Atmospheres of Extrasolar Giant PlanetsMark S. Marley and Jonathan FortneyNASA Ames Research CenterSara SeagerCarnegie Institute of WashingtonTravis BarmanUniversity of California at Los AngelesThe key to understanding an extrasolar giant planet’s spectrum–and hence its detectabilityand evolution–lies with its atmosphere. Now that direct observations of thermal emission fromextrasolar giant planets are in hand, atmosphere models can be used to constrain atmosphericcomposition, thermal structure, and ultimately the formation and evolution of detected planets.We review the important physical processes that influence the atmospheric structure andevolution of extrasolar giant planets and consider what has already been learned from the firstgeneration of observations and modeling. We pay particular attention to the roles of cloudstructure, metallicity, and atmospheric chemistry in affecting detectable properties throughSpitzer Space Telescope observations of the transiting giant planets. Our review stresses theuncertainties that ultimately limit our ability to interpret EGP observations. Finally we willconclude with a look to the future as characterization of multiple individual planets in a singlestellar system leads to the study of comparative planetary architectures.1. INTRODUCTIONAtmospheres of planets serve as gatekeepers, control-ling the fate of incident radiation and regulating the lossof thermal energy. Atmospheres are also archives, preserv-ing gasses that reflect the formation and the evolution ofa planet. Thus a complete characterization of an extraso-lar giant planet entails understanding its thermal evolutionthrough time, bulk and atmospheric composition, and atmo-spheric structure. To date transit spectroscopy has probedthe chemistry of the upper atmosphere of one EGP, andbroad band measurements of the flux emitted by two ex-trasolar giant planets were reported in 2005. Many moresuch observations will follow as we await the direct imagingand resultant characterization of many EGPs around nearbystars.This review focuses on the physics of giant planet at-mospheres and the models which describe them. We firstapproach these planets from a theoretical perspective, pay-ing particular attention to those aspects of planetary modelsthat directly relate to understanding detectability, character-ization, and evolution. We stress the modeling uncertaintiesthat will ultimately limit our ability to interpret observa-tions. We will review the observations of the transiting giantplanets and explore the constraints these observations placeon their atmospheric structure, composition, and evolution.Unlike purely radial velocity detections, direct imaging willallow characterization of the atmosphere and bulk compo-sition of extrasolar planets, and provide data that will shedlight on their formation and evolution through time. We willexplore what plausibly can be learned from the first genera-tion of EGP observations and discuss likely degeneracies ininterpretation that may plague early efforts at characteriza-tion.2. OVERVIEW OF GIANT PLANET ATMOSPHERESThe core accretion theory describing the formation of gi-ant planets (Wetherill and Steward, 1989; Lissauer, 1993)suggests that any planet more massive than about 10 Earthmasses should have accreted a gaseous envelope from thesurrounding planetary nebula. This leads to the expecta-tion that any massive planet will have a thick envelope ofroughly nebular composition surrounding a denser core ofrock and ice. For this review we implicitly adhere to thisviewpoint. Because subsequent processes, such as bom-bardment by planetesimals, can lead to enhancements of theheavier elements, we don’t expect the composition of theplanetary atmosphere to precisely mirror that of the nebulaor the parent star. Observed enhancements of carbon in so-lar system giant planets (Figure 1), for example, range froma factor of about 3 at Jupiter to about 30 times solar abun-dance at Uranus and Neptune.Departures from nebular abundance provide a windowto the formation and evolution history of a planet. The nearuniform enrichment of heavy elements in the atmosphereof Jupiter (Owen et al., 1999) has been interpreted as ev-idence that planetesimals bombarded the atmosphere overtime (e.g., Atreya et al., 2003). Direct collapse of Jupiter1110JupiterSaturnUranusNeptuneRatio to SolarC N OSPFig. 1.— Measured atmospheric composition of solar system giant planets (neglecting the noble gasses) expressed as a ratio to solarabundance (Lodders, 2003). Jupiter and Saturn abundances are as discussed in Lodders (2004), Visscher and Fegley (2005), and Flasaret al. (2005). Uranus and Neptune abundances are reviewed in Fegley et al. (1991) and Gautier et al. (1995).from nebular gas would not result in such a pattern of en-richment. A major goal of future observations should beto determine if most EGPs are similarly enriched in atmo-spheric heavy elements above that in the atmosphere of theirprimary stars.Other outstanding questions relate to the thermal struc-ture, evolution, cloud and haze properties, and photochem-istry of extrasolar giant planets. For discussion it is usefulto distinguish between cooler, Jupiter-like planets and thosegiant planets that orbit very close to their primary stars, the‘hot Jupiters’. While the ultimate goals for characterizingboth types of planets are similar, the unique atmospheresof the two classes raise different types of questions. Forthe hot Jupiters, most research has focused on the horizon-tal and vertical distribution of incident stellar radiation intheir atmospheres and the uncertain role of photochemicalprocesses in altering their equilibrium atmospheric compo-sition. The available data from the transiting hot Jupitersalso challenges conventional atmospheric models as theiremergent flux seems to be grayer than expected.Cooler, more Jupiter-like planets have yet to be directlydetected. Consequently most research focuses on predict-ing the albedos and phase curves (variation of brightness asa planet orbits its star caused by the angular dependence ofatmospheric scattering) of these objects to aid their eventualdetection and characterization. As with the solar system gi-ants, most of the scattered light reflecting from extrasolarEGPs will emerge their cloud decks. Thus developing anunderstanding of which species will be condensed at whichorbital distances and–critically–the vertical distribution ofthose condensates is required to
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