The preplanetary clumps grew by accretion from objects that might be imagined to be the size of baseballs and basketballs to planetesimals, embryonic proto-planets several hundred miles across. The early solar system must have consisted of millions of planetesi-mals.
While smaller than mature planets, the planetesimals were large enough to have sufficiently powerful gravitational forces to affect each other. The result was near misses and collisions that merged planetesimals into bigger objects, but also fragmentation, as collisions resulted in chunks of some planetesimals breaking off. As we saw in the last chapter, the formation of the moon likely happened at this point in the history of the solar system.
The larger planetesimals, with their proportionately stronger gravitational fields, captured the lion's share of the fragments, growing yet larger, while the smaller planetesimals joined other planets or were "tossed out." A certain number of fragments escaped capture to become asteroids and comets.
Unlike the planets, whose atmospheres and internal geological activity (volcanism and tectonics) would continue to evolve matter (the earth, for example, has rocks and minerals that vary greatly in age), asteroids and comets remained geologically static, dead; therefore, their matter, unchanging, marks well the date of solar system birth.
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