Planet formation accretion

Formation of a planet by accretion and the fractionation of the molten rock to form a metal core... 

The extent to which the dust from the ISM survives planet formation intimately depends on the details of the core collapse and the formation of the accretion disk. 

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). 

For theories of planet formation and disk chemistry, the mid-plane temperature, Tm, is more important than the surface temperature. Also, to solve Eq. (3.4) or Eq. (3.6) we need to know v, which depends on Tm, not on Te. For irradiation-dominated disks Tm is roughly equal to the surface effective temperature Te. For disks in which accretional heating is important this is not the case. A rough estimate for the mid-plane temperature valid for any case is T4 = (3/8)T4viscrR + r4irr, where T,visc and 7). T are given above and tr = E/cr with /cr the Rosseland mean opacity of the gas-dust mixture at the temperature Tm. The expression for Tm is therefore not explicit, as it is also used for kr. For realistic dust opacities it requires a numerical iterative solution procedure. For a gray opacity, or for piecewise power-law opacities, such as those in Bell et al. (1997), it can be solved analytically. 

The low Fe abundance in the lunar mantle suggests the Moon-forming impact happened late in Earth s accretion (Canup Asphaug 2001). It may have been the last collision with another embryo. Simulations of terrestrial-planet formation find that low-velocity, oblique impacts are common (Agnor et al. 1999), so that planets like Earth and Venus are likely to experience at least one such impact during their formation. This suggests large satellites may be a common outcome of terrestrial-planet formation. 

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). 

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. 

Clearly, type-I migration presents a problem for models of planet formation, both in terms of accreting fully formed planets before they migrate into the Sun and in terms of their survival once fully formed. However, it is likely that a sufficiently massive planet would have cleared a gap in the disk gas. Once this gap extended beyond the Lindblad resonances, type-I migration ceased. At present, there is considerable uncertainty about how massive a planet must be to clear a gap in the disk. This depends sensitively on the way in which waves damped in the nebula, and on the disk viscosity, both of which are poorly constrained. A recent estimate is that a body with a mass of 2-3M would have cleared a gap at 1 AU, while at 5 AU, a body 15M would do the job (Rafikov, 2002). 

Lissauer J. J. (1987) Time-scales for planetary accretion and the structure of the protoplanetry disk. Icarus 69, 249-265. Lissauer J. J. (1993) Planet formation. Ann. Rev. Astron. Astrophys. 31, 129-174. 

SNC meteorites (Harper et al., 1995 Borg et al., 1997). This isotopic anomaly requires early differentiation of mantle and crust. Because hafnium and mngsten fractionated into silicate and metal, respectively, the short-lived Hf- W system can be used to determine that martian core formation occurred within —13 milhon years of the planet s accretion (Kleine et al., 2002 Yin et al., 2002). Correlation between Nd and isotope anomalies, as well as the initial Os/ Os ratios for martian meteorites (Brandon et al., 2000), indicate synchronous differentiation of core, mantle, and cmst (Figure 14). On Earth, core formation took substantially longer, convection has stirred the mantle sufficiently to erase any evidence of early isotopic heterogeneity, and cmst formation continues throughout geologic history. 

Finally, on the theoretical front, the main issues are still those first raised in detail in the 1970s by Lewis the relationship of the solar nebula, planet formation and satelhte system formation to the composition of the satellites. The current information suggests that more complex models are required, including such factors as variations in the carbon chemistry in different systems and the effects of planetary and satelhte migration during formation and accretion. 

Wetherill (1990) has provided the standard model of planetary formation, based upon the planetesimal hypothesis. This model states that planets grow within a circumstellar disk, via pairwise accretion of smaller bodies known as planetesimals. It should be noted that the process of planet formation is a fundamentally different process from that of star formation. Stellar formation begins with the process of gas condensation, whereas planetary formation begins with the accumulation of solid bodies, and gas accretion takes place only at a late stage in some of the larger planets (Lissauer, 1993). 

Comet accretion models. Noble gases, as well as water, carbon, and nitrogen, could have been supplied to the inner planets by accretion of volatile-rich icy comets scattered inward from the outer solar system. Although noble gas isotopic distributions in comets are unknown, solar isotopic compositions would be expected in cometary gases acquired from the nebula. There is experimental evidence that the relative elemental abundances of heavier species (Xe, Kr, and Ar) trapped in water ice at plausible comet formation temperatures ( 30 K) approximately reflect those of the ambient gas phase, and trapped noble gas abundances per gram of water are substantial (Bar-Nun et al. 1985 Owen et al. 

Most planetary systems are pervaded by dust due to the planet formation process, where through coagulation of dust and gas accretion in the disks that develop during the collapse and infall of massive protostar envelopes planets are formed. By studying the structure and dynamics of this dust, which is very bright at the Far Infrared wavelengths, one can gain information on how such systems were formed. Once the planets are formed, as their motion influence the distribution of the dust, planetary orbits can be traced. 

Of the two models, homogeneous accretion is generally favoured. H. Wancke from the Max Planck Institute in Mainz (1986) described a variant of this model, in which the terrestrial planets were formed from two different components. Component A was highly reduced, containing elements with metallic character (such as Fe, Co, Ni, W) but poor in volatile and partially volatile elements. Component B was completely oxidized and contained elements with metallic character as their oxides, as well as a relatively high proportion of volatile elements and water. For the Earth, the ratio A B is calculated to be 85 15, while for Mars it is 60 40. According to this model, component B (and thus water) only arrived on Earth towards the end of the accretion phase, i.e., after the formation of the core. This means that only some of the water was able to react with the metallic fraction. 

However, one other interpretation has been discussed in the literature to explain the [Fe/H] excess observed for stars with planets. In fact, it has been suggested that the high metal content is the result of the accretion of planets and/or planetary material into the star (e.g. [12]). In such a case, the observed metallicity excess would itself be a by-product of the planetary formation process. 

The results presented above, showing that the probability of finding a planet is a strong function of the stellar metallicity, thus favor the core-accretion model as the main mechanisms responsible for the formation of giant planets (although they do not completely exclude the disk instability model - see e.g. [19]). Indeed, it has even be shown that according to the core-accretion model it is possible to predict the observed [Fe/H] distribution of planet-host stars [9]. 

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