The Solar System began when part of a molecular cloud — enriched by elements forged and scattered by earlier stars — collapsed under its own gravity. As matter piled toward the center, temperature and pressure rose until hydrogen fusion ignited and the Sun became a star. The remaining material, carried by rotation, flattened into a disk encircling it.

Within that disk, dust grains collided and stuck together; over millions of years pebbles, boulders, and planetesimals grew. In the hot inner region, where volatiles were driven off, rocky planets formed; in the cold outer region, where ice and gas could condense, giant planets took shape. The process was not smooth but full of collisions, and orbits only settled toward today's arrangement over hundreds of millions of years.

The model rests on observation: radioactive isotope ratios in meteorites date the system to about 4.57 billion years, and disks of the same kind have been directly imaged around distant young stars. Earth itself condensed from material in the inner disk — the stage for every event that follows was set here.