How do we know Earth is 4.5 billion years old?

We know Earth's age by reading radioactive clocks, and the best ones come from meteorites. Uranium slowly decays into lead at a steady, known rate, so by measuring how much lead has built up in a sample, scientists can work out how long the clock has been running. Done on meteorites, leftover building blocks of the Solar System, it gives 4.54 billion years, give or take about 50 million.

Why not use Earth's own rocks? Because our planet keeps recycling itself. Plate tectonics, weathering and hot water have mixed and remelted its crust for billions of years, resetting the clocks. The oldest crystals ever found on Earth, tiny zircons from Western Australia, are 4.4 billion years old, a little younger than the planet itself.

The number was won by an obsessive chemist. In the 1950s Clair Patterson kept getting too much lead in his measurements, and discovered that lead was everywhere: in his lab, his chemicals, even his own hair. So he built one of the first laboratory clean rooms, sealing it from the air and distilling every chemical. In 1956 he published an age of 4.55 billion years from meteorites, essentially the figure still used today. Shocked by how much lead pollution he'd found, he went on to campaign against it, helping to get leaded petrol banned.