Nebular theory

Bob pushes the thoughts out of his mind and enjoys the grass. For centuries, scientists hypothesized that the Sun and planets were born from a rotating disk of cosmic gas and dust. The flat disk constrained the nascent planets to have orbits lying roughly in the same plane. This nebular theory was proposed as far back as 1755 by the philosopher Immanuel Kant. 

The equilibrium-condensation model assumes that solids thermally equilibrated with the surrounding nebular gas, and any uncondensed elements were somehow flushed from the system. Planets accreted from these solids would then have compositions dictated by condensation theory. Because temperature and pressure decreased away from the Sun, the condensed solids would have varied with heliocentric distance. Figure 14.7 shows planets... 

The astrophysical models of protoplanetary disks based on optical observations and laboratory experiments and meteoritic measurements provide the basis for theories of nebular evolution. The best and most precise relevant measurements are from meteoritic analysis. Meteorites from the Asteroid Belt of our Solar System are the best record of the evolution of the solar nebula from a gas-dust mixture to an organized planetary system. The addition of cometary and solar-wind sample analysis complement these data. Combination of fundamental laboratory-based experiments and modeling efforts has led to a highly resolved understanding of the chemical conditions and processes in the primordial solar nebula (see Chapter 6). In this chapter an overview of recent advances in our understanding of the chemical and isotopic evolution of the early Solar System and protoplanetary disks is presented. 

These models produced a zoned Earth with an early metallic core surrounded by silicate, without the need for a separate later stage of core formation. The application of condensation theory to the striking variations in the densities and compositions of the terrestrial planets, and how metal and silicate form in distinct reservoirs has been seen as problematic for some time. Heterogeneous accretion models require fast accretion and core formation if these processes reflect condensation in the nebula and such timescales can be tested with isotopic systems. The time-scales for planetary accretion now are known to be far too long for an origin by partial condensation from a hot nebular gas. Nevertheless, heterogeneous accretion models have become embedded in the textbooks in Earth sciences (e.g.. Brown and Mussett, 1981) and astronomy (e.g.. Seeds, 1996). 

The unequal distribution of angnlar momentnm between the planets and the sun remains an unsolved problem, bnt not considered serions enough to finally abandon the nebular hypothesis. Rival theories such as the disintegration of one of a binary pair of stars to form the planets also leave many questions unanswered and a qnantitatively correct model has not been formulated. However, after the time of Newton observational astronomy moved out of the solar system to examine the stars in the Milky Way, also known as the Galaxy, based on the Greek word for milk. 

Lambert, Johann Heinrich (1728-1777) Swiss-born mathematician, physicist and astronomer in Augsburg and Berlin he studied hght intensity and also developed a theory of the generation of the universe that was similar to the nebular hypothesis that Thomas Wright and Immanuel Kant had (independently) developed. 

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