Like all planets in the solar system, Earth grew billions of years ago as a result of the accumulation of matter falling from outer space. To this day, matter continues to land on the surface of the Earth in the form of meteorites. In fact, some of the water on the planet stems from comets and asteroids that have impacted Earth. But a 10- to 15-kilometer-large asteroid impacted Earth 65 million years ago, causing the extinction of many living species on the planet, including dinosaurs.