So You Think You'd Like to be a Planet
Suppose, for example, that you are one of those 100 nanometer sized particles of rocky materials that populate the galaxy as a result of Supernovae and other major Stellar events. It's lonely out there in the cosmic void, so you would like to get together with friends to form a planet. How does one go about this? First thing is, you've got to hang where the cool crowd does - in a large, cool (say 10 C above absolute zero), cloud of gas and dust. There's usually a bunch of them around the plane of the disc of any respectable spiral galaxy. You and your cloud may need a little push to get started - a nearby supernova or a density wave in the spiral galaxy, for example, but once your local cloud is gravitationally bound, you're on your way. Once the cloud starts contracting, it begins to break up into small pieces. It's important at this point to stay in the thick of things, since the early stars get the gas - and dust. Be cool though, and hang on to some of you...