The solar system is the group of planets, moons, and space debris orbiting around our Sun. It is held together by the gravitational pull of the Sun, which is nearly 1,000 times more massive than all the planets put together. The solar system was probably formed from a huge cloud of interstellar gas and dust that contracted under the force of its own gravity five billion years ago.
The planets are divided into two groups. The four planets closest to the Sun are called "terrestrial," from the Latin word terra, meaning "land," because they are small and dense and have hard surfaces. The four outer planets are called "Jovian" because, like Jupiter, they are giant planets made largely of gas and liquid. Between Mars and Jupiter and beyond Neptune there are belts of very small bodies and dwarf planets called the asteroid belt and the Kuiper belt.