Orbital resonance and the missing planet
The main asteroid belt orbits between Mars and Jupiter, occupying a region where astronomical theory would predict a planet should have formed. Instead, Jupiter's gravity created resonance traps: certain orbital periods near Jupiter became destabilized. Any planetesimal drifting into a 2:1 orbital resonance with Jupiter (completing two orbits for Jupiter's one) gets periodically yanked outward, accelerating away from formation. The 3:2 resonance and other multiples created similar gravitational vacuums. These gaps are visible as distinct minima in the asteroid density map. Over millions of years, Jupiter's resonant perturbations cleared the region of large bodies, preventing planetary assembly.
Today, the asteroid belt contains only about 4% of the moon's mass, scattered across millions of objects ranging from dust-grain size to Ceres (940 km diameter). This sparse population contrasts sharply with the dense planetary system that should have consolidated there if Jupiter had been farther away.
Migration and current structure
Modern solar system models propose that Jupiter and Saturn migrated inward during the first few million years, scattering smaller bodies outward and fragmenting any growing proto-planets in the asteroid belt region. This Grand Tack scenario explains not only the belt's low mass but also the scattered disk beyond Neptune and the Late Heavy Bombardment that peppered the inner planets 3.8 billion years ago. Today's asteroid belt is a fossil record of this violent early history, with resonance gaps acting as signposts of where Jupiter's gravity remains strongest.