In the new animated adventure, “Planet 51,” the residents of the title planet would be more at home in “Leave It to Beaver” than “Alien.” Sure they have green skin, but their yards are enclosed by picket fences, television is in black and white, and families are made up of two parents and two well-behaved kids. Life is good.

Their only enemies are the Humaniacs: Giant one-eyed invaders whose sole mission is to firebomb Planet 51 and make zombie slaves of its survivors.

Then one day, as two families enjoy a sunny afternoon barbecue, a space shuttle lands in the backyard. Astronaut Chuck Baker (voiced by Dwayne Johnson) steps out, humming music from “2001: A Space Odyssey,” so oblivious he doesn’t notice the green people until he steps on a rubber ducky. He takes one look at the frightened suburbanites and hightails it out of there. After a disastrous, far-from-stealthy race through town, he winds up curled in the fetal position at the planetarium. Chuck has the good fortune not to be found by the military, which would like to cut out his brain, but Lem, the planetarium’s junior assistant curator, who reluctantly agrees to shelter him.

Lem is voiced by Justin Long and looks pretty much like you’d expect Justin Long to look like, were he an alien. Having landed his job the day before, Lem feels that his life is finally perfect. He envisions his adult life with a house and two kids. The kids will grow up, have kids of their own, and visit on the holidays. How’s he going to get these kids? He has an eye on Neera (Jessica Biel), the girl next door, who has been waiting years for Lem to come courting. (Neera seems gutsy, so you’d think she’d ask Lem out herself, but her planet is stuck in the 1950s. Oh, well.)

Like E. T., Chuck just wants to go home. Lem, busy helping him, neglects Neera even as she drifts closer to Glar, a guitar player who is “into something new called ‘protesting.’ ” Their favorite chant is “We’re real-ly upset. We’re real-ly upset,” and they direct it at the military’s cutout-his-brain-first, ask-questions-later strategy. Lem pretends to support the strategy because Chuck is hiding in the back of his car and a soldier is moving in for inspection. Neera is disappointed in Lem for blindly following the crowd.

I have a lot of questions about Planet 51, the place. If the men don’t need to wear pants, why do the women wear skirts? How does a culture bypass the wheel and go straight to hovercrafts? In an atmosphere where rocks are precipitation, how can an Earthling breathe? Kids probably won’t ask these questions, but after the tenth Cold War joke (e. g., students hiding under their desks during an alien-attack drill), adults will start to wonder if Planet 51 has a Russia. Then again, if a film can make you care about the sociopolitical structure of a fictional planet, it must be doing something right.

It’s not “E. T.,” but so what? “Planet 51” has a lot to offer: An adorable hero (Long is as awe-shucks irresistible here as in those Mac commercials); clever supporting characters, especially a puppy-like robot (think WALL-E) and a Planet 51 “dog” (think the alien from “Alien”); and a budding friendship between people from different species that’s no less touching for having been done before.

There’s even a nice moral about refusing to live in fear of the unknown. For kids and adults alike, that’s a lesson that doesn’t grow old.

PLANET 51

Three stars

STARRING: Justin Long, Dwayne Johnson, Jessica Biel

DIRECTOR: Jorge Blanco

RUNNING TIME: 90 minutes

RATING: PG for mild sci-fi action and some suggestive humor

THE LOWDOWN: An American astronaut causes “War of the Worlds”-level panic on a distant planet.