Gas giants

Hydrogen is the primary component of Jupiter and the other gas giant planets. At some depth in the... 

The gas giant planets Jupiter, Saturn, Uranus and Neptune. The planet Pluto has a status of its own, and has recently been renamed a dwarf planet. 

Although Uranus and Neptune also belong to the group of gas giant planets, they are constructed differently from Jupiter and Saturn ... 

The formation of the planets around the proto-sun initially started as a simple accretion process, aggregating small particles to form larger particles. This process was common to all planets, even the gas giants Jupiter and Saturn and to a lesser extent Neptune and Uranus. The planetesimals form at different rates and as soon as Jupiter and Saturn had reached a critical mass they were able to trap large amounts of hydrogen and helium from the solar nebula. The centres of Jupiter... 

A new reservoir of comets may have formed at around 5 AU in a local orbit around Jupiter or at least perturbed by its gravitational attraction. A comet close to Jupiter would simply have been captured, delivering its chemical payload to the ever-increasing gas giant. Some comets would merely have been deflected towards the inner terrestrial planets, delivering a similar payload of water and processed molecules. Cometary impacts such as the spectacular collision of the comet Shoemaker-Levy 9 with Jupiter would have been common in the early formation phase of the solar system but with a much greater collision rate. Calculations of the expected collision rate between the Earth and potential small comets deflected from the snow line may have been sufficient to provide the Earth with its entire... 

The density estimates in Table 7.1 show a distinction between the structures of the planets, with Mercury, Venus, Earth and Mars all having mean densities consistent with a rocky internal structure. The Earth-like nature of their composition, orbital periods and distance from the Sun enable these to be classified as the terrestrial planets. Jupiter, Saturn and Uranus have very low densities and are simple gas giants, perhaps with a very small rocky core. Neptune and Pluto clearly contain more dense materials, perhaps a mixture of gas, rock and ice. 

There are several basic features to note about the chemical abundances of the solar system. First, the Sun, and thus the solar system, consists dominantly of hydrogen and helium, with these two elements making up >98% of the mass of the solar system. Outside of the Sun, hydrogen and helium are found primarily in the gas-giant planets. 

Among places where condensates accreted into significant solid bodies, such as planets, habitable realms have always been rarer than places that were either too cold or too hot for life to exist. Much of our Solar System s mass is still far too hot for life. Most of the deep interiors of the gas giants and rocky planets are too hot, as is, of course, the Sun itself. Most of the surface area of solid bodies in the Solar System are too cold - the icy satellites of the outer planets and the myriad comets and Kuiper Belt Objects on the far outer fringes of the Solar System. In this sense, places like the surfaces of Earth and Mars and Europa s subsurface ocean are indeed very rare places. 

Planet formation followed the planetesimal and protoplanet formation in the final stage of the disk evolution (Chapter 10). Gas giants, Jupiter and Saturn, captured disk gas due to their large gravities, and other planets, including Earth, may also have some evidence of disk-gas capture. In the second part of this section, we will seek constraints on the timing of dust and gas dispersal in the proto-solar disk from planets (Section 9.3.2). 

Myr through runaway accretion (Pollack et al., 1996). However, such a nebula is likely to be marginally gravitationally unstable, a situation that could result in the rapid formation of gas giant planets in a few thousand years by the formation of self-gravitating clumps of gas and dust (Boss, 1997, 2000, 2002b). 

Boss A. P. (2000) Possible rapid gas giant planet formation in the solar nebula and other protoplanetary disks. Astrophys. J. 536, L101-L104. 

These observations have led to the development and refinement of a theory in which the planets formed from a disk-shaped protoplanetary nebula (Laplace) by pairwise accretion of small solid bodies (Safranov, 1969). A variant of the standard model invokes the gravitational collapse of portions of this disk to form gas giant planets directly. It should be pointed out that the standard model is designed to explain the planets observed in the solar system. Attempts to account for planetary systems recently discovered orbiting other stars suggest that planet formation is likely to differ in several respects from one system to another. 

Boss A. P. (2001) Gas giant protoplanet formation disk instability models with thermodynamics and radiative transfer. Astrophys. J. 563, 367-373. 

Comet-like materials are presumed to be the budding blocks of Uranus and Neptune (the ice giants) they may have played a role in the formation of Jupiter and Saturn (the gas giants) and they also played some role in transporting outer solar system volatile materials to inner planets (Delsemme, 2000). The inner solar system flux of comets may have been much higher in the past and comets may have played a role in producing the late heavy bombardment on terrestrial planets (Levison et al., 2001). Comets also exist outside the solar system and there is good evidence that they orbit a major fraction of... 

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