Some planets are just neutral spots for hanging out and the like. This is either because it fits their lore, they are not part of the plot yet or just because we havn't gotten around to making them conquerable or leveling planets. Some of these are:
-Reytha
-Dagobah
-Hoth
-Naboo
-Honoghr
-Manaan (there are sharks here you can kill)
-Felucia (there are rancor for high level chrarcters)
-Kuat
-Kessel
-Ilum
-New Kisge (may not actually be connected yet)
Some planets(the coloured ones) are conquerable by the rebels and empire. From memory, these are:
-Bespin
-Cerea
-Corellia
-Iridonia
-Rhen Var
-Corulag (also has rancor for high level characters)