The engine is geared towards making this easy by parenting. Though it may not be conceptually correct in your case, it's the most godot friendly way of doing it. Just add the ship as a child node of the planet when it lands, and detach it from the planet when it leaves.
As long as the parent-child relation is a chain of spatial nodes (if 3D) or Node2d (if 2D) they will share and propagate transforms.
If for a reason you want a node to be a child of a Spatial or Node2d without inheriting their movement, you just have to make sure that the "independent" node is child of a bare "Node" (the gray/white ones in the UI, instead of the blue/red). This would interrupt the transform propagation