Sirius IX was the inhabited ninth planet in the Sirius system of the Alpha Quadrant. The system was approximately eight light years from Sol. The planet had at least a thousand inhabitants, who allegedly had a unique biochemistry.
At some point before stardate 4978, Harry Mudd sold a love potion to a thousand people on Sirius IX. When they became ill, Mudd quickly left the planet. (TAS: "Mudd's Passion"; TNG: "The Naked Now" production art)