flattens or rises weakly—an indication that a second formation mechanism becomes available to produce objects perhaps 10 MJ and smaller. This second process is, by presumption, the planetlike process of solid and then gaseous accretion.
Additional attempts to confirm this suggestion include plotting the masses of detected SMOs against their orbital eccentricity, since the planetlike process should tend to produce objects in circular orbits (a consequence of the averaging out of the differing orbital angular momenta of the accreting planetesimals). Indeed there seems to be a correlation such that the lower-mass candidates tend to have small or near-zero eccentricity. However, this correlation is not perfect and suffers both from the statistics of small numbers and the “sin i” ambiguity of radial-velocity detections. Various processes after formation can reduce or eliminate the initial eccentricity, particularly for low-mass objects in very close orbits about their parent star. Conversely, an SMO significantly more massive than Jupiter, formed in a disk, can have its eccentricity pumped up by gravitational interactions with the disk itself. Eccentricity, therefore, appears not to be a reliable indicator of formation mechanism.
The formation of low-mass SMOs very close to their parent stars is perhaps the most active area of research on formation processes. Currently no consensus exists on a preferred mechanism. Formation in place is difficult because SMOs (whether formed by starlike direct fragmentation or the solid-gas, two-step planetlike process) are distended during collapse. As a result, tidal stresses will tend to tear them apart if they approach to within 0.05 AU of their parent stars—the present distance of 51 Pegasi B from its primary. However, accretion involving very refractory solids may keep envelope opacity and hence physical radius small, thus, perhaps, avoiding this problem.
Most models invoke formation at distances farther from the parent star, followed by transport inward (see presentation by M.J. Duncan in this chapter). This transport may involve gravitational scattering involving one or two other giant planets, or interaction with the gaseous or particulate disk. Inward migration in a gaseous disk is a consequence of the gravitational interaction between planet and disk, such that a gap is formed in the disk and torques result in inward evolution of the planet and gap. How the migration is stopped remains a lively issue of discussion: the inner edge of the disk, stellar rotational torques, and mass transfer all may play a role. Likewise, inward migration of a giant planet in a purely particulate disk is possible through gravitational interactions, although significant movement requires a mass of particulates comparable to the mass of the planet itself.
Formation models do not represent an academic exercise since, as noted above, all models have something to say about the likelihood of forming additional planets (including terrestrial ones) in systems containing SMOs. It must be emphasized that the discovery of SMOs comparable in mass to Jupiter does not imply the presence of other planets in that system. The existence of close-in giant planets does not preclude, on stability grounds, the presence of a planetary system like our solar system. However, the formation of such close-in objects or their migration inward would disrupt or delay the formation of other planets. Thus, the timing of the migration (early or late) in the history of the disk, and its effect on the disk, are critical to assessing the viability of smaller planets in those systems. The probability or frequency of giant