"General, I know what you're afraid of, and it's not Chuck. It's not monsters or aliens. It's the unknown. I've spent my whole life running from it, and I think maybe you have too, but I'm telling you the unknown isn't something to be afraid of. It can be your best friend. And just when you think that it means the end of everything you know, it's really just the beginning."
— Lem, to General Grawl
Planet 51, a Spanish/American All-CGI Cartoon feature from 2009, is an Affectionate Parody of tropes from both Science Fiction and Sit Coms of The Fifties.The premise: Human astronaut Captain Charles "Chuck" Baker, accompanied by an exploration robot named Rover, lands on the eponymous planet, which is supposedly uninhabited. However, Planet 51 is populated, by green-skinned humanoids whose world strongly resembles 1950s suburban America. The aliens are also paranoid about being invaded by another planet, so Chuck's appearance causes unintentional havoc, and he's soon being hunted by the military. Chuck befriends an alien teenager named Lem, a worker at a local observatory; Lem protects the astronaut as he struggles to return to his ship, which will automatically take off after a preset time limit whether Chuck is aboard or not.
Artistic License - Astronomy: An In-Universe example. According to Lem's observatory lecture at the beginning of the film, "the universe is nearly 500 miles long, and it contains—you're not going to believe this—over 1000 stars!"
This is apparently a running thing with the aliens' unit measurements - they seem to just use lower numbers for things in general: at one point the professor postulates that a manned space flight would cost "hundreds of dollars."
Actually, it's all probably deliberate considering just how little the aliens actually know about space. They haven't even been able to launch satellites into orbit yet.
Possibly Fridge Brilliance: he mentions at some point that the only thing he does with the star ship is to push some buttons, like a monkey could do. Chuck isn't exactly the brightest bulb of the set...
Tertiary Sexual Characteristics: The female aliens have built-in, organic "high heels" on their feet. Word Of God (aka character designer Ignacio GŁejes, in the book The Art of Planet 51) explains that this is because "we discovered that if the females had that extra heel on their feet, they would move differently than the male aliens, with swaying hips."
Cannot Spit It Out: Lem has a hard time asking Neera out on a date. This is partially due to his own shyness, and partially because Glar is a Moment Killer who bursts into song whenever Lem is about to ask Neera out. Technically, Lem never completes his question to Neera, because at the end of the film she says "yes" before he can finish, and gives him a "Shut Up" Kiss.
Hidden in Plain Sight/Status Quo Is God: How do you hide an alien astronaut? Paint an eyeball on his space suit and enter him in a costume contest promoting the Alien Invasion movie that just happens to be premiering today. Double for Status Quo because even though the "invasion" has the military in a tizzy, the movie premiere isn't even delayed!
Hoist by His Own Petard: Professor Kipple extracts the brains of the soldiers that have been tricked by Chuck into believing they have been brainwashed. At the end of the film, the two soldiers practice the same operation on the Professor.
Product Placement: At one point Chuck pulls a Twix bar out to convince Lem to clear the way to his ship. note It's not actually uncommon for any kind of explorer to take a supply of chocolate with them, because its high callorie count allows an average person to keep their hunger in check even in small quantities (no matter if it's an M&M's packet or a Toblerone bar), and it also takes a long time to rot (it may melt depending on circumstances, but that doesn't render it inedible). Professor Kipple later finds the discarded wrapping and interprets it as being a death threat in alien language (while the barcode would be the surrender terms).
There's also a Volkswagen badge on the front of Glar's van.
Rule of Funny: How likely is it that an alien civilization which has had little contact with Earth (and even that hasn't been revealed to the public) would evolve into a near-perfect duplicate of a specific human culture? How likely is it that such a world would have a popular science fiction franchise called Humaniacs? And how likely would it be that they have developed a perfect copy of the English language? Who cares?
Sci-Fi Writers Have No Sense of Scale: Parodied. No one can take seriously anyone who would even remotely suggest that the Universe was only 500 miles wide, since the planet they were on has to be much bigger than that in order to have an atmosphere capable of sustaining life, and a typical star is at least three orders of magnitude larger than 500 miles.