hi

I think they used a much more complex thing than simple LOD, like a spherical chunked terrain that looks like : http://imageshack.us/a/img202/3064/62479621.png and http://imageshack.us/a/img717/541/35220357.png , but it is not supported by any game engine natively, I'm afraid you have to write your own mesh generain and texturing, if you want both good performance and high terrain detail. or maybe by setting a couple of planet and terrain entities invisible/visible can do the trick in 3DGS too, to simulate a planet from any distance. anyway, it is an interesting topic in game creation.