Scientists believe the solar system began about 5 billion years ago, perhaps when a nearby star exploded and caused a large cloud of dust and gas to collapse in on itself. The hot, central part of the cloud became the sun, while some smaller pieces formed around it and became the planets. Other fragments became asteroids and comets, which also orbit the sun. The early solar system was a turbulent mix of hot gas and rocky debris.
In the solar system everything is affected by the sun's gravity. The planets and a variety of other objects, including comets, move the way they do because of the sun's gravitational attraction. Our planet, Earth, is the third out from the sun. The planets are all different. Their differences are largely the result of their different distances from the sun.
We call the planets that are closer to the sun, including the Earth, the inner planets. They are small rocky worlds. The outer planets are much larger and are made from much lighter materials. All but two planets, Mercury and Venus, have moons in orbit around them. Work Cited
Levy, David. The Nature Company Guides: Skywatching. San Francisco: Time-Life Books Co., 1994.