EARTH

The Earth Grew by Collision

~4.54 billion years ago

Hadean

There was no moment the Earth appeared: around the young Sun, dust stuck together into pebbles, pebbles gathered into bodies a hundred kilometres across, and those bodies went on colliding and merging, each impact leaving a slightly larger world. Geologists call the process accretion. Meteorites time it: they preserve the decay of hafnium into tungsten, a natural clock showing that most of the Earth's mass was in place within a few tens of millions of years of the Solar System's birth.

Why it matters

This is the zero point of the timeline: every other date refers back to it. And how the Earth was born matters as much as when. A world assembled by impacts starts out molten, and its crust, its oceans and its atmosphere all came out of the cooling that followed. Life came later still.

Dating & uncertainty

The figure of 4.54 billion years dates the material the Earth formed from, not the moment it stopped growing. Accretion tailed off over tens of millions of years, and the last few per cent of the planet's mass arrived with the collision that formed the Moon; a single birth date is a convention, not a measurement.

Sources

1. Yin et al. (2002), Nature · A short timescale for terrestrial planet formation from Hf-W chronometry of meteorites
2. Kleine et al. (2002), Nature · Rapid accretion and early core formation on asteroids and the terrestrial planets from Hf-W chronometry
3. Halliday & Canup (2022), Nature Reviews Earth & Environment · The accretion of planet Earth

Earth history as one day

00:00:0024 h before midnight

if Earth’s 4.54 billion years were compressed into a single day