Tom Hands - February 24, 2017

On the formation and resonant configurations of Kepler's 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 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. 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. Using both N-body models and two
dimensional hydrodynamical simulations, I explore this problem in detail, and show
that under the right conditions, super Earths can indeed migrate through mean-motion
resonances. I demonstrate that systems with giant outer planets such as Kepler-90
are more likely to exhibit tighter resonances, and that interactions with the
protoplanetary disc itself can lead to the breakdown of otherwise-stable resonances.