In the past, the asteroid belt was thought to be the remains of a shattered planet. Of course, that was all pseudoscience, the mass of the asteroid belt being 4% of the moon. I've been wondering, what would happen if this did occur. What would an asteroid belt with around 50% of the Earth's mass be like? How often would asteroids hit earth? Would it influence Mars?
The mass of a planet? There probably was, billions of years ago, and perhaps still is, but the debris of this planet-in-the-making has been shattered and dispersed by impacts. We know from fragments which have struck the Earth that the planet which was forming there had grown large enough to have a molten interior where the heavy metals, mostly iron and nickel, had sunk to the centre and differentiated from the rocky mantle. A massive impact must have shattered this forming planet about 4 or 5 billion years ago, and the debris has become so dispersed that it is difficult to assess how much there originally was. Some was kicked out of the asteroid belt completely, the rest is still there but much of it so finely shattered and dispersed that we can't see it (the asteroid belt covers an enormous area, much larger than the orbit of the Earth-moon system).
The resulting absence of a body with substantial gravitational field to attract comets & other objects has enabled them to pass on by, perhaps to impact the Earth and inner planets. Bodes Law says here should be a planet there, but whether it would have been a Mars-sized planet or not, nobody knows. The differentiation of nickel-iron from the rocky material becomes hard to explain without a fairly substantial planetesimal there to separate them, and in our solar system at least, Bodes Law seems fairly reliable.