New Study Says Primordial Asteroid Belt was Empty

"The standard picture is that the building blocks of the Solar System -- what we call planetesimals, generally thought of as 10-100 km-scale bodies -- started off in a smooth distribution across the Sun's planet-forming disk. The problem is, that puts a couple of times Earth's mass in the asteroid belt, where there is now less than a thousandth of an Earth mass. The challenge in this picture is therefore to understand how the belt lost 99.9% of its mass (but not 100%)."

"What we found is that the growth of the rocky planets is not 100% efficient. A fraction of planetesimals is gravitationally kicked outward and stranded in the asteroid belt. The orbits of captured bodies matches closely those of S-type asteroids. The efficiency of implanting S-types in the belt is quite low, only about 1 in 1000.  However, recall that the belt is almost empty.  There is a total of about 4 hundred-thousandths of an Earth-mass in S-types in the present-day belt.  Our simulations typically implanted a few times that amount. Given that some are lost during later evolution of the Solar System, this matches both the distribution and amount of S-type asteroids in the belt.