Planet-Planet Scattering Alone Cannot Explain the Free-Floating Planet   Population
  Recent gravitational microlensing observations predict a vast population of free-floating giant planets that outnumbers main sequence stars almost twofold. A frequently-invoked mechanism for generating this population is a dynamical instability that incites planet-planet scattering and the ejection of one or more planets in isolated main sequence planetary systems. Here, we demonstrate that this process alone probably cannot represent the sole source of these galactic wanderers. By using straightforward quantitative arguments and N-body simulations, we argue that the observed number of exoplanets exceeds the plausible number of ejected planets per system from scattering. Thus, other potential sources of free-floaters, such as planetary stripping in stellar clusters and post-main-sequence ejection, must be considered. 
Dimitri Veras, Sean N. Raymond
astro-ph.EP