Your turn
Each lab is one step from the video. Change things and see what happens.
Each lab isolates one step of the model: accretion, differentiation, the giant impact and the cooling that made oceans. Pictures are not to scale.
Tap the buttons and slide the sliders.
Tap to look through the lens. How old is the crystal inside?
A Jack Hills zircon, ~250 microns across, dated to 4.4 billion years by uranium–lead decay.
Tap to look closer. What is inside?
The crystal is drawn much bigger than life.
Drag through the dust. Do the clumps grow and crash?
Grains stick, planetesimals grow and gravity takes over. Stir the disc and fast-forward.
Drag your finger. Do the bits stick?
A toy model: gravity and sticking are exaggerated. More in how do planets form?
Cool it down or heat it up. When does the iron sink?
Only a molten planet can sort itself: dense iron–nickel sinks, silicates rise.
Make it hot. Where does the heavy metal go?
Layer thicknesses in the cut-away are not to scale.
Drag Theia, let go, and watch the crash make a ring, then a Moon.
Aim the impactor; the debris disc coalesces into a moon. Real impacts were oblique and far slower to settle.
Pull the little planet and let go!
Sizes and timing squashed to fit. More in how did the Moon form?
Slide through time at this one spot. When do the oceans fill?
Magma ocean, crust and outgassing, a few hundred million years of rain, then today’s coast.
Slide time. When does it rain?
Time is squashed: the slider is not to scale.
Earth was made by lots and lots of crashes, a very, very long time ago.
Long ago there was no Earth. There was a big cloud of dust. It spun around, and the middle became our Sun.
Tiny bits of dust bumped and stuck together, like fluff balls under your bed. The clumps grew into rocks. The rocks crashed into each other and grew bigger. One of them became Earth!
All the crashes made young Earth so hot it melted. Heavy metal sank to the middle. Lighter rock floated to the top. That is why Earth has layers, like a peach with a stone inside.
Another small planet crashed into Earth. Hot rock flew up into a ring, and the bits stuck together to make our Moon.
Slowly Earth cooled down. Volcanoes puffed out steam. Then it rained and rained, and the rain filled the first oceans. Life began in the sea.
Want more? See how planets form, how the Moon was made, or how volcanoes erupt.
Earth formed about 4.54 billion years ago from the leftovers of the Sun's birth.10 A huge cloud of gas and dust collapsed and spun into a flat disc. The middle became the Sun. In the rest of the disc, specks of dust stuck together into pebbles, then boulders, then space rocks kilometres across called planetesimals. These crashed together and grew, a process called accretion, until a few became planets.12
The crashes made young Earth so hot that much of it melted. Heavy iron sank to the centre to make the core, and lighter rock floated up to make the mantle and crust.23 A Mars-sized body probably smashed into Earth and the debris formed the Moon.6 Then Earth cooled, water gathered and oceans formed.58
In short: Earth wasn't made in one go. It was built up, crash by crash, over tens of millions of years.5
(For the whole story of the dust disc and how every planet was built, see how do planets form?)
Accretion simply means growing by adding material. In the young solar system, grains of dust circled the Sun at slightly different speeds, so they kept bumping into each other and sticking, a bit like fluff balls gathering under a bed.5 Once a clump was big enough, maybe a few kilometres across, its own gravity started to matter. It could pull in smaller rocks nearby, and the bigger it got, the faster it grew.2
It was violent. Computer models show planetesimals crashing into each other, sometimes shattering, sometimes merging.1 Every incoming rock hit hard enough to melt part of the growing planet.2 Not everything got swept up: the leftovers survive today as asteroids and comets.2
If you cut Earth in half, you'd find layers like an onion's. That's because young Earth was once hot enough to melt. In melted rock, heavy things sink and light things float, like pebbles sinking in honey. Dense iron and nickel sank to the centre and lighter rocky minerals rose toward the surface. Scientists call this sorting by weight differentiation.23
| Layer | What it's made of | How thick |
|---|---|---|
| Crust | Solid rock (granite on land, basalt under the sea) | about 30 km on land, 5 km under oceans |
| Mantle | Hot rock that flows very slowly | about 2,900 km |
| Outer core | Liquid iron and nickel | about 2,300 km |
| Inner core | Solid iron and nickel, about 5,400 °C | about 1,221 km radius |
Figures from NASA's Earth facts page.4 Earth's metal core is about 7,000 km across, bigger than the planet Mercury.3
The liquid outer core matters to us today. As that liquid metal churns, it produces Earth's magnetic field, which shields us from the solar wind.34
Most scientists think the Moon was born in a collision. Around 4.5 billion years ago, something about the size of Mars (or maybe a series of objects) crashed into the young Earth. The crash threw enough molten and vaporised rock into space to build the Moon.6 The impactor is often called Theia.5 Rock ages suggest it happened roughly 60 million years after the solar system began, and some studies put it as late as 175 million years.65
How do we know? Apollo astronauts brought back 382 kg (842 pounds) of Moon rock. Its oxygen isotopes, a kind of chemical fingerprint, match Earth's rocks far too closely to be a coincidence. The samples also show that the Moon was once covered by an ocean of magma, which is just what a huge, hot impact would leave behind. And the Moon has only a tiny iron core, 1.6–1.8% of its mass compared with about 30% for Earth, which fits a Moon made mostly from rocky outer layers blasted off in the crash.6
What's still debated: scientists agree on an impact, but not on the details: how big Theia was, how fast it was going, what it was made of, or whether there was one crash or several.5 Older ideas, that the Moon was captured as it passed by or formed right alongside Earth, fell out of favour after the Apollo samples came back.6
(The crash gets its own explainer: how did the Moon form?)
