Jorge Blanco's computer-animated family comedy has a cute idea. On a distant planet that's vaguely Earthlike in both looks and inhabitants, the off-course American astronaut who lands there is perceived as a fearsome alien threat. The funhouse-mirror premise plays out as expected: The nerdy kids are thrilled; the military grab their guns; and before the intergalactic reconciliation, comic cultural misunderstandings abound. It's too thin a gimmick to sustain a full-length movie, so one's tolerance for cute, green rubbery creatures (instead of humans) going through familiar motions will determine one's enjoyment. (If you must identify with an Earthling, there's the affable, square-jawed spaceman, voiced by Dwayne Rogers.) The 1950s stylings of Planet 51 are charming, if occasionally undermined by oh-so-modern but unnecessary toilet humor. Starts Fri., Nov. 20.