I think the game actually refers to them as "Areas". Planet Strike doesn't use the elevator concept of Aliens of Gold. I called them Levels because that's what Mapedit calls them. =/
To get to the secret levels there are teleports on four of the main levels, but they are in order was they appear in the game.
And yeah, there are only 20 main levels. Kind of cheap considering how many AOG had. Also 18 and 19 are small, dinky little things.