As a wise frog once remarked, it’s not easy being green. Not only are the emerald-colored extraterrestrials in “Planet 51’’ terrified by the appearance of a human astronaut on their home turf, they’re imprisoned in a cruddy, reductive version of “Happy Days’’ and forced to spout inane dialogue. Young children and adults with high pain thresholds will enjoy the movie during its brief pause on the way to your On Demand menu.
The planet of the title is home to a complacent race of noseless humanoids who speak English and whose culture has made it up to the “Grease’’ era of drive-in restaurants and cheesy sci-fi invasion movies. When an actual rocket ship comes to ground, the inhabitants panic, sending cocky American Chuck Baker (voiced by Dwayne “The Rock’’ Johnson) running for cover. He’s taken in by teenage misfit Lem (Justin Long), who hides him from a warlike general (Gary Oldman) and a mad scientist (John Cleese) after the human’s brain.