Outcomes of Sub-Neptune Collisions

Abstract

Observed high multiplicity planetary systems are often tightly packed. Numerical studies indicate that such systems are susceptible to dynamical instabilities. Dynamical instabilities in close-in tightly packed systems, similar to those found in abundance by Kepler, often lead to planet-planet collisions. For sub-Neptunes, the dominant type of observed exoplanets, the planetary mass is concentrated in a rocky core, but the volume is dominated by a low-density gaseous envelope. For these, using the traditional perfect merger assumption to resolve collisions is questionable. Using both N-body integration and smoothed-particle hydrodynamics, we have simulated sub-Neptune collisions for a wide range of impact parameters ($b^{\prime}$) and impact velocities ($v_{\rm{im}}$) to study the possible outcomes in detail. We find that the majority of the collisions with kinematic properties similar to what is expected from dynamical instabilities in multiplanet systems may not lead to mergers of sub-Neptunes. Instead, both sub-Neptunes survive the encounter, often with significant atmosphere loss. When mergers do occur, they can involve significant mass loss and can sometimes lead to complete disruption of one or both planets. Sub-Neptunes merge or disrupt if $b^{\prime}<b_{\rm{c}}^{\prime}$, a critical value dependent on $v_{\rm{im}}/v_{\rm{esc}}$, where $v_{\rm{esc}}$ is the escape velocity from the surface of the hypothetical merged planet assuming perfect merger. For $v_{\rm{im}}/v_{\rm{esc}}\lesssim2.5$, $b_{\rm{c}}^{\prime}\propto(v_{\rm{im}}/v_{\rm{esc}})^{-2}$, and collisions with $b^{\prime}<b_{\rm{c}}^{\prime}$ typically leads to mergers. On the other hand, for $v_{\rm{im}}/v_{\rm{esc}}\gtrsim2.5$, $b_{\rm{c}}^{\prime}\propto v_{\rm{im}}/v_{\rm{esc}}$, and the collisions with $b^{\prime}<b_{\rm{c}}^{\prime}$ can result in complete destruction of one or both sub-Neptunes.