Earth has been slowly cooling since it formed 4.5 billion years ago, and sometime between 1.5 and 0.5 billion years ago, the core began to crystallise into a solid ball of mainly iron and nickel. The core is growing by around one millimetre per year, and at that rate, Earth won’t have time to fully cool and solidify before the Sun reaches the end of its life in around five billion years, when it’ll expand and potentially engulf the planet we live on. If some currently unknown mechanism caused Earth to cool much sooner, it would have serious long-term consequences for most life on the planet. Without the electric dynamo of the molten outer core, Earth’s magnetic field would fade to zero, and the stream of charged particles from the Sun, known as solar wind, would begin stripping away the atmosphere, as may have happened to Mars long ago.