EARTH
The Formation of the Earth
~4.54 billion years ago
The Earth formed about 4.54 billion years ago by accretion, the process in which grains of dust around the young Sun stuck together into ever larger bodies that collided and merged.
Accretion is the process by which the Earth formed. The young Sun was surrounded by a disc of gas and dust, known as the solar nebula. In that disc, grains aggregated into pebbles, then into bodies some hundred kilometres across, called planetesimals. The planetesimals continued to collide and merge, and each collision produced a more massive body. Researchers read the chronology of this growth in meteorites. A radioactive form of the metal hafnium decays into another metal, tungsten. Its half-life, the time in which half of it decays, is short on the scale of the solar system. Researchers use this decay to date the separation of the Earth's core, its centre, from the mantle, the rock that surrounds the core. This dating shows that most of the Earth's mass was assembled only a few tens of millions of years after the formation of the solar system.
Why it matters
The formation of the Earth is the starting point of the timeline, and every other date refers back to that event. A planet assembled by collisions begins in a molten state. The Earth's crust, which is its solid outer layer, its oceans and its atmosphere all result from the cooling that followed. Life appeared only much later.
Dating & uncertainty
The age of 4.54 billion years is that of the material of which the Earth is made, and not that of the end of its growth. Accretion lasted several tens of millions of years, and the last per cent of the Earth's mass arrived only with the collision that formed the Moon. A single date of birth for the Earth is therefore a convention and not a measurement.
Earth history as one day
00:00:0024 h before midnight
if Earth’s 4.54 billion years were compressed into a single day