The large number of exoplanets found to orbit their host stars in very close orbits have significantly advanced our understanding of the planetary formation process.

 It is now widely accepted that such short-period planets cannot have formed in situ, but rather must have migrated to their current orbits from a formation location much farther from their host star. In the late stages of planetary formation, once the gas in the protoplanetary disk has dissipated and migration has halted, gas giants orbiting in the inner disk regions will excite planetesimals and planetary embryos, resulting in an increased rate of orbital crossings and large impacts. They find that a mechanism is revealed by which the collision–merger of planetary embryos can kick terrestrial planets directly into orbits extremely close to their parent stars.