The key idea is that Jupiter’s strong gravity disrupted the region between Mars and itself during the early formation of the solar system. As Jupiter formed, its gravity stirred up the orbits of nearby planetesimals, raising collision speeds and ejecting material from that zone. Gravitational resonances with Jupiter also destabilized orbits, so material couldn’t peacefully accrete into a single rival planet. The result is a belt of many small bodies that never grew into a planet. Neptune’s gravity isn’t the main factor clearing that region; it’s far away from the Mars–Jupiter gap. A single large collision isn’t how the belt formed—it's leftover debris from a region that Jupiter’s perturbations prevented from becoming a planet. The Sun’s radiation alone wouldn’t have been the dominant force clearing the area given the early disk dynamics.