there are a few good documentaries like journey to the edge of the universe,i watched it 3 times,and lost planets( i dont quite remember the name of this documentary)it says that on some planets it can rain molten iron,or you can find planets with diamonds the size of a small tower,or even bigger.it also said that there are planets and stars running a drift trough the universe till they find a home,mostly this happens when a bigger star enters the system and slingshots a other star out of the system.some scientists say that these planemos can have life if they had lots of CO2.
edit: i think its this
http://channel.nationalgeographic.co...ideos/07059_00