The aliens were originally from Mars.
In season 3, episode 1, Ralph and Maxwell are taken into the spaceship, and the alien shows them the planet that used to be his home. He says it used to be beautiful (a "garden planet") and now it's a desert. When the spaceship approaches the planet, we see an unmistakable shape: Olympus Mons, a volcano on Mars. There can be no other in-universe explanation: the planet has a Martian landmark because it is Mars. The guess is supported by real-life geological findings, backing the idea that Mars had water on the surface until 1 billion years ago, which made life possible.- As a corollary, the translator devices were purposefully programmed not to translate the native name of the planet (a series of chirps) as a challenge for humans to figure it out themselves, and eventually, go there with their own means. Figuring out the answer is a test of intelligence; figuring out a means to go there is a test of teamwork and dedication to pursue a goal.
