Delta IX was the inhabited ninth planet of its star system.

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)

This planet was only mentioned in writing.
According to Star Trek: Star Charts ("United Federation of Planets I") and Stellar Cartography: The Starfleet Reference Library ("Federation Historical Highlights, 2161-2385"), the Delta system was located in the Alpha Quadrant.

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