hot Jupiters orbiting less brighter stars with varied levels of success in H 2 O detections. Swain et al. These efforts demonstrate that single transits are inadequate to make high-confidence H 2 O detections for most hot Jupiters currently known with the exception of those transiting the brightest stars such as HD b. Consequently, more recent observations using multiple visits to reduce the observational uncertainties have led to good quality transmission spectra with clear H 2 O detections for hot Jupiters orbiting even moderate brightness stars Kreidberg et al.
In addition to the presence of molecular abundances, thermal emission spectra also provide constraints on the disk-averaged temperature profile of the dayside atmosphere. However, with the exception of a few cases WFC3 observations to date have generally revealed thermal spectra with subdued molecular features, if any. Considered on its own, the spectrum is consistent with an isothermal temperature structure in the atmosphere. On the other hand, considering together with existing photometric observations in the range 0.
Subsequent studies have observed WFC3 thermal emission spectra for several other hot Jupiters Wilkins et al. Robust detections of molecular features in thermal emission spectra using HST WFC3 have been reported for the dayside atmospheres of only two hot Jupiters to date. Most recently, Haynes et al. Going beyond detecting the presence of molecules, the exoplanetary spectra discussed above have also been used to derive the molecular abundances which in turn can be used to constrain atmospheric processes, elemental abundances, bulk compositions, and formation conditions.
Molecular abundances are derived from exoplanetary spectra using detailed atmospheric retrieval methods which lead to joint statistical constraints on the chemical composition and temperature profile of an exoplanetary atmosphere given an observed spectrum; see e. Madhusudhan et al. Atmospheric retrieval methods for exoplanets typically comprise of a 1-D atmospheric model coupled with an optimization algorithm to estimate the free parameters of the model given the data.
The molecular abundances and the pressure-temperature P - T profile are free parameters in the model; typically there are over ten free parameters depending on the number of molecules included and the adopted parametrization of the temperature profile. For the optimization algorithm, a number of methods have been tried over time ranging from grid-search in the early days Madhusudhan and Seager to Bayesian approaches such as the Markov Chain Monte Carlo MCMC method in subsequent years Madhusudhan et al.
Chemical abundances have been retrieved for several giant exoplanets to date. Initial statistical constraints on atmospheric abundances e. Madhusudhan and Seager ; Lee et al. On the other hand, while robust HST WFC3 spectra have been observed for over a dozen planets now, most of them have uncertainties large enough to be consistent with featureless spectra Mandell et al.
H 2 O has been detected in all the six giant exoplanets listed above, albeit with different abundances. One important feature in almost all the transmission spectra with robust H 2 O detections to date is that the amplitude of the H 2 O absorption feature is typically smaller than model predictions for a clear i. This was first noted for the hot Jupiter with the most-precise transmission spectrum, HD b Deming et al. However, the derived low H 2 O abundances are degenerate with the possibility of clouds or hazes at the terminator in these atmospheres Sing et al.
Constraints on molecular abundances have also been reported in the dayside atmospheres of hot Jupiters using thermal emission spectra. While the veracity of the Spitzer photometric observations of WASPb were a subject of substantial debate in the intervening years Cowan et al. Most recently, H 2 O was detected at the day-night terminator region of the planetary atmosphere using a HST WFC3 transmission spectrum, however the abundance of H 2 O detected was subject to model assumptions as discussed above Kreidberg et al.
In another example, joint constraints on the H 2 O abundance at both the terminator as well as the dayside atmosphere were obtained using transmission and emission spectra for the hot Jupiter WASPb resulting in an H 2 O abundance of 0. Wilkins et al. As discussed in Sect. Simultaneously constraining the abundances of all these molecules requires a long spectral baseline with observable bandpasses where these molecules have strong spectral features. One of the main reason is that typically only WFC3 spectra are available in transmission, implying that stringent constraints are possible only on the H 2 O abundance.
For example, if a sub-solar H 2 O abundance is observed in a hot Jupiter assuming a cloud-free atmosphere , the low H 2 O can be caused either by a low overall metallicity i. On the other hand, the presence of clouds only makes the inference more degenerate. The presence of high-temperature condensates as hazes and clouds in hot exoplanetary atmospheres may contribute significantly to the optical depth in transmission spectra Seager and Sasselov ; Fortney Therefore, determinations of chemical compositions from transmission spectra can be strongly degenerate with the presence of clouds or hazes in the atmospheres.
In transmission spectra, the amplitude of a spectral feature is directly proportional to the height of the atmospheric annulus through which the star light traverses. The presence of an opaque cloud deck at a given height in the atmosphere means that only the layers above the cloud deck contribute to the transmission spectrum. Thus for a cloud deck located high enough in the atmosphere the amplitude of a spectral feature can be significantly diminished thereby confounding estimates of the corresponding molecular abundances.
High-precision observations of several transiting exoplanets have revealed such spectra with diminished features. Scattering due to haze particles have also been inferred from visible transmission spectra of hot Jupiters which show an inverse power-law dependence on wavelength with parameters different from that due to pure gaseous Rayleigh scattering. Similar inferences have been made for several other hot Jupiters in recent years Sing et al.
