The details are a mystery, but the basic story is that the Sun formed when part of a huge cloud of dust and gas (a ‘molecular cloud’) was triggered into a runaway collapse by some cosmic event (like a supernova say). As it collapsed it span faster and faster, spinning out a disk (like a chef spinning dough into a pizza). The disk was dense enough for solid particles to form and stick to each other, making bigger and bigger lumps until they reached the size of a planet. We can see this process going on right now for other young stars and planets in our galaxy..
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