It implies that the planet must have a rocky core. They all move at the same speed. They share Jupiter's orbit. They are the largest asteroids. They do not have elliptical orbits.
Irons remain very hot long after they fall.
Within the nebula that formed the Solar System, most of the solid material condensed into planetesimals, which were the building blocks for planets. They are transparent and let us see deeper, darker layers. They are shadows. The core contracts and the outer layers expand. The core expands and the outer layers contract.