Why are there no planets in the asteroid belt?

Planetary scientist David Rothery took on this planetary puzzle...

David - I think you’re pretty much on the ball there David. Most scientists are pretty sure that it was Jupiter that stopped a planet growing where the asteroids now are. There clearly was planet-forming material all the way from near the Sun outwards to Jupiter and beyond and it would clump together into larger and larger bodies, eventually planet-sized bodies. But too close to Jupiter and Jupiter’s gravity would stir up the orbits of the bodies just inside its orbit, which is where the asteroid belt is, and cause the collisions between such bodies to be too energetic to allow materials to clump together, so no very large bodies grew. The largest bodies there today are less than about 1,000 kilometres across. So they’re stirred up and most of the material has, in fact, been lost because of Jupiter. We couldn’t make a planet now if we stuck all the asteroids together. It’s about a hundredth of an Earth mass in terms in total. It’s a surprisingly small amount even though there are thousands of bodies there.

Chris - Where’s it all gone? Has it become asteroids and impactors that rained down on us in the early phases of the solar system then?

David - If we’ve lost about an Earth mass or more of material from where the asteroid belt is. A lot of it’s been scattered out of the solar system. Some scattered inwards and would have hit Mars and the Earth and Venus and Mercury, and doubtless, some has been gobbled up by Jupiter. We’d scarcely notice it because Jupiter is 318 times the mass of the Earth. But Jupiter stirred stuff up and that’s what stopped a planet growing between Jupiter and Mars.