My wild and crazy theory:
(without reading any spoilers or leaks)
The beast planet is Unicron's first minion. After Unicron's original defeat at the hands of the original Primes, he fled and slumbered, he "creates" this planet like how Unicron "created" Earth.
Was awakened when he felt his master's "revival". After sensing Unicron's "death", he creates the Predacons to take revenge on his master's killers.
To protect humans, Optimus moves the Autobots to the beast planet to fight the Predacons, and to defeat the beast planet's "spark".
*bows*