If I were to rough this out do what the fellow above suggested.
Use a photo editor to make the ring, anything that you want to be transparent should be black and whatever part of the texture map is white will be opaque.
render the planet
render the above mentioned quad with the above mentioned texture,
To make transparency the map you can search Google for "GIMP alpha masks, or specular maps" The techniques you find should work equally well for making the rings texture map.