Planet planetesimal building blocks

Anhydrous planetesimals formed within the inner solar system, unlike the ice-bearing bodies discussed in the next chapter. These objects, composed of rock and metal, were the primary building blocks of the terrestrial planets. Relics of that population may survive today as asteroids that dominate the inner portions of the main belt. 

Many of the observations described in the previous section can be explained by the planetesimal theory, in which rocky planets form by the aggregation of huge numbers of asteroid-sized building blocks called planetesimals (Safronov 1969). This... 

Both comets and asteroids are surviving planetesimals from the solar nebula, preserved building blocks of planets. Comets are generally considered to be relic materials from the outer regions of the solar nebula, while asteroids are considered to be materials that formed or ended up near the asteroid Belt, mostly between 1.9 AU and... 

Planetesimals The building blocks of the planets. Small rocky or icy bodies formed in the primordial solar nebula. They range greatly in size from one kilometer up to the size of small planets. 

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