A high concept gets low execution in “Planet 51,” a lame-brained toon that even kids will recognize as an insipid goof on sci-fi conventions. Any number of things have been and could still be done ...

The animated film Planet 51 turns 1950's sci-fi about Earth being invaded by little green men on its ear. Life on the titular planet very much resembles the stereotypical vision of 1950's America, but ...