Planets all start with a giant clump of gas, dust, and other materials. Because of gravity, the gas and dust start to bunch up and start to form a little depression. That depression in space grows and grows, becoming a star. A star, like the Sun, gives off heat, light, and particles that are filled with energy. After a star forms, there is always material that is left over. That material is called space dust. As the space dust orbits, or circles around the newly formed star, it starts crashing into other pieces of space dust. Over time, giant clumps of space dust or protoplanets start to form. Then, the protoplanets start crashing and colliding into each other to create round planets. Planets are round because of gravity and how the material is squished together. Gas planets, on the other hand, form in almost the same way as a star, but are small enough to be considered a planet. Gas planets do not give off as much energy as a star does.