More proof that the asteroid belt started out as a planet?
That's what I thought, too! I read Immanuel Velikovsky's "Worlds in Collision" decades ago, and ever since then it made sense that a planet breakup formed the asteroid belt.
Negative:
A model of early solar system formation (and there is evidence supporting such) describes that metal, such as Nickel-iron, rock, and ice condensed out from the accretion disk created as our solar system formed. The metals condensed out first (this is why many of the asteroids are Nickel-iron) Followed by rocky material and ice. These tiny particles then collided creating small boulders and asteroids.
Once these small asteroids and boulders have enough mass, gravity becomes the driving force. Thusly the planets and moons are formed. However, since Jupiter is so large and the total mass of the asteroid belt is so tiny, the material forming the asteroid belt never was "allowed" to form a small planet or moon because of the gravitational perturbations from Jupiter. Remember the asteroid belt has less mass than 1 tenth of our moon.
Finally the solar wind from the newly formed star (our sun) would blow all of the remaining gas into interstellar space leaving us with the planets, moons, comets, asteroids, etc. circling our little star.
Note: This is a really simplified version. There is much (volumes of data) I did not include.
Also:
Jupiter has a profound effect on the asteroid belt.
Since Jupiter has a semimajor axis of 5.2 AU (I AU is the distance from the Sun to the Earth) it ends up with an orbital period of 11.86 years. Also, since the asteroids are not all at the same distance from the sun, their orbital periods differ in a direct relationship to their distance from the sun. This results in some of them having an orbital period of one half of Jupiter. This puts those particular asteroids in a 2:1 orbital resonance with Jupiter. The result of this resonance is gaps called Kirkwood’s gaps.
The rub is why did not this asteroid belt form a small planet? The reason is the gravitational force of Jupiter. It perturbs the asteroids giving them random velocities relative to each other.
Another effect of both Jupiter and the Sun on the asteroid belt is a group of asteroids that both precede and follow Jupiter in its orbit by 60 degrees. These asteroids are known as the Trojans.