Yst was an Earth-like planet that existed 700 lightyears away from the Unicron Singularity until Unicron possessed it as his new body. Its original size is unclear, but the form that Unicron created from it was only as big as an average Transformer. Some shrinkage was seen to occur in this process, but how much is unknown.
Fun Publications Cybertron comic
After the Planet X agent Soundwave freed Unicron's essence from the black hole, it travelled to Yst. Penetrating the planet itself, Unicron's essence cast the planet's civilization into war and chaos, eventually leading to the planet's destruction and the rebirth of Unicron's body.
- The inhabitants of this world resemble the Cardassians from Star Trek. Garak better not have died.