Formation of the giant planets

To the accuracy of the measurement of molecular weights for the giant planets, only hydrogen and helium have significant abundances. The relative proportions of these elements, expressed as the molar fraction He/H, are 0.068+0.002 for Jupiter, 0.068+0.013 for Saturn, 0.076+0.016 for Uranus, and 0.100+0.016 for Neptune (Lunine, 2004). None of these ratios are like those of the nebula (0.085, Table 4.1). 

Diagram on the left shows the composition of the solar nebula (abundances in wt. %). Diagram on the right expands metals (astronomical jargon) into ices (water, methane, and ammonia) and rock (all other remaining elements). Jupiter and Saturn formed mostly from nebular gases, Uranus and Neptune formed mostly from ices, and the terrestrial planets formed primarily from rock. 

Phase diagram for hydrogen, showing the conditions under which hydrogen changes from molecular (H2) to metallic (H+). Below the gray He saturation curves, He and H are immiscible. Adiabats for Jupiter and Saturn cross the saturation curve once H becomes metallic, but the Uranus (and presumably Neptune) adiabats do not reach such high pressures. 

Models of the interiors of the giant planets depend on assumed temperature-pressure-density relationships that are not very well constrained. Models for Jupiter and Saturn feature concentric layers (from the outside inward) of molecular hydrogen, metallic hydrogen, and ice, perhaps with small cores of rock (rocky cores are permissible but not required by current data). Uranus and Neptune models are similar, except that there is no metallic hydrogen, the interior layers of ice are thicker, and the rocky cores are relatively larger. 

Masses (relative to Earth) versus thermal equivalent radii for extrasolar planets. Thermal equivalent radii are the mean orbital distances from their stars normalized to the Sun s luminosity, which corrects for different stellar properties. Planets in our solar system (open circles) are identified by their first letters. Modified from Lunine et al. (2009). 

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Models for the formation of the giant planets suggest that a rocky planetary embryo of about ten Earth masses can form rapidly, within 10s years. Once this embryo is established these massive planetary embryos accumulate two Earth masses of solar nebular gas over 107 yr (Kortenkamp et al., 2001). 

Planet formation unfolds differently beyond the snowline, where water condensation enhances the surface density. Here massive cores (> 5-10 MEarth) may form rapid enough to accrete directly and retain nebular gas. These massive cores, if formed prior to the dispersal of the gas disk, rapidly reach Jupiter masses, forming giant planets. An alternative mechanism that may be responsible for the formation of some giant planets is gravitational instability in a massive, marginally unstable disk (e.g. Boss 2007 Mayer etal. 2007). 

The effect of the giant planets and the formation of the Asteroid Belt... 

The giant planets, especially Jupiter and Saturn, significantly influenced accretion in the inner Solar System, with important consequences for the properties of the terrestrial planets, described in Section 10.4.1. The influence of the giant planets is especially strong in the Asteroid Belt. Given that meteorites are our primary samples of primitive Solar System material, understanding the role of dynamical and collisional processes in the formation and evolution of the Asteroid Belt is of fundamental importance for theories of planet formation (Section 10.4.2). 

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., Wetherill G. W., and Haghighipour N. (2002) Rapid formation of ice giant planets. Icarus 156, 291—295. Briceno C., Vivas A. K., Calvet N., Hartmann L., Pacheco R., Herrera D., Romero L., Berlind P., and Sanchez G. (2001) The CIDA-QUEST large-scale survey of Orion OBI evidence for rapid disk dissipation in a dispersed stellar population. Science 291, 93-96. 

Models of planetary evolution assume that at the time of planetary formation the solar system had a single universal and well-mixed composition from which aU parts of the solar system were derived (see Podosek, 1978). Information as to the elemental and isotopic characteristics of this primordial composition is presently available from the Sun, meteorites, and the atmospheres of the giant planets (Wider, 2002). In the case of the Sun, distinction is usually made between the present-day composition, which is available via spectral analysis of the solar atmosphere and capture of the solar wind, either directly in space or by using metallic foU targets, and the proto-Sun (the composition at the time of planetary accretion) whereby the lunar regolith and/or meteorites are utilized as archives of ancient solar wind. As discussed below, the distinction is only really important for helium due to production of He by deuterium burning. 

Safranov, V. S. (1972). Ejection of bodies from the Solar System in the course of the accumulation of the giant planets and the formation of the cometary cloud. In lAU Symposium 45, 329-34, ed. G. A. Chebotarev, E. 1. Kazimirchak-Polonskaya, ... 

The giant outer planets formed from an agglomeration of billions of comets. The comets we observe today are the left over pieces like the asteroids are the left over pieces from the formation of the inner planets (Mercury, Venus, Earth and Mars). 

The term "Solar System ice" denotes in general a solidified volatile and/or mixtures of solidified volatiles. The Solar System ices are mostly water ice H2O, but also solidified CO2, CO, NH3, N2, SO2, CH4 and many other simple molecules as well as the organics. On the surfaces of many of the satellites of the giant planets (Jupiter, Saturn, Uranus, and Neptune) water ice is the dominant component. Therefore, in the following we will adopt the common assumption that a satellite is composed of water ice, and silicates. The radii and the masses of all satellites but the smallest ones are well known. Therefore their densities are known as well, see Table 1. Taking into account that the densities of water ice are about 940,1190, and 1360 kg m at phases I, II, and VI, respectively, and that the density of the silicates is (3400 400) kg m" the mass ratio C of silicates to total mass of the satellite can be estimated. This is rather simple for large satellites however, the estimate can fail for smaller satellites because of the possible bulk primordial porosity left fi-om an epoch of formation. 

The giant planets possess low surface temperatures and have atmospheres that extend several thousand miles. The markings on Jupiter, the largest planet, consist of cloud formations composed of methane containing a small amount of ammonia. The atmosphere of Jupiter absorbs the extreme red and infrared portions of the spectrum. These absorptions correspond to the absorption spectra of ammonia and methane, suggesting the presence of these gases in Jupiter s... 

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... 

Cosmochemislry places important constraints on models for the origin of the solar nebula and the formation and evolution of planets. We explore nebula constraints by defining the thermal conditions under which meteorite components formed and examine the isotopic evidence for interaction of the nebula with the ISM and a nearby supernova. We consider how planetary bulk compositions are estimated and how they are used to understand the formation of the terrestrial and giant planets from nebular materials. We review the differentiation of planets, focusing especially on the Earth. We also consider how orbital and collisional evolution has redistributed materials formed in different thermal and compositional regimes within the solar system. 

The lifetime of protoplanetary disks determines the time available for planet formation with the loss of the dusty gas disks no raw material is left to form planetesimals or giant planets. Thus, disk mass as a function of time is perhaps the single most important constraint on the formation of both the rocky and the giant planets. The most readily observable, albeit imperfect, indicator of disks is the presence of excess emission above the stellar photosphere, emerging from small, warm dust grains. 

The giant planets ceased growing when the flow of gas onto their envelopes was cut off. This may have been the result of gap formation or because the nebula dispersed. The latter seems unlikely, since the timescale for gas accretion onto a Jupiter-size planet is small compared to the lifetime of the nebula. However, hydrodynamical simulations suggest that gas would continue to flow onto Jupiter after it cleared a gap in the disk (Lubow et al., 1999), so this explanation is problematical too. In addition, it has been suggested that some gas would remain at the same orbital distance as the planet after it cleared a gap if the disk viscosity was low (Rafikov, 2002), and this would also be accreted by the planet eventually. 

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