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.

CELESTIAL MECHANICS: The Frenchman Pierre Simon Laplace (1749-1827) was the first scientist to make an attempt to compute all the motions of the Moon and the planets by mathematical means. In his five volume work, Traite de mechanique celeste (1799-1825), Laplace treated all motion in the solar system as a purely mathematical problem, using his work to support the theory of universal gravitation. His idea, for which he was severely criticized during the following century, was that the heavens were a great celestial machine, like a timepiece that, once set in motion, would go on forever.

ORBITING THE SUN: Some of the planets, including Earth, orbit the Sun in ellipses that are close to being circles. Others have more eccentric orbits. Comets have the most eccentric orbits, which are very elongated. The distance between the planets and the Sun is measured in terms of "astronomical units" or AU; each unit is equal to the average distance between Earth and the Sun, or 93 million miles (149.6 million km). This drawing shows the orbits nearly to scale