No one really believes the Asteroid Belt is a "failed planet."

Much more likely, it's a planet that was destroyed, in the recent past. If it had been a collection of debris when the solar system formed, enough time would have elapsed to have let gravitational attraction fuse the asteroids into a planet.

http://www.bbc.com/news/science-environment-30916692

On Wed, 21 Jan 2015 15:42:59 -0800 (PST), RichA
wrote:

Much more likely, it's a planet that was destroyed, in the recent past. If it had been a collection of debris when the solar system formed, enough time would have elapsed to have let gravitational attraction fuse the asteroids into a planet.

It is certain beyond reasonable doubt that the asteroids are material
that failed to form planets. The asteroid belt is held in place by
gravitational resonances, which also prevent it from forming any sort
of planetissimals. Not that it likely would in any case, given its
extremely low mass (about that of Saturn's small moon, Rhea, even less
if you toss the four minor planets that make up half its mass) and
large volume. You could spend thousands of years crossing back and
forth across the asteroid belt and never encounter an asteroid. It's
basically empty space.

Regardless of how the asteroids formed and came to their present
location, it is certain that they were never a single coalesced body.