Answer:
As the proto-planet grew in size its increased gravitational field would have attracted even more objects its surface. The composition of these colliding bodies would have included metal-rich fragments (i.e.., iron meteorites), rocky fragments (i.e., stony meteorites), and icy fragments (i.e., comets). Although accretion was much more prevalent in the early stages of the Earth's history, these extraterrestrial collisions are still occurring today, exemplified by shooting stars and fireballs in the night sky, and by the occasional impact of larger bodies on the Earth's surface.
The very large amount of kinetic energy inherent in these moving bodies is instantly converted to heat energy upon impact, thus providing a component to the Earth's internal heat source.
