Copyright © 2007 Dorling Kindersley
Our local star, the Sun, and everything that circles around it is known as the Solar System. The Sun’s gravity holds planets, asteroids, comets, dust, and other bodies in oval paths, or orbits, around it. This gravity is so powerful that some objects are in orbits trillions (thousands of billions) of miles out from the Sun.
The Earth is one of nine PLANETS in the Solar System. Most planets have natural satellites (moons) circling them. Swarms of mini-planets, called asteroids, also travel in the Solar System, and much farther out are icy lumps that become glowing comets when they approach the Sun.
The Sun and planets were born in a huge cloud of cold, swirling gas, called the solar nebula. The cloud collapsed under its own gravity into a fast-spinning, ball-shaped mass. The center part became denser and hotter and eventually began shining brightly as the Sun. Rocks, dust, and gases circling in a disk around the Sun began lumping together, eventually forming the planets.
A planet is a world that orbits a star. There are nine planets circling around the Sun—Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto. They take different amounts of time to orbit the Sun, from only 88 days for Mercury to nearly 250 years for Pluto.
The four planets closest to the Sun are largely made up of rock, like the Earth, and are called the terrestrial (Earth-like) planets. The next four planets are much larger and made mainly from hydrogen and helium. They are often called gas giants. Pluto is different still, a deep-frozen world of rock and ice.