That's not too bad a start at all
Firstly take a look through the file sharing section. I've uploaded a fully procedural planet a few years back, pretty sure there are a bunch of others in there too. Just do a search for "planet". Take a look at them, pull them apart and see how they work.
Also head over to
"New World Digital Art", they have some excellent pre-sets including one for an earth type planet. There is also a planet in development which should be released soon that is phenomenal!
What your doing is fine, your approach is correct. "All" you have to do is add in MUCH more detail and variation.
In the terrain group add some hills and mountains as a child of a surface layer preceded by a compute terrain. You can then give them a minimum height in the surface layer.
Try using the "merge" node to blend different terrains and textures together. Also the warp to create twisted mountain ranges.
Hope this helps some.
Cheers
Richard
