Architecture of Planetary Systems Based on Kepler Data: Number of   Planets and Coplanarity
  We investigated the underlying architecture of planetary systems by deriving the distribution of planet multiplicity (number of planets) and the distribution of orbital inclinations based on the sample of planet candidates discovered by the Kepler mission. The scope of our study included solar-like stars and planets with orbital periods less than 200 days and with radii between 1.5 and 30 Earth radii, and was based on Kepler planet candidates detected during Quarters 1 through 6. We created models of planetary systems with different distributions of planet multiplicity and inclinations, simulated observations of these systems by Kepler, and compared the properties of the transits of detectable objects to actual Kepler planet detections. Specifically, we compared with both the Kepler sample's transit numbers and normalized transit duration ratios in order to determine each model's goodness-of-fit. We did not include any constraints from radial velocity surveys. Based on our best-fit models, 75-80% of planetary systems have 1 or 2 planets with orbital periods less than 200 days. In addition, over 85% of planets have orbital inclinations less than 3 degrees (relative to a common reference plane). This high degree of coplanarity is comparable to that seen in our Solar System. These results have implications for planet formation and evolution theories. Low inclinations are consistent with planets forming in a protoplanetary disk, followed by evolution without significant and lasting perturbations from other bodies capable of increasing inclinations. 
Julia Fang and Jean-Luc Margot
astro-ph.EP