The Moon started out much closer to us. Today it drifts away by about 3.8 cm (1.5 inches) every year.6 (Why we see it change shape is another story: why does the Moon have phases?)
Right after it formed, Earth was far too hot for oceans. The magma ocean left after the Moon-forming impact meant any water could only exist as vapour in the air.5 So where did today's oceans come from? There are two main sources, and scientists still argue about how much each one gave:7
As Earth cooled over a few hundred million years, the water collected into oceans.8 Today oceans cover about 71% of Earth's surface, and Earth is the only planet in our solar system with liquid water on its surface.4
The best age for Earth is 4.54 billion years, accurate to about 1%.10 Oddly, it doesn't come from Earth's oldest rocks. Plate tectonics has recycled Earth's first crust, so no original rocks are left.10 Instead, scientists use radiometric dating. Radioactive elements like uranium slowly turn into lead at a known, steady rate, so they work like a clock.1011 More than 70 meteorites, leftovers from the planet-building era, give ages between 4.53 and 4.58 billion years. Combine these with lead from Earth's oldest ores and you get 4.54 billion.10
| Clue | Age |
|---|---|
| Solar system begins (meteorites) | 4.53–4.58 billion years |
| Earth (best estimate) | 4.54 billion years |
| Oldest Moon rocks | 4.4–4.5 billion years |
| Jack Hills zircon, Western Australia (oldest known bit of Earth) | 4.4 billion years |
| Acasta Gneiss, Canada (oldest known rock) | 4.03 billion years |
| Life begins in the oceans | about 3.8 billion years ago |
Sources: USGS,10 NASA Astrobiology and Britannica,1112 NASA Earth facts.4
The oldest known material that formed on Earth is a tiny crystal of zircon from the Jack Hills of Western Australia, about 4.4 billion years old.1213 It's roughly 250 microns wide, about a quarter of a millimetre,11 and it was found inside a much younger rock.12 Zircon is tough enough to survive being worn away and buried again and again, so it keeps its age like a time capsule.11 The chemistry of these zircons hints that Earth had cooled enough for liquid water, and perhaps oceans, by around 4.4 to 4.3 billion years ago, surprisingly soon after the magma ocean.111314
If you could visit Earth in its first few hundred million years, you wouldn't recognise it. First came a glowing ball with a magma ocean hundreds of miles deep.5 The air had no oxygen to breathe: it was probably first hydrogen and helium, then volcanic gases such as water vapour, ammonia and carbon dioxide.8 The newborn Moon hung much closer in the sky,6 and asteroids kept slamming into the surface.28 Oxygen only built up much later, after bacteria that make food from sunlight evolved, around 2.2 to 2.7 billion years ago.8
Yes. Astronomers see many flattened, spinning discs of gas and dust around young stars. They look like our own solar system's first stages, billions of years ago.15 Discs are a normal by-product of a star being born (see how are stars formed?), so building planets from the leftovers seems to happen all over the galaxy. For the very beginning of the story, before there were any stars at all, see how did the Big Bang happen?
About 4.6 billion years ago, a cloud of gas and dust collapsed. Most of it became the Sun, and the rest spun into a flat disc. In the disc, dust stuck together into bigger and bigger rocks, which crashed together until they built the planets, including Earth.
About 4.54 billion years old, give or take about 1%. The age comes from radioactive dating of meteorites and of lead in Earth's oldest ores, because Earth's very first rocks have been recycled by plate tectonics.
Nothing like today. Constant crashes and radioactive heat kept young Earth so hot that much of it was molten, with a magma ocean at the surface and no liquid water. Oceans only formed after it cooled.
The leading idea is a giant impact: a Mars-sized body, often called Theia, crashed into the young Earth. The blast threw molten rock and vapour into orbit, and that debris clumped together into the Moon. The details, like Theia's size and whether there were several impacts, are still debated.
Scientists are still working it out. Some water was probably released as steam by volcanoes, and more was likely delivered by water-rich asteroids and perhaps comets. Measurements of comet water, such as from ESA's Rosetta mission, suggest asteroids delivered more than comets.
Dust grew into planet-sized bodies over tens of millions of years. The last giant crash, the one that made the Moon, happened roughly 60 to 175 million years after the solar system began.
Tiny zircon crystals from the Jack Hills in Western Australia, about 4.4 billion years old. The oldest known intact rocks, the Acasta Gneiss in Canada, are about 4 billion years old.