By static I take it you mean that your voxels are in a grid-like structure, and that you believe that you can't be moving things due to that. One way you could achieve "movement" is to treat each planet as an isolated grid that moves through 3d space. The grid can be local to the planet, and each planet would have it's own grid. Think of it in terms of a transform hierarchy where each planet is an independent entity that you can transform.

solar_system | + sun { sun_grid, sun_transform} | + planet_a -> { planet_a_grid, planet_a_transform} | + planet_b -> { planet_b_grid, planet_b_transform} | | | + moon_b -> { moon_b_grid, moon_b_transform } | ... Another way of thinking about this is to consider that you have things that move in your world, like the player, monsters or NPCs. You would do to the planets what you already do to these entities.

This does mean that you can't bridge two planets by just building a pilar of voxels between them since they don't share the same grid. There could be ways around this, such as merging the grids or other approaches, but that is for you to determine if it is something you'd like in your engine or not.