Right now we are using custom terrain compilation tools to produce planets viewable in OT. These are a bit complicated to use and ever changing, so for now only we can produce planets using data that people give us in sufficient detail and quality.
And that is actually the hard part. OT doesn't specialize on generating coarse terrain structures (yet), it assumes terrain elevation data are at least in around 100m resolution and it only refines it below that. The same algorithms will work on coarser data, but it doesn't look that nice.
Generating planet data at 100m via some external tools is quite problematic at that scale; so far only the ME-DEM project was able to pull it off with basic elevation and color data. There was another promising project, but given the whole process is demanding (weeks and months of processing at that scale) it wasn't finished.
Setting up and importing the planet once you have the data is sophisticated but we can do it relatively quickly, we can do that.
I'm a terrain gen freak myself, and I'd like to create a planet creator tool based on OT one day, that would allow to skip the complicated step of generating the input data, and integrate it into in-game tools instead that enable artistic input at various levels of detail and automatic procedural refinement. However, as you may guess, that's by no means an easy task ...