It all starts with a cloud of dust and gas in the shape of a disc. As it turns out, in the center of this cloud there is usually a star in the process of formation, gathering material through the force of gravity. Around this protostar, the gas and dust particles start to condense into larger and larger chunks of matter revolving around the protostar. Once the protostar gathers enough material to ignite, it becomes a star, and its solar wind (which are high energy particles produced by the nuclear reactions in the star) blows away any dust that didn't become part of a planet. That's how planetary systems are formed, our solar system being just one of possibly millions around stars in our galaxy (so far, more than a hundred have been discovered).

Planets are formed by a process of cooling.In the early stages of formation of the solar system, there were many clouds of dust and hot gas revolving around the sun. Over time, some of these cooled down and formed planets, just as hot fudge cools down and becomes a solid. Other, smaller clouds may form satellites that revolve around the planets, like Earth's moon.