Planet earth is billions of years old, far older than any life that has existed on the planet. How do scientists know how old the planet is? Through the study of geology! Geologists study the physical structure of planet Earth as well as its history, and every process at work on it. From the shifting movement of Earth's tectonic plates to the eruption of volcanoes, geologists study it all! Each of these discoveries is used to help our understanding of our home planet.