Earth

From Ben 10 Planet, the wiki resource for Ben 10!

Earth is the homeplanet of humans. It appears as the setting in every Ben 10 episode. An odd thing about the planet is that a great deal of alien activity happens on earth which isn't a trans-galactic level civilization. This has led to the majority of the plot lines within the Ben 10 series, as well as a major story arc within Ben 10 Alien Force. Earth, as well as a number a planets, is regarded as a "Cesspool" of the galaxy by one of the bounty hunters within the episode The Galactic Enforcers (episode).

The Earth begins to have a substantial population of aliens sometime between Ben being 15 and 30. When exactly isn't disclosed and may well be non-cannon due to the events of Ben 10 Alien Force. Before then