The Maximum Density of a Collisionally-Produced Planet is A Function of its Mass and Orbital Period

Abstract There are many different theoretical explanations for the formation of high-density Mercury-like planets, but concrete evidence for any of these formation mechanisms remains elusive. A popular explanation for dense planets is the collisional hypothesis, which states that iron-rich planets can be formed as the products of high-energy, mantle-stripping impacts. Planetesimal collision simulations predict that higher-velocity collisions can form higher-density planets. Motivated by the characteristics of the high-density, short-period (P ≈ 0.3d) GJ 367 b, we study the results of previously-published smoothed-particle hydrodynamics (SPH) simulations on exoplanet collisions, combining these with models describing the likely collision velocities of these objects, to investigate the relationship between the core mass fractions (CMFs) of exoplanets, their masses, and their orbital periods. We predict that collisionally-produced super-Mercuries should be more common (and more dense) at low masses and short orbital periods. This correlation may enable us to pinpoint the formation mechanism of super-Mercuries as the population of observed targets grows. Afterwards, we connect our hypothesis to the observed Mercury-like population of high-density exoplanets, and find that GJ 367 b is the best exoplanetary candidate for collisional formation.