What you will learn in How the Solar System Formed

1. 1. The collapsing cloud

   The Solar System began as a cloud of gas that was 98.5% hydrogen and helium. As it fell in on itself it spun faster: shrunk to a tenth of its size, a hundred times faster.

2. 2. Why it flattened into a disc

   The cloud was round, not flat. Collisions cancelled its up-and-down motions, but nothing could cancel its total spin, so it settled into a disc with every orbit going the same way round.

3. 3. The frost line

   Close to the young Sun only rock and metal could stay solid. Beyond the frost line, where the disc was colder than 182 K, water froze too. One million years in, that line was at 3.26 au.

4. 4. From dust to planets

   Dust stuck into pebbles, pebbles into kilometre-sized planetesimals, and those into protoplanets. Beyond the frost line there was 2.17 times as much solid to build with, enough for cores that grew into giants.

5. 5. What was left over

   The asteroid belt is what never became a planet, stirred by Jupiter and mostly empty. The median asteroid in a sample of 7,668 has its nearest neighbour 16 million km away.