A potential degeneracy in such inferences arises from the fact that the presence of star spots may also cause similar optical spectra with inverse power law slopes e. McCullough et al. The black circles with error bars show the data available at that time, observed using multiple instruments on HST and Spitzer. The grey curve shows a model spectrum for a dust-free atmosphere. Such studies reported rather low geometric albedos for several hot Jupiters in the optical, e.
Demory et al. Furthermore, based on the asymmetry in the visible phase curve they also suggested the presence of an inhomogeneous cloud cover in the atmosphere of Kepler-7b. In another study, Evans et al. Most recently, Martins et al. Heng and Demory ; Esteves et al. However, as discussed in Sects. Sudarsky et al. Robust detection of molecules in exoplanetary atmospheres have also been made using high-resolution Doppler spectroscopy in the near infrared.
This technique involves the detection of molecular lines in the planetary spectrum that are shifted in wavelength due to the radial velocity of the planet Brogi et al. Thus the spectrally shifted molecular lines of the planetary atmosphere are easily identifiable compared to those in the stellar spectrum as well as those in the telluric spectrum which is static. A template planetary spectrum including the sought after molecule is cross-correlated with the observed spectrum to detect the Doppler shift in the molecular lines with phase thereby revealing the presence of the molecule.
Critical to this method, however, is the high resolution of the observed spectrum so that individual molecular lines can be resolved. Moreover, as the planetary signal diminishes with the increased resolution the method has been successfully applied only to planets orbiting the brightest stars.
This wavelength range offers the optimal conditions because in the near-infrared the planet-star flux ratio increases with wavelength, however the background noise also increases with wavelength; therefore the K-band around 2. Snellen et al. Direct imaging offers another avenue to characterize atmospheric compositions of exoplanets and works preferentially for a complementary region in planetary parameter space. Exoplanets detected to date via direct imaging are all young gas giants at large orbital separations.
Together these factors make young gas giants particularly favorable to near-infrared spectroscopy via direct imaging. The advantage of direct imaging is that the detection of a planet simultaneously results in observation of its atmospheric thermal emission spectrum, i. This is in contrast to transiting exoplanets where the detections are generally made using photometric transit surveys and RV confirmations, while atmospheric spectra are obtained for optimal targets using follow-up observations with specialized instruments.
On the other hand, the challenge with directly imaged planets is that generally only the atmospheric spectrum is available with little information about any other planet property; the mass, radius, and hence gravity, temperature, age, are all unknowns in modeling the planets. Therefore, typically, planetary evolution models are required along with atmospheric models to robustly constrain the atmospheric and physical parameters of the planets.
Moreover, spectra are available at only one point of the orbital phase which means thermal phase curves are not observable precluding constraints on atmospheric properties with longitude. Nevertheless, the possibility of high-resolution absolute near-infrared spectroscopy for directly-imaged planets means that chemical signatures can be robustly detected in emission spectra using ground-based instruments. Chemical detections have been reported for a few directly-imaged exoplanets in recent years.
Given the expected temperature range of young giant planets, the dominant molecular species expected are H 2 O, CO, and CH 4. All these molecules have been detected in one or more directly-imaged planets. Most of the detections have been reported for planets in the well characterized and nearby HR system. The presence of CH 4 and other candidate molecules e. Janson et al. In addition to molecular detections, spectra of directly imaged planets have also been use to place constraints on their elemental abundance ratios just as have been pursued for transiting hot Jupiters. However, determining elemental abundance ratios for directly imaged planets is challenging since most of the planetary properties e.
Nevertheless, several studies have attempted to fit static models to spectra of directly-imaged planets to report nominal constraints on the elemental abundances. Using a model grid to fit spectroscopic observations of HR c, Konopacky et al.
Lee et al. More recently, Todorov et al. Atmospheric elemental abundances of solar-system giant planets have led to important constraints on the origins of the solar system. However, the abundance of O is not known for Jupiter. On the other hand, indirect constraints based on kinetics models explaining the observed CO abundance suggest an O abundance of 0. Taking the currently available lower-limit on O at its true abundance, implying more carbon than oxygen, would require unusual formation conditions in the early solar system. Instead, using core accretion models Mousis et al. Thus, accurately measuring the O abundance is critical to constrain the formation conditions of Jupiter, and of the outer solar system in general.
The H 2 O abundance is similarly unknown for any other giant planet in the solar system. In particular, the H 2 O abundances play a central role in constraining exoplanetary formation conditions. Since O is cosmically the most abundant heavy element it is expected that H 2 O is one of the most dominant volatile in interstellar and planet-forming environments van Dishoeck et al. Measurements of such molecular abundances allow estimations of elemental abundances ratios involving H, C, O, and N.
Such elemental abundances can in turn provide crucial clues regarding exoplanetary atmospheric processes, interior compositions, and formation mechanisms, just as pursued for solar system planets. This fortuitous opportunity makes hot giant exoplanets the perfect laboratories to investigate the origins of giant planets and the diversity of their atmospheres and interiors.