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 then chose to call the movie "Planet 51" after Area 51, the supposed center of American UFO investigation.
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.
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.
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."
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 43:06, 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.
The alien dog creature may be a reference to movie Alien. It's face resembles that of a Xenomorph, it attached itself to one of the aliens face and when it's dog house is shown the name on it reads "Ripley" which is the main character of the alien series