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Saturn formed beyond the snow line in the primordial solar nebula, and that made it possible for it to accrete a large mass. Disk instability and core accretion models have been proposed for Saturn’s formation, but core accretion is favored on the basis of its volatile abundances, internal structure, hydrodynamic models, chemical characteristics of protoplanetary disk, etc. The observed frequency, properties, and models of exoplanets provide additional supporting evidence for core accretion. The heavy elements with mass greater than 4He make up the core of Saturn, but are presently poorly constrained, except for carbon. The C/H ratio is super-solar, and twice that in Jupiter. The enrichment of carbon and other heavy elements in Saturn and Jupiter requires special delivery mechanisms for volatiles to these planets. In this chapter we will review our current understanding of the origin and evolution of Saturn and its atmosphere, using a multi-faceted approach that combines diverse sets of observations on volatile composition and abundances, relevant properties of the moons and rings, comparison with the other gas giant planet, Jupiter, and analogies to the extrasolar giant planets, as well as pertinent theoretical models.
The science of extra-solar planets is one of the most rapidly changing areas of astrophysics and since 1995 the number of planets known has increased by almost two orders of magnitude. A combination of ground-based surveys and dedicated space missions has resulted in 560-plus planets being detected, and over 1200 that await confirmation. NASA's Kepler mission has opened up the possibility of discovering Earth-like planets in the habitable zone around some of the 100,000 stars it is surveying during its 3 to 4-year lifetime. The new ESA's Gaia mission is expected to discover thousands of new planets around stars within 200 parsecs of the Sun. The key challenge now is moving on from discovery, important though that remains, to characterisation: what are these planets actually like, and why are they as they are?
In the past ten years, we have learned how to obtain the first spectra of exoplanets using transit transmission and emission spectroscopy. With the high stability of Spitzer, Hubble, and large ground-based telescopes the spectra of bright close-in massive planets can be obtained and species like water vapour, methane, carbon monoxide and dioxide have been detected. With transit science came the first tangible remote sensing of these planetary bodies and so one can start to extrapolate from what has been learnt from Solar System probes to what one might plan to learn about their faraway siblings. As we learn more about the atmospheres, surfaces and near-surfaces of these remote bodies, we will begin to build up a clearer picture of their construction, history and suitability for life.
The Exoplanet Characterisation Observatory, EChO, will be the first dedicated mission to investigate the physics and chemistry of Exoplanetary Atmospheres. By characterising spectroscopically more bodies in different environments we will take detailed planetology out of the Solar System and into the Galaxy as a whole.
EChO has now been selected by the European Space Agency to be assessed as one of four M3 mission candidates.
About 40% of the extrasolar giant planets discovered so far have orbital distances smaller than 0.2 AU. These “hot Jupiters” are expected to be in synchronous rotation with their star. The ability to measure their radii prompts a careful reexamination of their structure. I show that their atmospheric structure is complex and that thermal balance cannot be achieved through radiation only but must involve heat advection by large-scale circulation. A circulation model inspired from Venus is proposed, involving a relatively strong zonal wind (with a period that can be as short as 1 day). It is shown that even this strong wind is incapable of efficiently redistributing heat from the day side to the night side. Temperature variations of 200 K or more are to be expected, even at pressures as large as 10 bar. As a consequence, clouds should be absent on the day side, allowing more efficient absorption of the stellar light. The global chemical composition of the atmosphere should also be greatly affected by the presence of large temperature variations. Finally, stellar tides may also be important in their ability to deposit heat at levels untouched by stellar radiation, thereby slowing further the cooling of the planets.
Brown dwarf atmospheres form molecules, then high temperature condensates (corundum, titanates, silicates, and iron compounds), and then low temperature condensates (ices) as they cool down over time. These produce large opacities which govern entirely their spectral energy distribution. Just as it is important to know molecular opacities (TiO, H2O, CH4, etc.) with accuracy, it is imperative to understand the interplay of processes (e.g. condensation, sedimentation, coagulation, convection) that determines the radial and size distribution of grains. Limiting case models have shown that young, hot brown (L) dwarfs form dust mostly in equilibrium, while at much cooler stages (late T dwarfs) all high temperature condensates have sedimented out of their photospheres. But this process is gradual and all intermediate classes of brown dwarfs can partly be understood in terms of partial sedimentation of dust. With new models accounting for these processes, we describe the effects they may have upon brown dwarf spectral properties.
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