You could also model your own backdrop by deforming a rectangle or square in a modeler, creating several instances of it, and turning then into a group (Map the first square before creating instaces). This will give you a backdrop with as many layers as you need to apply background, alpha, specular, grunge/dirt textures etc. to produce the background atmosphere you need for
your scene.
You could create a  ground plane for your backdrop using a sperical or square base object and the same process. Using deformers in your modeler, displacement painting, etc. create morphs to simulate the type of land mass that suits your needs.

Sebastian