Our solar system began forming about 4.6 billion years ago within a concentration of interstellar dust and hydrogen gas called a molecular cloud. The cloud contracted under its own gravity and our proto-Sun formed in the hot dense center. The remainder of the cloud formed a swirling disk called the solar nebula.

Within the solar nebula, scientists believe that dust and ice particles embedded in the gas moved, occasionally colliding and clumping together. Through this process, called "accretion," these microscopic particles formed larger bodies that eventually became planetesimals with sizes up to a few kilometers across. In the inner, hotter part of the solar nebula, planetesimals were composed mostly of silicates and metals. In the outer, cooler portion of the nebula, water ice was the dominant component.

ASC NASA PD