The Solar System
The solar system is the Sun and everything held by its gravity: eight planets, five officially named dwarf planets, hundreds of moons, and millions of asteroids and comets. It formed about 4.6 billion years ago when a cloud of interstellar gas and dust collapsed into a spinning disk, and the Sun gathered more than 99% of the matter. Nearest the Sun are the rocky terrestrial planets — Mercury, Venus, Earth and Mars — followed by the giants Jupiter, Saturn, Uranus and Neptune. Beyond Neptune lie the Kuiper Belt and the vast Oort Cloud. In 2006 the International Astronomical Union defined what a planet is, reducing the census from nine to eight and reclassifying Pluto as a dwarf planet; NASA's New Horizons has since shown Pluto as a complex world.