The lead character, for instance, is an over-confident alpha male spaceman, already done to perfection in the person of Buzz Lightyear, the comic hero of "Toy Stories" one, two, and 3-D.
And its retro-fifties, alien-paranoia context was worked pretty thoroughly in "Monsters Vs. Aliens." There are uncanny similarities, from the images (one-eyed metal monoliths) to entire sequences (aliens attack a couple parked on a lovers' lane).
"MvA" beat "Planet 51" to the punch, and its punchier. A nice story might have helped, but "Planet 51" is stuck with a predictable yarn about a stammering nerd (Justin Long) trying to work up the nerve to approach the beautiful girl next door (Jessica Biel) when the spaceman crash-lands into his life.
Long's character tries to hide the astronaut from militant government forces, and does so by pretending to be stridently anti-alien. In doing so he, um, alienates his pro-alien girlfriend, and the rest of "51" has him trying to hide the spaceman and make things right with the girl.
It's for very young audiences, and may tide them over until "Avatar" comes along.