Leftover planetesimals that formed in the region of the solar system now occupied by the jovian planets are called

Comets

Explanation:

Comets are planetary celestial bodies consisting of ice and dust, sometimes rocky particles  formed in the region of the solar system. Long-period comets propagate towards the Sun by gravitational perturbations caused by passing stars. Some comets usually hyberbolic comets, move through the inner Solar System  prior to entering the interstellar region. Short period comet lies beyond the orbit of the Neptune.

The Jovian planets include Jupiter, Saturn, Uranus, and Neptune.

Therefore, leftovers of comets (planetesimal bodies) formed in the region of the solar system that are now occupied by the Jovian planets is due to the dusty particles  associated with the comets.

jupiter

explanation:

it is so massive and has such a strong gravitational pull its likely prevented the formation of a planet between itself and mars in the asteroid belt

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