While there is still much to learn about the processes that led to the formation of the planets, it is now fairly clear they formed from the collisions and sticking together of smaller objects that have been dubbed "planetesimals." In the more distant regions of the solar system, where the temperatures are colder and where gas could accumulate, giant planets like Jupiter were able to form relatively quickly. Jupiter's gravitational effects, in turn, were able to keep many of these planetesimals from forming into planets. The result was a large region of space where these "leftover" planetismals remained, which is the part of the solar system we now call the main asteroid belt.
It is clear that if we want to understand our origins, we need to study these asteroids, which are relatively primitive bodies that preserve clues as to the conditions that existed during the early days of the solar system.
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