the orbit would have to move in the block grid, unless each planet was it's own chunk.
Though if the planets where objects (like boats or carts) they could move freely, but you couldn't destroy them or build on them.
The gravity one would have to be that you can walk on any surface and I'm not sure how else the concept would work for things like trees or mobs.
you have mods that can move blocks right?
well if you just treat each sphere as a point mass, and calculate its direction and speed every time (and store those)
then every step you move every single sphere to its new location and voila simulated orbits.
Act first, then think, then try to find a way to cover up the horrible mess you made.
Si Hoc Legere Scis Nimium Eruditionis Habes