planets are thought to be gas giants on the basis of their masses and densities. Presumably, more gas giants are observed because they are large, and large planets are much easier to detect, leaving open the question of how many terrestrial planets remain hidden from Earth in distant planetary systems. A few “super-Earths,” with masses of several to 10 Earth masses, may be terrestrial planets, but no measurements of the radius or density of these objects has confirmed this. Gas-giant planets appear to be more likely with stars that have proportions of heavier elements (heavier than H, He, and Li) as high as the Sun (Fischer and Valenti, 2005), suggesting that heavy-element concentrations in the circumstellar disk influence the rate or efficiency of planet formation.
Measurements of the masses, orbital distances, and orbital eccentricities (Figure 1.3) of extrasolar planets provide clues about processes that may help determine what the final planetary system looks like. A particularly interesting class of planets, that of gas-giant planets in orbits extremely close to (less than 0.1 AU)1 their host stars—sometimes called “hot Jupiters”—are significant because models have been unable to account for why they form so close to the star (Butler et al., 2006). These hot Jupiters are thought to be telling us that large planets can drift inward toward their star as they form. Models also suggest that planets can under some circumstances drift away from the star, so the ultimate location of the planets may have little to do with where they originally formed. Extrasolar planets more than a few tenths of an AU distant from their host star often have quite eccentric orbits, which contrasts with the Solar System where all of the planets except Mercury have nearly circular orbits.
The Solar System is composed of radically different types of planets. The outer planets (Jupiter, Saturn, Uranus, and Neptune) are distinguished from the inner planets by their large size and low density. The outer planets are the primary products of the planet formation process and comprise almost all of the mass held in the planetary system. They are also the types of planet that are most easily recognized orbiting other stars. The inner planets (Mercury, Venus, Earth, and Mars) are composed mostly of rock and metal, with only minor amounts of gaseous material. There are “standard models” for the formation of both types of planets, but they have serious deficiencies and large uncertainties.
According to the standard model for outer-planet formation, the formation of giant planets starts with condensation and coalescence of rocky and icy material to form objects several times as massive as Earth. These solid bodies then attract and accumulate gas from the circumstellar disk (Pollack et al., 1996). The two largest outer planets, Jupiter and Saturn, seem to fit this model reasonably well, as they consist primarily of hydrogen and helium in roughly solar proportions, but they also include several Earth masses of heavier elements in greater than solar proportions, probably residing in a dense central core. Uranus and Neptune, however, have much lower abundances of hydrogen and helium than Jupiter and Saturn and have densities and atmospheric compositions consistent with a significant component of outer Solar System ices.
An alternative to the standard model is that the rock and ice balls are not needed to induce the formation of gas-giant planets; they can form directly from the gas and dust in the disk, which can collapse under