Aartis is the home planet for humans in the game; however, the humans who live there were then infected by the Thera Virus. Like Thera, Aartis is in the Milky Way galaxy. It was discovered by astronomers using the Kepler telescope and it was once known as Kepler 187b. It has a population of 15,563,981,129 people as of 3104 and was the primary setting of SAS: Zombie Assault 4 until the release of Boreas.
The Earth is never mentioned in the game, but it is possible that it was overrun by zombies during the Biotox incident in SAS: Zombie Assault 3 and refugees fleeing Earth at the time tried to find the next most hospitable planet. They turned to Aartis as a refuge until the zombie epidemic on Earth gets under control. This theory is a reference to the movie Jason X, in which Earth has become a polluted wasteland by the year 2455 and humans left Earth to colonize a planet designated as "Earth 2."