Outcomes of Grazing Impacts Between Sub-Neptunes in Kepler Multis

TL;DR

This study investigates the outcomes of grazing collisions between sub-Neptune exoplanets in Kepler multis, using detailed hydrodynamics to improve models of planetary collisions and their effects on system stability.

Contribution

It introduces realistic hydrodynamic collision simulations for sub-Neptune planets, providing improved prescriptions for dynamical models beyond the sticky-sphere approximation.

Findings

Collisions often lead to mergers, scatterings, or binary formations.

Remnant densities depend strongly on core mass fractions.

Collisions tend to stabilize planetary systems.

Abstract

Studies of high-multiplicity, tightly-packed planetary systems suggest that dynamical instabilities are common and affect both the orbits and planet structures, where the compact orbits and typically low densities make physical collisions likely outcomes. Since the structure of many of these planets is such that the mass is dominated by a rocky core, but the volume is dominated by a tenuous gas envelope, the sticky-sphere approximation, used in dynamical integrators, may be a poor model for these collisions. We perform five sets of collision calculations, including detailed hydrodynamics, sampling mass ratios and core mass fractions typical in Kepler Multis. In our primary set of calculations, we use Kepler-36 as a nominal remnant system, as the two planets have a small dynamical separation and an extreme density ratio. We use an N-body code, Mercury 6.2, to integrate initially unstable systems and study the resultant collisions in detail. We use these collisions, focusing on grazing collisions, in combination with realistic planet models created using gas profiles from Modules for Experiments in Stellar Astrophysics and core profiles using equations of state from Seager et al. (2007), to perform hydrodynamic calculations, finding scatterings, mergers, and even a potential planet-planet binary. We dynamically integrate the remnant systems, examine the stability, and estimate the final densities, finding the remnant densities are sensitive to the core masses, and collisions result in generally more stable systems. We provide prescriptions for predicting the outcomes and modeling the changes in mass and orbits following collisions for general use in dynamical integrators.