Two families of planets: the pattern that needs explaining

1. Compared with each other, the four planets nearest the Sun, Mercury, Venus, Earth and Mars, are small and rocky.
2. The four planets furthest from the Sun, Jupiter, Saturn, Uranus and Neptune, are large and gaseous.
3. That split is too neat to be a coincidence, and the accretion model is the account of how the Solar System formed that explains it.
4. The model rests on three ideas you need to be able to name: gravity, the many elements present in interstellar clouds, and the rotation that flattens the cloud into a disc.

It starts with a cloud that already contains everything

1. The Solar System began as a patch of an interstellar cloud, far larger and far thinner than anything in the Solar System today.
2. That cloud contained many different elements, not just hydrogen and helium gas but heavier elements locked up in solid grains of dust.
3. This mixture matters, because it means both the rocky raw material and the gaseous raw material were available from the start, so the difference between the planets came from where they formed, not from what was on offer.
4. Those heavier elements had themselves been made inside earlier generations of stars and scattered into space, which is why the cloud was chemically rich rather than pure hydrogen.

Gravity does the building, and it speeds itself up

1. Every particle in the cloud has mass, so every particle attracts every other particle gravitationally.
2. Wherever the cloud happened to be slightly denser, that region pulled in its neighbours, and the process then feeds on itself.
   1. Gravity pulls nearby dust grains and gas towards the dense region.
   2. They collide and stick, building clumps.
   3. Each clump now has more mass, so it pulls harder.
   4. Pulling harder sweeps up more material still, so the growth accelerates.
3. The clump that ended up at the centre gathered by far the most material and became the Sun.
4. The clumps that survived further out kept orbiting and kept growing, and those are the planets.

Rotation flattens the cloud into a disc, and the disc has a hot end

1. The original cloud was already rotating slowly, and as gravity pulled it inwards that rotation got faster, in the same way a spinning skater speeds up on pulling their arms in.
2. Material could fall freely towards the axis of rotation but not sideways across the spin, so the cloud collapsed into a flat rotating accretion disc rather than a sphere.
3. That is why the planets today all orbit in roughly the same plane and the same direction: they inherited the motion of the disc they grew in.
4. The disc was not the same everywhere, and that difference is the whole explanation for the two families of planets.
5. Close to the forming Sun the disc was hot.
   1. Light gases stayed as gas and were largely driven away, so mainly rocky and metallic grains were left to accrete.
   2. There was far less of that solid material to collect, so the inner planets ended up small and rocky.
6. Far from the Sun the disc was cold.
   1. Gases and ices survived there, so the growing planets had a much larger supply of material within reach.
   2. Once a core grew big enough its gravity could hold on to gas as well, so the outer planets ended up large and gaseous.