The film was originally going to be titled "Planet One," until the owners of Planet One, which makes children's and teen TV programs in the United States, threatened to sue. The film's producers had no knowledge of the company's existence, and apoligized. They then chose to call the movie "Planet 51" after Area 51, the supposed center of American UFO investigation.
Even though much on the alien planet is round in design, nothing has wheels. Aztecs were one of the few ancient civilizations that hypothesized about alien life, and never used wheels.
(at around 1h 8 mins) One of the soldiers in the secret army base has a helmet that reads "Corn to Mill," a parody of the helmet graffiti in Full Metal Jacket (1987) which reads "Born to Kill."
The dog in the movie is called Ripley (shown by the name on the doghouse), with the appearance close to that of the Xenomorph from Alien (1979), and is obviously an homage to Ellen Ripley, the protagonist from the Alien franchise.
Production lasted sixteen months, roughly half the amount of time it takes to make a Pixar movie.
(at around 59 mins) One of the hippies is holding a sign, which reads "Make like, not war."
Lem, the name of the main alien character voiced by Justin Long, is also the abbreviation for Lunar Excursion Module. NASA's LEM was used as part of the Apollo program to land astronauts on the moon.
(at around 24 mins) When Chuck and Lem first meet at the planetarium, Chuck tells Lem about his plans when arriving on their planet. In the Brazilian release, Chuck says he was planning on thrusting the American flag on the planet's soil, playing some golf and coming back to Earth on time for the Rio 2016 Olympics, because golf would be one of the selected sports. The movie was released in Brazil shortly after the International Olympic Committee (IOC) announced that the 2016 Olympics would be held in Rio de Janeiro.
The film finished production by June 2009. New Line Cinema had picked up the U.S. distribution rights in November 2007, with plans to release the film in summer 2009. However, New Line Cinema sold the rights to TriStar Pictures through Sony. According to Variety, "Warner decided to let the picture go after the producers insisted on a November release, when Warner is releasing its sixth Harry Potter film." The new distributor moved the U.S. release date to November 2009.
(at around 18 mins) When the two alien characters in costume walking by one compliments, "You make a good toothbrush," the dialog slowly fades out, but you can hear the other respond with, "What I do really well as is a suppository."
(at around 1h 17 mins) When the launch pad room in Base 9 is falling apart, you can see a Tie Fighter cockpit in the background.
Chuck's command module orbiting the planet is called Odyssey. This is the same name of the command module used in the Apollo 13 mission.
(at around 43 mins) A "Strange Tales" comic book is visible behind Lem and Baker. That was the name of the Marvel anthology series that started in the 1950s with horror morality tales. In the 1960s, it changed to science fiction stories about aliens trying to take over the earth.
When Glar and his fellow protesters are on the elevating platform. The camera shows four soldiers watching them. The second one from the left is wearing a headband that looks like the one that Ralf Maceio wears in the "Karate Kid franchise.
(at around 45 mins) Neera tells Lem, "the times, they are a different," on the courthouse steps, spoofing the Bob Dylan song, with the lyrics, "the times, they are a-changing."
(at around 1h 10 mins) When Lem & his gang go to save Chuck from Base 9, Skiff pulls on a Coke bottle that opens up the secret entrance. This is a reference to Spies Like Us (1985) and the Pepsi dispenser that allows access to the underground base.
Dwayne Johnson initially turned down the main role, feeling he had no business being in an animated film since he wasn't a voice-actor. Eventually, he was cast in the movie after he read the script and enjoyed some of the jokes. However, he was slightly annoyed that most of the other actors present weren't professional voice-actors, meaning he couldn't get input on how his performance was.