Geology is the study of how the planet Earth formed and how it changes. Scientists called geologists study rocks, soils, mountains, volcanoes, rivers, oceans, and other parts of the planet. Some geologists study fossils, the marks or remains of dead organisms. The study of fossils helps scientists learn how life developed on Earth. Geologists also investigate other objects in the solar system, including Earth's moon, other planets and their satellites, and asteroids and meteorites. The word geology comes from Greek words meaning study of Earth.