As the cloud rotated, it became a flat disc perpendicular to its axis of rotation. Most of the mass concentrated in the middle and began to heat up. Meanwhile, the rest of the disc began to break up into rings, with gravity causing matter to condense around dust particles. Small fragments collided to become larger fragments, including one collection about from the center: this would become the Earth. As the Sun condensed and heated, nuclear fusion started, and the solar wind cleared out most of the material in the disc, which had not yet condensed, into larger bodies.