Planet formation
How were the planets formed?
The planets formed from gas and dust around the young Sun. Small pieces joined into larger bodies, and gravity helped some grow into planets. Different materials and conditions produced rocky planets and giant planets.
A disc, then worlds
The young Sun was surrounded by a flat cloud of gas and dust, spinning. Grains stuck. Pebbles stuck. After a few million years there were bodies the size of towns, then of moons, then of planets. Leftover gas either got blown out or got pulled onto the biggest worlds.
You can still see the disc in the finished system: almost everything goes the same way, and almost everything sits in the same thin plane. The exceptions — Venus spinning backwards, Uranus on its side, Triton going the wrong way — each need an extra event.
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They did not finish where they sit
Models in which the giant planets drifted inward and back out — dragging small bodies, emptying part of the asteroid belt, scattering the Kuiper belt — reproduce a great deal of what we see. Which version happened, and when, is not agreed. The zoom ladder shows where they are now, not the road they took.