The one that didn’t make it
Not all the matter in the solar system has been swept into the Sun or the planets. There are many other objects out there, such as comets, meteorites and asteroids, although their combined mass is trivial, less than half the mass of the Moon. In the main, these do not concern us here; however, there is one part of their story which is relevant to this chapter's planetary interests.
We have known for a long time that meteorites found on Earth are either stony or iron-based. Some of the stony ones look like unaltered material from the early solar system. But others look more like terrestrial rocks. The only explanation for how they got this way is that they must have been built into a planet that later broke up. Similarly, the iron ones contain patterns that show they must have cooled very slowly, probably in the core of a planet.There are even a few “stony irons” that seem to have come from the boundary between the iron core and the rocky outer layer of a planet.
This long-gone planet from the early solar system must have been tiny compared to the Earth. To judge from the leftover parts that arrive on the Earth today, it never developed oceans and weather of its own, much less life. The rocks are quite unlike the sedimentary types formed by water and weather at the surface of the Earth.