Proven in Quantum Physics when two particles become entangled, the spin of one directly correlates with the spin of the other. Is this possible on a larger scale with the spin of planets? Could earth be entangled with another planet?
This more on the lines of an extended comment than an answer.
You should ask it in Physics.SE, they're not likely to appreciate how you've put it though. It's definitely a good question. It was after all, historically speaking, the spin of planets and of any macroscopic object that inspired the idea of QM Spin.
But you can reverse the question and ask whether there is an aggregate QM Spin of particles in the same way say you can add together the angular momentum of particles - not spin itself. You may want to confirm at least this on Physics.SE. I expect it is - since it will probably be subject to some conservation law.
Entanglement has been demonstrated practically on very small macroscopic scales. But the question is would it scale to planetary distances and planetary sizes. I expect theoretically yes. In that the similar mathematical reasoning will go through. But I expect it physically not - due to the meaning of infinite precision. That is there are likely to be some extremely small numbers entering the calculation - and one has to be careful as to the physical meaning of them.
Philosophically its a subtle question as it directly includes the interpretation of QM. What would entanglement mean in the instance the observer was on one of the planets?