  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.
