Theta Scorpii is a star system many light years away from Cybertron. It is a hostile area, filled with hazards including plasma storms and asteroids. Even Megatron would think twice about going there.
During the Great War, Optimus Prime launched the Allspark off of Cybertron towards Theta Scorpii to keep it out of Megatron's hands. Many years later, Optimus and Wheeljack traveled to Theta Scorpii in Ultra Magnus's ship in order to retrieve it.