Your main problem is the terraforming technology. It's much easier to destroy entire environments like The Mass, but manipulating an atmosphere to support life, on top of transporting millions of new species to support a new ecosystem is definitely tier 3 material.
An easy solution, thought up by our great friend Munroe would be your original homeworld was irradiated by solar flares, and you simply migrated to a new one nearby. Simple fix where you can discover your little custom energy source. It's not a big deal.