The two families of planets are not an accident. They are fossils of how the solar system formed, about 4.6 billion years ago, from a cloud of gas and dust. Astronomers have since watched the same process happening around young stars.
Why it works
A cloud of gas and dust, slowly turning, begins to collapse under its own gravity. As it shrinks it spins faster, because its angular momentum (which goes as $r^2\omega$ for each spinning part) stays the same, like a skater pulling in her arms. Spinning fast, it cannot collapse along its equator as easily as along its axis, so it flattens into a disk. Most of the mass falls to the center and becomes the Sun. In the disk, grains of dust stick together into pebbles, then into kilometer-sized planetesimals, which collide and grow into planets. What could freeze depended on temperature: near the young Sun only rock and metal could condense, but beyond the frost line, a few AU out, ices froze too. With much more solid material out there, large cores formed quickly and pulled in hydrogen and helium gas: the giants.
Key idea
The solar nebula collapsed and spun up into a flat disk; the Sun formed at the center and the planets formed beside it, in the disk. Rock and metal condensed everywhere, ices only beyond the frost line, which is why small rocky planets lie inside and giants outside.
The evidence
- All eight planets orbit the Sun in the same direction, in nearly the same plane, and the Sun spins the same way: the marks of one spinning disk.
- Rocky planets lie inside, giants outside, as the temperature of the disk predicts.
- The oldest meteorites date to 4.57 billion years ago, the same for every one: the planets' building blocks formed together.
- Telescopes such as ALMA have imaged dusty disks around young stars, some with gaps where planets seem to be forming.
Worked example
Worked example: Spinning up
A slowly turning cloud shrinks to one-tenth of its radius. How many times faster does it spin?
- Angular momentum stays the same: $r_1^2\omega_1 = r_2^2\omega_2$.
- $\frac{\omega_2}{\omega_1} = \left(\frac{r_1}{r_2}\right)^2 = 10^2 = 100$.
- A barely turning cloud becomes a rapidly spinning disk.
Answer: $100$ times faster.
Common mistake
An old idea is that the planets formed from material torn or thrown out of the Sun. The evidence points the other way: the planets formed in the disk around the Sun, at the same time as the Sun. Check: material from the Sun would be mostly hydrogen and helium everywhere, yet the inner planets are rock and metal, sorted by temperature just as the disk model predicts.