Yst

Yst is an Earth-like planet that exists 700 lightyears away from the Unicron Singularity.

Fiction

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.

From Teletraan I: The Transformers Wiki, a Wikia wiki.