Sterile and Fertile Planetary Systems - Statistical Analysis of   Multi-Planet Systems in Kepler's data
  The Kepler mission has discovered a large number of planetary systems. We analyze the implications of the discovered single/multi-exoplanet systems from Kepler's data.   As done in previous works, we test a simple model in which the intrinsic occurrence of planet is an independent process, and with equal probability around all planet producing stars. This leads to a Poisson distribution for the intrinsic number of planets around each host. However, the possibility of zero/low mutual inclination is taken into account, creating a correlation between detecting different planets in a given stellar system, leading to a non Poisson distribution for the number of transiting planets per system. Comparing the model's predictions with the observations made by Kepler, we find that the correlation produced by planarity is insufficient and a higher correlation is needed; either the formation of one planet in the system enhances the likelihood of other planets to form, and/or that some stars are considerably more fertile than others. Kepler's data presents evidences that both correlations might play a part, in particular a significant dependency in the radial distribution of planets in multi-planet systems is shown. Followup observations on Kepler planet's hosts can help pinpoint the physical nature of this correlation. 
Amir Weissbein, Elad Steinberg and Re'em Sari
astro-ph.EP