Step 3: Building the Core and Crust

In step 1 dust grew into planetesimals, rocky bodies a few kilometres across. Now those bricks need to become a building — and the process is far more violent than any construction site on Earth.

Runaway growth

Once a planetesimal is big enough, its gravity starts to matter. It pulls in nearby bodies, gets bigger, and pulls even harder. The largest bodies in a region grow fastest, a process called runaway growth. Within perhaps a few hundred thousand to a few million years, the inner solar system was filled with dozens of protoplanets — worlds roughly the size of the Moon to the size of Mars.

The general name for this growth by collecting material is accretion.

The age of giant impacts

Protoplanets then compete for space. Their orbits cross, and for tens of millions of years they smash into each other. These giant impacts are the final stage of building a rocky planet.

Earth's last big collision is thought to have produced the Moon. According to the leading hypothesis, a Mars-sized body (often called Theia) struck the young Earth about 4.5 billion years ago. Debris thrown into orbit gathered together to form the Moon. Scientists still debate the details, because the Moon's rocks are surprisingly similar in chemistry to Earth's.

Melting the planet

All this construction generates enormous heat:

• Impacts turn the energy of motion into heat.
• Radioactive elements, especially short-lived aluminium-26 in the earliest bodies, release heat as they decay.
• Gravity squeezing material together adds more.

The young Earth was probably covered, at least at times, by a global magma ocean hundreds of kilometres deep. That sounds like a disaster, but it is exactly what a planet builder needs for the next step.

Sorting by weight: differentiation

When a planet is molten, its ingredients can move. Heavy iron and nickel sink towards the centre; lighter silicate rock floats upward. This sorting is called planetary differentiation, and it gives rocky planets their layered structure.

Earth's layers today, from the inside out:

Layer	What it is made of	Approximate thickness
Inner core	Solid iron–nickel	radius about 1,220 km
Outer core	Liquid iron–nickel	about 2,260 km
Mantle	Hot, slowly flowing silicate rock	about 2,900 km
Crust	Light rock: basalt under oceans, granite-like rock on continents	5–70 km

Chemical clocks in meteorites and Earth rocks suggest that Earth's core finished separating within the first few tens of millions of years.

Why the core matters so much

A core is not just ballast. Earth's liquid outer core churns as it slowly cools, and moving molten iron generates a magnetic field. That field creates a protective bubble, the magnetosphere, which deflects much of the charged particle stream from the Sun. Mars once had a magnetic field too, but its smaller core cooled and the field shut down billions of years ago.

Lesson for builders: a planet needs to be big enough to stay warm inside for a long time.

A crust that keeps moving

As the surface cooled, a solid crust formed. On Earth it broke into large plates that move a few centimetres a year — about as fast as fingernails grow. Plate tectonics builds mountains, opens oceans and, crucially, recycles carbon between the surface and the interior, which helps keep the climate stable over billions of years. Earth is the only planet known to have it today.

What size should you build?

• Too small (like Mars or the Moon): cools quickly, loses its magnetic field, struggles to hold an atmosphere.
• Earth-sized: stays geologically active for billions of years.
• A few times Earth's mass: a “super-Earth”. These are common around other stars. Some may be excellent homes; others may grow thick hydrogen envelopes and become mini-Neptunes. Where the boundary lies is an active research question.

Builder's checklist for step 3

• Planetesimals grow into protoplanets by accretion
• Giant impacts finish the job — and may give you a moon
• Heat melts the planet so iron can sink into a core
• A big, hot core powers a magnetic field
• A crust forms on top — ideally one that keeps moving

You now have a solid, layered world. It still has no air to breathe: Step 4: Adding an atmosphere.