How exactly was the Earth built?

It happened 4.5 billion years ago, when the solar system was full of orbiting space rocks. This debris clumped up in a snowball effect, getting bigger and bigger. The planet is still growing today. By Alexander E Gates

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It isn't easy to figure out how the Earth was built, because it happened 4.5 billion years ago, and no one was there to watch.

So scientists have had to look at what the Earth looks like now and at all of the other planets, moons and debris in the solar system.

They've concluded that the Earth was built in the same way that you would build a big snowball to make a snowman. The mass that would become our home rolled through planetary debris - rocks floating in space - for more than 100 million years, adding more and more material, until it grew into a full-size planet.

How do scientists like me know that this is what happened? First, studies of the size, composition and location of asteroids and comets, many of which are as old as the Earth, indicate that 4.5 billion years ago the solar system looked the way Saturn looks today, with rings of space rocks orbiting around the Sun.

There’s still one such ring around the Sun — it’s called the asteroid belt and lies between Mars and Jupiter, with the Sun's gravity holding the rocks in orbit.

All of the other bodies that we know as planets today began as similar rings of space debris.