Please note that the content of this book primarily consists of articles available from Wikipedia or other free sources online. The Solar System consists of the Sun and the astronomical objects gravitationally bound in orbit around it, all of which formed from the collapse of a giant molecular cloud approximately 4.6 billion years ago. The outer region of the Solar System is home to the gas giants and their large moons. The four outer planets, or gas giants, collectively make up 99 percent of the mass known to orbit the Sun. Learn more about the outer Solar System, its formation and evolution in the following pages.