How did Earth form?

From a cloud of dust to a planet in 4.6 billion years

It all began 4.6 billion years ago with a huge cloud of gas and dust. Some push — perhaps the shock wave of a star exploding nearby — squeezed the cloud, and it began collapsing under its own weight. At the centre matter packed together and heated up — the Sun ignited. The leftover dust and gas flattened into a spinning disc.

In the disc, dust grains stuck together into pebbles, then into kilometre-sized 'planetesimals'. They collided and merged, growing into planets within a hundred million years. Close to the hot Sun only rock and metal remained — which is why Mercury, Venus, Earth and Mars are rocky; farther out, ice survived and the gas giants grew.

The young Earth was a ball of molten lava. Heavy iron sank to the centre and formed the core — the source of our magnetic field. Then a Mars-sized body named Theia struck Earth: the debris gathered in orbit and formed the Moon. That is why Moon rocks look so much like Earth's.

Once the planet cooled, water vapour from volcanoes fell as rain — for hundreds of millions of years! — and filled the oceans. Icy comets may have brought water too. 3.8 billion years ago the first living cells appeared in the ocean. The oxygen they released changed the air and paved the way for life as we know it.

How do we know? The age of meteorites (4.56 billion years) is measured with radioactive 'clocks'; Moon rocks were brought back by Apollo; telescopes still see 'newborn' discs around other stars — HL Tauri, for example. Earth's history is a vast detective story that scientists are still unravelling.