Astronomers have good reason to believe that when the solar system was young, that bodies of many different sizes formed from the rotating disk of gas in the early solar 'nebula' orbiting the infant sun. The process began with the assembly of micron-sized dust grains into centimeter-sized particles, and thecoagulation of these granules into kilometer-sized bodies. Even larger bodies assembled from these thousands of kilometer-sized chunks by direct collision at slow speeds until hundreds of bodies ranging in size from 100 to several thousand kilometers accumulated. Nine of these were to become the known planets, but a second population of these large bodies would eventually be trapped into orbit as satellites. When the solar system was still only a few hundred million years old, these large bodies impacted the surfaces of the still larger bodies leaving behind large impact 'basins' and craters, while in other collisions some of these large bodies were shattered into rubble. Perhaps as many as 5-10 of these bodies may have existed in the asteroid belt because meteorites recovered and studied come from only 5-10 distinct chemical families, plus they already show signs of differentiation into 'iron-nickel 'carbonaceous' or 'stony' types which indicated an origin in larger bodies that have fractionated internally. Astronomers think that as the last of these large bodies were absorbed by the existing planets, they left their marks by forming the earth's moon, and tilting the rotation axis of the smaller planets so that they no longer are nearly perpendicular to the plane of the ecliptic or their own orbital planes. This would account for the large axial tilts of Venus, Earth, Mars, and Uranus and Neptune.

This process seems very plausible given what we know of the dynamics and contents of the early history of the solar system.