Multiple-Planet Scattering and the Origin of Hot Jupiters
  Exoplanets show a pile-up of Jupiter-size planets in orbits with a 3-day period. A fraction of these hot Jupiters have retrograde orbits with respect to the parent star's rotation. To explain these observations we performed a series of numerical integrations of planet scattering followed by the tidal circularization. We considered planetary systems having 3 and 4 planets initially. We found that the standard Kozai migration is an inefficient mechanism for the formation of hot Jupiters.   Our results show the formation of two distinct populations of hot Jupiters. The inner population of hot Jupiters with semimajor axis a < 0.03 AU formed in the systems where no planetary ejections occurred. This group contained a significant fraction of highly inclined and retrograde orbits, with distributions largely independent of the initial setup. However, our follow-up integrations showed that this populations was transient with most planets falling inside the Roche radius of the star in <1 Gyr. The outer population of hot Jupiters formed in systems where at least one planet was ejected. This population survived the effects of tides over >1 Gyr. The semimajor axis distribution of Population II fits nicely the observed 3-day pile-up.   The inclination distribution of the outer hot planets depends on the number of planets in the initial systems and the 4-planet case showed a larger proportion (up to 10%), and a wider spread in inclination values. As the later results roughly agrees with observations, this may suggest that the planetary systems with observed hot Jupiters were originally rich in the number of planets, some of which were ejected. In a broad perspective, our work therefore hints on an unexpected link between the hot Jupiters and recently discovered free floating planets. 
C. Beauge and D. Nesvorny
astro-ph.EP