The asteroid belt is a region between Mars and Jupiter that is populated by millions of asteroids, but none of these asteroids are large enough to be considered a planet.

The largest asteroid, Ceres, is only about 950 kilometers in diameter, which is much smaller than the smallest planet, Mercury, which is 4,880 kilometers in diameter.

It's theorized that the asteroid belt was once a protoplanet, but its formation was disrupted by the gravitational pull of Jupiter.