The Enthralling Tale of the Formation and Evolution of Compact Planetary Systems

Of the myriad of insights into exoplanetary systems provided by the Kepler mission, one of the most intriguing new discoveries is that of a class of compact planetary systems which include Kepler-11, Kepler-32 and Kepler-90. In such systems, ensembles of several planets are found in very closely packed orbits (often within a few percent of an astronomical unit of one another). These systems present a challenge for traditional formation and migration scenarios, since these planets presumably formed at larger orbital radii before migrating inwards. In particular, it is difficult to understand how some planets in such systems could have migrated across strong mean-motion resonances without becoming trapped, and remaining relatively well-spaced. It is also difficult to explain how such systems remain dynamically cold, as resonant interactions tend to excite orbital eccentricity and lead to close encounters. I present a dynamical study of the formation of these systems, using an N-body method which incorporates a parametrized model of planet migration in a turbulent protoplanetary disc. The study explores a wide parameter space, and finds that under suitable conditions it is possible to form compact, close-packed planetary systems via traditional disc-driven migration, albeit with an over-abundance of mean-motion resonances. I then extend the study to include Jupiter-mass planets exterior to the compact systems, and find that the dynamical effect of these companions can significantly modify the resonant structure of the compact planets. Finally, I extend this work to two dimensional hydrodynamical simulations in an attempt to model type I migration self-consistently. In particular, I find that clearing of the disc by photoevaporation can halt migration of compact systems, and also discover that planet-disc interactions can - under the right conditions - break mean-motion resonances.