Solar system formation and planetary astronomy

The Solar System began as a solar nebula: a giant molecular cloud, or a fragment of one, slowly rotating under its own weak gravity. Once the cloud became dense enough, gravity overcame pressure and it began to collapse. A cloud with even a small initial rotation cannot collapse into a point — angular momentum has to be conserved, so as the cloud shrinks it spins faster (the same effect that speeds up a spinning skater pulling in their arms), and it flattens perpendicular to its spin axis into a disk, because collapse along the spin axis has nothing left to conserve against while collapse in the disk plane is opposed by rotation. Almost all of the cloud's mass fell to the centre, where rising pressure and temperature eventually ignited hydrogen fusion and created the Sun.

The leftover disk — the protoplanetary disk — is where the planets built up, in stages: microscopic dust grains stuck together electrostatically into pebbles, pebbles collided and grew into kilometre-sized planetesimals, and planetesimals' mutual gravity let the largest ones sweep up the rest into full-sized planets. What each planet turned into depended heavily on where in the disk it formed. Close to the young Sun, temperatures were too high for anything but rock and metal to condense, so the inner planets are small, dense and rocky. Beyond a critical distance from the Sun — the frost line — temperatures were low enough for water, ammonia and methane to freeze into solid ice, adding enormously more raw material to build with; the giant planets that formed out there could grow massive enough to also gravitationally capture large amounts of hydrogen and helium gas directly from the disk.

Because every planet formed from the same flattened, rotating disk, their orbits all lie close to one plane and go around in the same direction — and because gravity is the only significant force acting between the Sun and each planet at their present separations, those orbits obey Kepler's laws (and Newton's law of gravitation, which explains why Kepler's laws hold) to excellent approximation.