I.

Introduction

Planet, a round body in space that orbits a star. To be a planet, a body must be big enough to settle into a rounded shape from the inward pull of its own gravitation. A planet shines by reflecting light and not by releasing nuclear energy the way a star does. Our solar system has eight major planets—Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune—and a number of small, dwarf planets, including Pluto, Eris, and Ceres. A planet-like body that revolves around a larger planet is called a satellite or moon rather than a planet. Planets are distinct from asteroids and comets, smaller bodies that also orbit stars.

The study of planets is called planetary science. Areas of research include the composition, structure, and evolution of planets and planetary systems. New discoveries have made scientists rethink the best way to define what objects should be called planets. Important properties that help determine what should be called a planet include the object’s size, its shape, its location, what it is made of, and the way it formed.

Ideas about planets have changed many times in the history of science. For thousands of years, most people thought that planets circled around Earth instead of the Sun. Many people also believed that planets gave off their own light and were made of a substance called ether that was different from ordinary matter found on Earth. The modern understanding of planets began with the pioneering scientists Nicolaus Copernicus and Galileo in the 16^th and early 17^th centuries. They showed that planets orbit the Sun and are made of ordinary matter like the Earth.