Sothis III was the inhabited third planet in its star system.
It was the homeworld of the Satarrans, a humanoid civilization who allegedly hated mysteries and were not as technologically advanced as the Federation. They possessed a unique device which allowed them to wipe the collective memory of the entire crew of a starship. They also possessed the capability to impersonate other humanoid appearances. By 2368, the Satarrans had been at war with the Lysians for decades. (TNG: "Conundrum", "The Chase")
This planet was only mentioned in dialogue.
According to the script for "The Chase", the pronunciation for Sothis was "SO-this".