The many theories of the origin of minor planets may be divided into two major categories:

Remnants of Planetary Accretion

In this scenario, the disk of material from which the major planets formed consisted of gas and dust containing all the constituent elements of the present solar system. Through a process of collisions and accretion, larger and larger masses were formed. As these attained a certain critical mass, they began to sweep the solar system clean of the remaining protoplanetary material, until the only particles left were largely confined to the plane of the solar system and located between the orbits of Mars and Jupiter - the region known as the ``asteroid belt''.

The ``Exploding Planet'' Theory

This theory seeks to explain the existance of the asteroid belt at a place where a planet might have been expected to form, and to account for the distinctive compositions of minor planets and comets, which comprise three types of material one might expect to find in a terrestrial planet, namely, metallic core, lightweight rocky crust, and oceans (ice.) The theory is that a planet once existed at the present location of the asteroid belt and, for a reason as yet undetermined, exploded, scattering debris in all directions, most of which was ultimately ejected from the solar system, except for that in the asteroid belt and the comets. The planet Mars is postulated to have been a moon of this exploded planet.