Unicron, in Transfomers: The Last Knight?
According to the Transformers: The Last Knight, app, Primus and his children the 13 Primes, defeated the Chaos Bringer, and imprisoned him in a ball of rock and sen that ball of rock hurling across space...
If this intended as extended backstory and marketing material, its not really a huge leap to conclude that Earth, much like in Transformers: Prime, is the body of Unicron.
Oh, right. The app also says Primus, is Cybertron, which is a tried and tested part of the Transformers mythos. Which also might help explain why Cybertron seems to headed for a collision with Earth in some of the trailers.