Development of the CAGC model

Planet formation theories and models have been proposed, debated, and refined for quite a few decades. An overall discussion of planet formation is presented by J. Lissauer in this book, and a brief historical overview of the CAGC model is offered in this section.

The earliest theories proposed that planets formed from mass thrown off the Sun after it had condensed into its current state (Descartes 1644; Kant 1755; Laplace 1796; W. Herschel 1811). A rudimentary version of an accretion theory was proposed by Buffon (1749), in which he considered a "building up" formation process rather than a condensing mechanism, proposing that a comet passing close to the Sun pulled matter off the Sun, which then accreted into planets. It was understood early on that the planets were formed from the parent star's environment.

Over the next century, the methods by which the Sun's thrown-off material evolved into the planets was debated by geologists and Darwinian evolutionists (e.g., Chamberlin 1899; Moulton 1905), as well as by physicists and mathematicians (e.g., Jeffreys 1917, 1918; Jeans 1919; Russell 1935). During this time, two models persisted to trade off as the prominent planet formation theory: the nebular hypothesis (e.g., Kuiper 1951; Cameron 1978), referred to today as the gas instability model, and the accretion model (Urey 1951; Perri & Cameron 1974). This period, and the scientific milestones relevant to planet formation, is provided in more detail by Brush (1990).

The earliest quantitative work was undertaken by Safronov during the 1960s. He developed a model based on the work of Shmidt (1944), who postulated that the Sun captured material (a "protoplanetary cloud") from interstellar space. Safronov (1969) created an analytical formulation for the accumulation of solid particles from the protoplanetary cloud into planets. The Safronov accretion model and the burgeoning capabilities of computers prompted extensive work on planet formation. Wetherill (1980) was one of the earliest researchers who adopted Safronov's planetary accretion model for a computer simulation of Earth and terrestrial planet formation.

In tandem to Safronov's work, a series of papers by Kusaka et al. (1970), Hayashi (1981), and Nakagawa et al. (1981) investigated the growth of solid particles in the solar disk. Mizuno et al. (1978) included the effect of the gaseous nebula on the buildup of planetesimals into planets, and then Mizuno (1980) extended that accretion model to the formation of Jupiter and Saturn. Their work was based on the computation of a series of protoplanetary models of increasing core mass, with a gaseous envelope in hydrostatic equilibrium that extends out to the protoplanet's tidal radius. Mizuno determined that there is a maximum core mass, called the "critical" core mass, Mcrit, for which a static solution for the envelope with a core mass greater than Mcrit was not possible. This value was determined to be Mcrit « 10 M®. They also found that Mcrit was insensitive to the distance from the Sun. The success of the study by Mizuno and his collaborators at Kyoto University marked a clear advantage of the accretion model over the gaseous condensation model.

About a decade before the discovery of the companion to 51 Peg, Bodenheimer & Pollack (1986) computed the first evolutionary calculation of gas giant planets based on the core accretion model. These models were based on an adapted stellar-evolution code with constant accretion rates. They found that the critical core mass was most sensitive to the rate of planetesimal accretion, namely, that Mcrit decreased as the planetesimal accretion rate was reduced. They corroborated Mizuno's results that Mcrit was not dependent on solar nebula boundary conditions, but that Mcrit was less sensitive to micron-sized grains in the envelope than was determined in Mizuno's calculation.

Wuchterl (1991a,b) used a radiation-hydrodynamics code rather than a quasi-static one used by previous investigators to analyze the core accretion model. He found that once the envelope mass became comparable to the core mass, a dynamical instability develops that results in the ejection of much of the envelope.

Within the last two decades, the CAGC model has become a sophisticated model, with computer simulations that explain many features of the gas giant planets (e.g., Paper 1; Bodenheimer et al. 2000; HBL05). Interior model calculations of the gas giant planets (e.g., Hubbard et al. 1999; Saumon & Guillot 2004) are an important component to the general investigation of gas giant planet formation. Based on actual observations of the giant planets in the Solar System (e.g., gravitational moments), substantial information about the presence of a solid core—and the size and composition of it—can be extracted when structural model parameters are matched with observed values (Marley et al. 1999). Though most of the theoretical work has been based on the gas giants in our Solar System, the understanding derived from these models has been extended to the extrasolar planets.

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