Abstract
The final stage of planet formation is dominated by collisions between planetary embryos. The dynamics of this stage determine the orbital configuration and the mass and composition of planets in the system. In the solar system, late giant impacts have been proposed for Mercury, Earth, Mars, and Pluto. In the case of Mercury, this giant impact may have significantly altered the bulk composition of the planet. Here we present the results of smoothed particle hydrodynamics simulations of high-velocity (up to ~5v esc) collisions between 1 and 10 M ⊕ planets of initially terrestrial composition to investigate the end stages of formation of extrasolar super-Earths. As found in previous simulations of collisions between smaller bodies, when collision energies exceed simple merging, giant impacts are divided into two regimes: (1) disruption and (2) hit-and-run (a grazing inelastic collision and projectile escape). Disruption occurs when the impact parameter is near zero, when the projectile mass is small compared to the target, or at extremely high velocities. In the disruption regime, we derive the criteria for catastrophic disruption (when half the total colliding mass is lost), the transition energy between accretion and erosion, and a scaling law for the change in bulk composition (iron-to-silicate ratio) resulting from collisional stripping of a mantle.
Highlights
IntroductionMore than 300 extrasolar planets have been discovered. Of these, more than 10 have masses 10M⊕
To date, more than 300 extrasolar planets have been discovered
The chemical composition of planets in the solar system generally reflects the gradient in the nebula, the final collisions forming each planet may involve embryos scattered from regions with different bulk composition; the final composition of each planet is thought to be dominated by the last few impact events (e.g., Wetherill 1994; Chambers 2004)
Summary
More than 300 extrasolar planets have been discovered. Of these, more than 10 have masses 10M⊕. With the launch of the COROT (Borde et al 2003) and Kepler (Borucki et al 2003) satellites, it is expected that many transiting super-Earths will be discovered in the few years. The radius determined from the transit, when combined with radial velocity measurements of the planet’s mass, can be used to determine a mean density and infer a bulk composition (Valencia et al 2007). The formation of super-Earths may involve very high velocity impacts between large bodies (e.g., 0.1 to several Earth masses). Collision outcomes will vary widely depending on the impact velocity, mass ratio between the bodies, and the impact angle (Agnor & Asphaug 2004; Asphaug 2009; Stewart & Leinhardt 2009). The conditions for imperfect merging are interesting as they provide an opportunity to further alter the bulk composi-
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