During the formation of the solar system, much of the rocky material in the solar nebula collapsed to form the inner, "rocky" planets (Mercury, Venus, Earth, and Mars). Outside of the orbit of Mars, however, a large amount of rocky material was never able to form a planet. Most scientists agree that this is due in part to the gravitational pull of Jupiter. The inner, rocky planets formed from the progressive collisions of proto-planets. In the asteroid belt, the collisions of protoplanets were of such high energy that the material failed to accumulate into larger bodies, instead breaking up into smaller and smaller pieces. So, the end result is a broad zone of rocky bits from dust up to pieces the size of the state of Montana. Although science fiction movies often portray the asteroid belt as densely populated by asteroids that pose a threat to spacecraft, in reality, asteroids are very far apart and the belt is easily passed by spacecraft. The total mass of asteroids in the asteroid belt is approximately 4% of the mass of the moon, so even if it had formed a planet, it would have been a very small one!

The asteroid belt is a remnant of the accretion disk surrounding the early Sun from which the planets formed. About 4568 million years ago, the Sun was just ending its build up stage and there was a pancake like disc with the sun at the center of small particles of rock and lots of Hydrogen and Helium gases. This disk is called the SOLAR NEBULA or PROPLYD proto planetary disk. In the next 30 to 70 million years, collisions between the solid particles (rocks) in this disk made the number of particles decrease and the size of a few particles grow to house size, then Santa Barbara size, then California size, then planet size. This process is called COLLISIONAL ACCRETION and it is how the planets formed. The asteroid belt is the left over stuff that never accreted into bigger bodies like Earth.

The formation of the asteroid belt in our solar system has several theories. One previous theory held that the asteroid belt formed from the remains of a planet that had been destroyed by a collision or explosion of some sort, but this is no longer the most likely case. Instead, the asteroid belt is believed to have formed in the same process as planets. However, unlike the planets, where tiny pieces of matter eventually fused together, the asteroids never became that large. The heavy mass of nearby Jupiter is believed to have interfered with the orbit of the asteroids, which prevented them from forming larger planets.

A similar method could also be responsible for asteroid belts in solar systems with several smaller planets close to the sun, followed by larger gas giants further out.

The main asteroid belt lies between the orbit of Mars and Jupiter. The solar system formed about 4.6 billion years ago when a cloud of dust and gas began to collapse in on itself due to the force of gravity. After the sun formed, it was surrounded by a disk-shaped cloud of left- over dust and gas, known as the protoplanetary disk. As these dust particles collided with each other they stuck together and formed bigger and bigger bodies; this process is known as accretion. Once these bodies are large enough (about a kilometer across), they can draw in other bodies through the gravitational attraction and become planetesimals. However, the effect of Jupiter´s gravity made it so that the planetesimals in what is now the asteroid belt were unable to form a planet. Instead, when they collided with each other they broke apart to form the asteroids we see today.