Since there's not much of a way to guess what size the planet in question originally was, or how it exploded, and therefore, how much of its mass would rightfully be expected to remain in a matching orbit, that seems like a rather doubtful datapoint to me.
Then just assume that the mass would be roughly the mass of the existing rocky planets.
The dynamic at play is the gravitational force of the protostar. The larger rocky fragments would fall towards the center while the smaller ones would be pushed away by the solar winds. As the fragments get larger due to attraction/collisions with other fragments, centrifugal force kicks in and the fragments begin to orbit on their own, sweeping up more dust and fragments in their paths. There is an equilibrium point where the fragments haven't fallen in and are too dense to be pushed away further when the protostar ignites and blows the dust away. These fragments became the asteroid belt, but weren't big enough to form a planet (the mass wasn't enough to attract the lumps to each other to form one single mass).
-PJ -PJ