How did the Sun and the planets form from a spinning cloud of gas?

The Sun and the planets formed together, about 4.6 billion years ago, out of a single cloud of gas and dust. Part of a giant cloud collapsed under its own gravity. Most of the material fell into the centre and became the Sun, while the rest flattened into a spinning disc around it. The planets, moons and asteroids all grew out of that disc, which is why they travel around the Sun near one flat plane.

The flattening was a side effect of spinning. Because of the conservation of angular momentum, the cloud spun faster as it shrank, like a skater pulling in her arms, and spinning material settles into a disc. The middle grew hot and dense, while the disc stayed cooler.

Inside the disc, tiny dust grains stuck together into clumps, and the clumps collided and merged into bodies called planetesimals, about ten kilometres across. Over a few million years, collisions made them grow bigger and bigger. Where the disc was warm, near the Sun, only metals and rock could stay solid, and these were scarce, so the inner planets stayed small. Farther out, past the frost line, ice could also stay solid, so there was much more building material. That is why the giants got so large that, today, Jupiter, Saturn, Uranus and Neptune hold nearly 99 percent of all the mass that orbits the Sun.

The party ended when the young Sun's solar wind blew the leftover gas and dust away, after three to ten million years. The idea behind all this, the nebular hypothesis, dates from the 18th century, when Swedenborg, Kant and Laplace proposed it.