If we land on another planet — or back here on Earth — and find something other than what we expect, who is the alien in that situation, the modern day Earthman, or the local? It’s a plot staple of the classic old Twilight Zone series and the basis of a lot of films, including the entire Planet of the Apes series. It’s the allegorical mirror of Pogo’s famous “we have met the enemy and he is us.”
With any movie, it’s always interesting to keep asking “why?” as it unspools. In the case of Planet 51, why have it set in a retro 1950s was the one question that stuck with me, and the answer I finally settled on wasn’t the visual style or the fact that we as a nation were wrestling with the possibility of men in space, but that it was during the mid 50’s that the Soviets launched Sputnik and completely changed the Cold War, dramatically escalating tensions and creating a period that was more characterized by fear of “the other” than anything hopeful.