M12-578 was an uninhabited planet in the Pegasus galaxy, intended to serve as the new home for Atlantis and its crew. As this planet has no Stargate, it can only be reached by ship. Atlantis was forced to flee Lantea when the Asurans launched an attack using an Stargate satellite. It was chosen due to the fact that it possessed a large ocean which could be landed on. While en route, however, the City-ship fell out of hyperspace prematurely, and after the capture of Dr. Elizabeth Weir on Asuras the planet was abandoned in favor of a location that she had no knowledge of. (SGA: "First Strike", "Lifeline")

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