Delta IX was the inhabited ninth planet in the Delta system, in the Alpha Quadrant.
In 2328, this was the point of destination of passengers traveling from the Donatu V Spaceport via commercial transports. Janice Lee and Tony Graf traveled on the SS Kogin, and Michael Westmore, Jr. and Lisa De Moraes traveled on the SS Wisconsin. (TNG-R: "Inheritance", okudagram)
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This planet was only mentioned in writing.
This planet's quadrant of origin is inferred based on the position of its star system as seen in the star chart appearing in the Star Trek: Picard episode "Maps and Legends".