Perturbation of Compact Planetary Systems by Distant Giant Planets
  We examine the effect of secular perturbations by giant planets on systems of multiple, lower mass planets orbiting Sun-like stars. We simulate the effects of forcing both eccentricity and inclination, separately and together. We compare our results to the statistics of the observed Kepler data and examine whether these results can be used to explain the observed excess of single transiting planets. We cannot explain the observed excess by pumping only inclination without driving most systems over the edge of dynamical instability. Thus, we expect the underlying planetary population for systems with a single transitting planet to contain an intrinsically low multiplicity population. We can explain the Kepler statistics and occurrence rates for R< 2 Rearth planets with a perturber population consistent with that inferred from radial velocity surveys, but require too many giant planets if we wish to explain all planets with R < 4 Rearth. These numbers can be brought into agreement if we posit the existence of an equivalent size population of planets below the RV detection limit (of characteristic mass 0.1 Mjupiter). This population would need to be dynamically hot to produce sufficiently strong perturbations and would leave the imprint of high obliquities amongst the surviving planets. Thus, an extensive sample of obliquities for low mass planets can help to indicate the presence of such a population. (...) Some of our simulations also produce planetary systems with planets that survive in the habitable zone but have no planets interior to them -- much as in the case of our Solar System. This suggests that such a configuration may not be altogether rare, but may occur around a few percent of FGK stars. 
Bradley M. S. Hansen
astro-ph.EP