What Kinds of Planets Are Out There?

In our solar system, we've got two kinds of planets. You have small, rocky planets with a thin atmosphere or no atmosphere. And then you have larger gas planets with some kind of solid or metal core surrounded by dense gases extending out for thousands of kilometers in radius. How do these different types of planets form? Well, the leading theory on the formation of the planets is that our solar system began as a huge cloud of interstellar gas and dust floating in space.