The Orbital Architecture of Exoplanetary Systems

The great diversity of extrasolar planetary systems has challenged our understanding of how planets form. During the formation process their orbits are modified while the protoplanetary disk is present. After its dispersal orbits may also be modified as a result of mutual gravitational interactions leading to their currently observed configurations in the longer term. A number of potentially significant phenomena have been identified. These include radial migration of solids in the protoplanetary disk, radial migration of protoplanetary cores produced by disk-planet interaction and how it can be halted by protoplanet traps, formation of resonant systems and subsystems, and gravitational interactions among planets or between a planet and an external stellar companion. These interactions may cause excitation of orbital inclinations and eccentricities which in the latter case may attain values close to unity. When the eccentricity approaches unity, tidal interaction with the central star could lead to orbital circularization and a close orbiting Hot Jupiter, providing a competitive process to direct migration through the disk or in-situ formation. Long-term dynamical instability may also account for the relatively small number of observed compact systems of super-Earths and Neptune class planets that have attained and subsequently maintained linked commensurabilities in the long term.