The circumstances under which the asteroid belt was discovered led to the assumption it represented material that never quite formed into a planet. What remains was thought to be a fraction of the region's original mass. Such thinking remains dominant today, but a new theory proposes the opposite – the space between Mars and Jupiter started off empty and gradually filled with escaping particles from elsewhere.
The first clue to the presence of tens of thousands of asteroids with orbits 2-3.5 times the Earth-Sun distance came when Ceres was found in 1801, inspired by the belief there should be a planet filling the gap in “Bode's Law”. Ceres was too small on its own to represent the missing planet, but as smaller asteroids were found, the idea arose that these were either the remains of a world that had broken apart or the pieces of something Jupiter's gravity had prevented from ever forming.
Recent versions of this theory have proposed a primordial belt with a mass at least as great as Earth's. Less than one two-thousandth of this remains, and while theories have been proposed as to where the rest went, not everyone is convinced.
“Rather than invoking a large mass in asteroids that requires later depletion, it is worth considering that the primordial asteroid belt may have simply been empty,” Dr Sean Raymond and Dr Andre Izidoro of the Université de Bordeaux write in Science Advances. “If this is true, the belt may represent a cosmic refugee camp, a repository for planetesimals implanted from across the solar system, none of which calls the asteroid belt home.”
The pair modeled the behavior of dust grains left over in the inner solar system after the planets had formed. Many of those that started out with orbits between 1 and 1.5 astronomical units (the distance between the Earth and Sun) ended up concentrating in the asteroid belt. Even some whose orbits started off near Venus ended up migrating out.
Although the processes are different, the authors also looked at dust that began its journey out near the gas giants. They concluded that a substantial fraction of this outer solar system debris may also have found its way to the asteroid belt, where Jupiter's influence kept it stable.
The fact that objects would tend to migrate to the asteroid belt from both locations explains why the asteroids are so different in composition and are unevenly distributed within the belt. S-type asteroids, mostly made of iron and magnesium-silicates, are most common towards the inner edge, while carbon-based C-types are more common further out. This makes sense if S-types represent the coalescing of inner solar system particles, while C-types are formed from components that migrated inwards.