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The giant outer planets–Jupiter, Saturn, Uranus, and Neptune (Figure 1.1)–are by far the largest planetary bodies in the solar system and together comprise 99.56% of the planetary mass. Although very far from the Earth, the enormous physical size of Jupiter and Saturn meant that these planets were easily visible to the ancients. However, the other two ˵giants″, Uranus and Neptune, are significantly smaller and so much farther from the Earth that they were unknown before the advent of telescopes, although Uranus is in fact just visible to the naked eye. Uranus was discovered by accident in 1781 by William Herschel (1738–1822) (later Sir William Herschel). Perturbations in the observed orbit of Uranus led John Couch Adams (1819–1892) and Urbain Jean Joseph Le Verrier (1811–1877) to independently predict the presence of a further planet, and Neptune was subsequently discovered close to its predicted position by Johann Gottfried Galle (1812–1910) in 1846. The mean observable properties of the outer planets are listed in Table 1.1. [ABSTRACT FROM AUTHOR]

According to current cosmological theories, approximately 14 billion years ago the universe was created at a single point in space-time in the ˵Big Bang″. As the universe expanded and cooled, numerous particle physics processes and particle-antiparticle annihilations occurred (e.g., Krane, 1996). After about t = 6 s, the universe consisted of some number N protons, N electrons, 0.16N neutrons, 109N photons, and 109N neutrinos, all at a temperature of approximately 1010 K. [ABSTRACT FROM AUTHOR]

With the exception of Jupiter, none of the atmospheres of the giant planets has been directly sampled. Even with Jupiter the only in situ measurements that are available were made from the Galileo entry probe, which sampled a single, probably not very representative, region of the planet, namely a 5 μm hotspot. Hence, the bulk of our knowledge of the composition, cloud structure, and dynamics of these planets has not come from direct measurements, but instead has come indirectly from analyzing features in their electromagnetic spectrum, measured by ground-based telescopes, Earth-orbiting telescopes, and from specific flyby and orbiting spacecraft missions. There are two main components of the observed spectra that provide atmospheric information: reflected sunlight from the cloud and haze layers, and thermal emission from the atmosphere itself. [ABSTRACT FROM AUTHOR]

In the previous chapter we saw how the ultraviolet (UV), visible, infrared (IR), and microwave spectra of the giant planets are formed, and how the absorption features of different gases (and theoretically aerosols) are visible in these spectra. Clearly much can be learned about the atmospheres from observing the spectra of these planets, and in this chapter we will review the measurements that have been made to date and how they may be used. [ABSTRACT FROM AUTHOR]

Subsequent to their formation from the collapse of the solar nebula, the atmospheres of all the planets, including the giant planets, have (to a greater or lesser extent) evolved over time through the action of a number of possible processes including thermal escape, cometary bombardment, and internal differentiation. In this chapter we will outline the principal evolution mechanisms and estimate their effects on the present-day composition of the giant planet atmospheres. [ABSTRACT FROM AUTHOR]

Much of what we know about the giant planets comes from observations of the conditions in the upper parts of their molecular-hydrogen/helium envelopes. Remote-sounding and in situ observations of the vertical profiles of pressure, temperature, and composition provide constraints on vertical motion, mixing, heat sources, deep composition, and photochemistry. These observations are complemented by observations of vertical cloud structure. [ABSTRACT FROM AUTHOR]

In Chapter 4 we discussed the vertical profiles of temperature, pressure, composition, and clouds of the giant planet atmospheres, and also briefly discussed transport mechanisms such as convection and eddy mixing. Of course the real atmospheres of planets are three-dimensional and thus to understand fully the observations of ground-based telescopes and spacecraft missions, we need to understand how planetary atmospheres move and how heat and material are transported, not just vertically, but horizontally as well. In this chapter we will consider the basic equations of fluid motion in rotating planetary atmospheres and see how these theoretical considerations relate to the observed winds, clouds, and storm systems observed in the atmospheres of the giant planets. [ABSTRACT FROM AUTHOR]

In this book, we have discussed how the giant planets and their atmospheres formed 4.6 billion years ago and have reviewed what is known about their atmospheres from existing measurements, models, and theory. The story of the way in which our understanding of these giant worlds has advanced is essentially one of how improvements in technology have allowed more and more detailed observations of these worlds, which in turn have allowed more advanced and accurate analysis. The rate of improvement in these data has rapidly increased with time, and thus we have learned more about these worlds in the last 25 years than in all of the previous centuries put together! Technology continues to develop at an extraordinary rate, and thus many exciting new measurements are expected in the next few years which will further improve our understanding of these giant worlds, and how they formed from circumplanetary disks. Some of the key questions concerning the atmospheres of the giant planets that still remain are What are the upper visible clouds of Jupiter and Saturn actually composed of when the spectral signature of ammonia ice seems to be generally so elusive?What actually is the composition of the main cloud deck on Uranus and Neptune?What is the nature of the chromophores on the different giant planets?To what level are different heavy elements enriched on the giant planets and how did the observed differences come about?What is the enrichment of heavy elements other than carbon on Uranus and Neptune?Were the heavy elements introduced to the giant planet atmospheres through amorphous or crystalline water ice?Why is ammonia so scarce down to several tens of bars on Uranus and Neptune and why does it appear depleted in the upper atmosphere of Jupiter?Is there really N2 in the atmosphere of Neptune and what are the abundances of other disequilibrium species on the ice giants?Why does Uranus have such a small source of internal energy?Why do Jupiter and Saturn have strong prograde equatorial jets, while Uranus and Neptune have retrograde jets?Why are almost all of the large ovals on the giant planets anticyclonic?Is the structure of our solar system actually typical compared with other star systems, or is it in some way unusual? [ABSTRACT FROM AUTHOR]

The discovery of the first extrasolar planet, 51 Pegasi, in 1995 has opened up a new and exciting area of planetary astronomy. Using a number of different techniques 170 extrasolar planets have now been discovered, and this number is rapidly increasing. Due partly to observational biases, just over half of these newly discovered planets are ‘Hot Jupiters' — Jupiter sized planets orbiting within 1 AU of their parent stars. However, the number of giant planets discovered in more distant orbits, as seen in our own Solar System, is increasing all the time as detection techniques improve and longer time-series are analysed. In this paper, the characteristics of the extrasolar planets discovered to date will be reviewed, together with the implications these characteristics have upon theories of the formation of planetary systems in general and our own Solar System in particular. Current and future methods for detecting extrasolar planets are also reviewed, together with prospects for detecting terrestrial planets and perhaps even searching for the signs of life itself. [ABSTRACT FROM AUTHOR]