Step 1: Gathering the Materials

Before you can build a planet, you need a construction site and a pile of materials. In nature, both are supplied by the same event: the birth of a star.

The quarry: a cold cloud of gas and dust

Galaxies are full of molecular clouds — vast, cold regions of gas (mostly hydrogen and helium) sprinkled with dust grains made of silicates, carbon compounds and ices. A single cloud can hold enough material for thousands of stars.

These clouds can float quietly for millions of years. Then something tips the balance: a shock wave from a nearby supernova, a collision with another cloud, or simply gravity slowly winning against the gas pressure. A dense knot inside the cloud begins to collapse under its own weight.

The spinning disk

As the knot collapses it spins faster, the same way a figure skater speeds up by pulling in their arms. Most of the material falls to the centre and becomes a young star. The rest cannot fall straight in — its spin flattens it into a thin, rotating pancake around the star called a protoplanetary disk.

This disk is your construction yard. By mass it is roughly 99% gas and only about 1% dust, but that 1% is what rocky planets are made from. Telescopes such as ALMA in Chile have photographed disks around young stars like HL Tauri, showing bright rings and dark gaps where planets appear to be forming right now.

Our own Sun had a disk like this about 4.57 billion years ago. The oldest solid grains found in meteorites date from that time, which is how we know the age of the solar system so precisely.

Sorting the materials: the frost line

Close to the young star, it is hot. Only rock and metal can stay solid there. Farther out, past a boundary called the frost line, it is cold enough for water, ammonia and methane to freeze into ice.

This one boundary shapes everything that follows:

• Inside the frost line there is little solid material to work with, so planets stay small and rocky — like Mercury, Venus, Earth and Mars.
• Outside the frost line ice adds a huge amount of extra building material. Planets there can grow large enough to pull in gas from the disk and become giants like Jupiter and Saturn.

In our solar system the frost line sat somewhere between the orbits of Mars and Jupiter. That is why the planets split so neatly into small rocky worlds on the inside and giants on the outside.

From dust to pebbles to planetesimals

Dust grains in the disk are tiny — about the size of smoke particles. Growing them into planets takes several stages:

1. Sticking. Grains drift and bump into each other at low speeds. Static electricity and surface forces make them cling together into fluffy clumps.
2. Pebbles. Clumps grow to millimetre and centimetre sizes. Here builders hit a famous problem: pebbles tend to bounce or shatter instead of sticking, and gas drag makes them spiral towards the star within a few thousand years.
3. Planetesimals. One leading explanation for how nature solves this is the streaming instability: pebbles bunch up in the gas like cars in a traffic jam, until a cluster is dense enough to collapse under its own gravity into a solid body kilometres across. These bodies are called planetesimals.

Planetesimals are the bricks of planet building. Once something is a few kilometres wide, its gravity starts pulling in neighbours, and growth speeds up dramatically. That is the subject of step 3.

The clock is ticking

There is a deadline. Observations of young star clusters show that most protoplanetary disks lose their gas within a few million years, blown away by the star's radiation and winds or swallowed by the star. Any planet that wants a thick hydrogen envelope — a gas giant — has to grow a big enough core before the gas runs out.

Rocky planets are under less pressure. They can keep growing from planetesimals and collisions for tens of millions of years after the gas is gone.

Builder's checklist for step 1

• A collapsing cloud that forms a star and a disk
• Rock and metal near the star, ice farther out
• Dust that grows into pebbles and then into planetesimals
• Enough time before the disk disappears

With materials on site, the next decision is where to build. Continue to Step 2: Choosing the orbit.