An international co-production by Spain and the United Kingdom, the film was originally acquired for North American distribution by New Line Cinema, but then sold to Sony Pictures before completion.

Chuck convinces Lem to help return him to his spacecraft before command module Odyssey in Planet 51's orbit departs for Earth in three days and leaves him stranded.

Lem enlists the help of his best friend Skiff, an eccentric science fiction aficionado with conspiracy theories about the so-called "Base 9" (Planet 51's equivalent of Area 51), to hide Chuck away from the army.

Chuck is later captured by Grawl's forces during a festive movie premiere in town and is slated to have his brain removed by alien scientist Professor Kipple.

After admiring Planet 51's view from space, Lem successfully asks Neera out on a date, while Grawl expresses his gratitude to Chuck for saving him.

In the mid-credits scene, Kipple climbs out of the underground base, but is taken back to his own lab for brain surgery by two of his own patients, whom he wrongly deemed to be mind-controlled by Chuck earlier.

Planet 51 is based on an original idea by Jorge Blanco, Marcos Martínez, Ignacio Pérez Dolset and Javier Abad.

The Spanish film company behind it, Ilion Animation Studios, made an offer to the existing entity for all ownership rights to their "Planet One" trademarks and related website URLs.

Since the film was intended to be a parody of American pulp science fiction shot in Eastern Europe, Stillman thought it would be hilarious to have the name hint about a writer whose works have nothing to do with "little green men" stereotypes.

The site's consensus reads: "Planet 51 squanders an interesting premise with an overly familiar storyline, stock characters, and humor that alternates between curious and potentially offensive.

[16] Roger Ebert of the Chicago Sun-Times gave 2+1⁄2 stars out of 4 and positively wrote of the film being "perfectly pleasant as kiddie entertainment, although wall-to-wall with pop references to the American 1950s.

"[17] Furthermore, some critics such as Markovitz of EW,[16] Steven Rea of The Philadelphia Inquirer,[18] and Brian Miller of The Village Voice[19] acknowledged Planet 51 as "an E.T.