On the Origin of the Kepler-36 System

The Kepler-36 system comprises two exoplanets: an inner terrestrial super-Earth (4.5 M_E) and an outer gaseous sub-Neptune (8.1 M_E). Both planets are orbiting very close to their host star (0.12 and 0.13 AU away) and to each other. Their orbital periods are in nearly a 7:6 ratio, a mean-motion resonance that is not usually populated. Finally, the inner planet is much denser than Earth (7.5 g/cm³), while the outer planet is substantially less dense (0.89 g/cm³), a surprise given the close proximity of their orbits.We examine origin scenarios for Kepler-36 featuring planetary migration due to dissipative forces within the circumstellar gas disk. In our model, the planets did not form in situ but rather moved inwards through a region of smaller planetesimals and embryos. We assume that the outer body formed beyond the frost line with significant gas accretion to account for its low density. This sub-Neptune migrated inward and captured an embryo of negligible relative mass into a 2:1 orbital resonance. Collisions between this embryo and other bodies of similar mass broke the resonance and allowed the two planets to converge while the inner one grew in mass and density.Using HNDrag, an N-body code, we have found several scenarios that might explain the observations. Although our successful scenarios occur with low probability, this is actually a strength since few known systems share the peculiarities of Kepler-36. Our model explains the main observed features of this system in the context of standard planetary formation theory.