Beltane IX was the inhabited ninth planet in its star system.
The planet was regularly visited by freighters. Because of this, Jake Kurland stole the Type 7 shuttlecraft Copernicus from the USS Enterprise-D to travel to Beltane IX and sign onto a freighter there. However, after Jake had unbalanced the dilithium reaction, the shuttlecraft lost power and it was nearly destroyed in the atmosphere of Relva VII. Fortunately, Captain Jean-Luc Picard talked Kurland through a procedure which involved "bouncing" the shuttle off the atmosphere and Kurland returned to the ship. (TNG: "Coming of Age")
This planet was only mentioned in dialogue.
According to the Star Trek: Star Charts ("United Federation of Planets I"), in 2378, Beltane IX was a Federation member.