Fusion. Every solid thing in the universe was created by lighter atoms of gas collecting into clouds. The gravity of the atoms gather them together until a star is formed. The pressure and energy of all that stuff force smaller atoms to bond together into larger heavier ones.
Eventually that shift from light elements to heavy ones unbalances the reactions of the star and it explodes outward, flinging those now fused elements out in a cloud of dust and leftover gas. That dust and gas gathers together bit by bit until eventually a planet is formed. Sometimes the remaining gas can form a new smaller star.
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