Look in ContinentDefs.xml
There are a lot of definitions for continent chunks like:
The texture files are in:
Galactic Civilizations III\Gfx\planets\
From what Paul said on a stream, the generic continents are randomly generated from chunks.
But some are not random.
At the top of ContinentDefs.xml you have the definitions for planets like earth and mars that are not random (they have
<ExcludeFromRandomGen>1</ExcludeFromRandomGen> in their definition)
the heightmaps for those are in Galactic Civilizations III\Gfx\planets\custom_heightmaps
Anyway this should get you started.