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The main belt is a large gathering of asteroids in what is known as an “asteroid belt,” which exists between the planets of Mars and Jupiter in the Earth’s solar system. This belt is made of hundreds of thousands of asteroids that vary a great deal in size and are thought to have originated in one of two ways. It was once believed that these various asteroids existed as an early planet or protoplanet that was destroyed by a collision with an asteroid, though modern theories hold that the rocks are leftover matter from the formation of the solar system.
Also known as the main asteroid belt, the main belt consists of asteroids of greatly varying sizes. While some of the asteroids are fairly large, many of the objects are only particles of dust floating in space. The massive size of the main belt also means that its composition is primary made up of empty space, with most asteroids being very far from each other. Collisions between asteroids are possible, however. There are a number of comets that seem to travel throughout this belt as well.
There are two basic theories regarding the formation of the main belt in the Earth’s solar system. The first theory, which has largely been abandoned, was that a protoplanet began to form in the solar system, between the orbits of Mars and Jupiter. While this was forming, a large asteroid or other object impacted the protoplanet and destroyed it with such tremendous force that its remains were scattered throughout the region that has become the main belt. The relatively low mass of the various objects throughout the belt, however, make such a formation unlikely and the energy released in such destruction would have left a lasting impression on the solar system.
Today, the preferred theory as to the formation of the main belt is that the various objects are leftover from the formation of the solar system. The tremendous mass of Jupiter exerts sufficient gravitational force to prevent the matter from forming a planet. These early rocks and particles would have assumed an orbit around the sun, much like the planets, but could not have formed together into a planet. The low mass of the belt supports this theory, as does the type of matter found among the various asteroids, which indicate separate bodies that were never part of a single planet.