Abstract

The origins of compact exoplanet systems consisting of super-Earths and mini-Neptune are not yet well understood. Using N-body simulations, we examine the in situ final stage assembly of multiplanet systems via the collisional accretion of protoplanets. Two different prescriptions for treating planetary collisions are adopted. In this talk, I will discuss a number of important issues: how do the results depend on the accretion prescription?; do the resulting systems resemble the Kepler systems, and do they reproduce the observed distribution of planetary multiplicities when synthetically observed?; do collisions lead to significant modification of protoplanet compositions, or to stripping of gaseous envelopes?; do the eccentricity distributions agree with those inferred for the Kepler planets? We find that the accretion prescription is unimportant in determining the outcomes. The final planetary systems look broadly similar to the Kepler templates adopted, but the observed distributions of planetary multiplicities or eccentricities are not reproduced, because scattering does not excite the systems sufficiently. Post-processing the collision outcomes suggests that they would not significantly change the ice fractions of initially ice-rich protoplanets, but significant stripping of gaseous envelopes appears likely. Hence, it may be difficult to reconcile the observation that many low-mass Kepler planets have H/He envelopes with an in situ formation scenario that involves giant impacts after dispersal of the gas disc. Finally, I will also talk about how the eccentricity dichotomy inferred from Kepler data for single and multiple planets may arise through perturbations provided by outer systems of ice/gas giants.