You need to make new meshes for each planet to add new textures.
I did this with a new moon "planet" (colonizable moon).
For instance, if you want to add a new texture to the Ice planets, what I did:
Look for Planet_IceX.mesh (where X is a number, 0-3, etc).
Copy one, rename it Planet_Ice4.mesh (or whatever you want really, I just kept it sequential).
Open the file, and change the bolded parts (it's at the top of the file):
Material
DiffuseTextureFileName "
C:\PlanetIce01-cl.dds"
SelfIlluminationTextureFileName "
C:\PlanetIce01-da.dds"
NormalTextureFileName ""
Change them to reference your texture names.
Then you have to add the new mesh to the planet_ice.entity (found in the gameinfo folder), find:
TXT
entityType "Planet"
meshInfoCount 4
meshInfo
typeNameStringID "IDS_PLANETTYPE_ICE"
asteroidTemplate "Sparse"
dustCloudTemplate "IceSafe"
meshName "Planet_Ice_0"
cloudColor ffdafcff
nullMeshRadius 0.000000
nullMeshParticleEffect ""
hudIcon "HUDICON_PLANET_ICE0"
smallHudIcon "HUDICONSMALL_PLANET_ICE0"
infoCardIcon "INFOCARDICON_PLANET_ICE0"
mainViewIcon "MAINVIEWICON_PLANET_ICE0"
undetectedMainViewIcon "MAINVIEWUNDETECTEDICON_PLANET_ICE0"
picture "PICTURE_PLANET_ICE0"
Copy everything from "meshInfo to the "picture "PICTURE_PLANET_ICE0" line, and paste it AFTER the last mesh item.
Once you do that, chage the "meshName "Planet_Ice_0"" line to "meshName "Planet_Ice_4"" (or whatever you named the new mesh).
Don't forget, for every new mesh you add to the planet's entity file, you need to change the meshInfoCount number. Add 1 to it for every meshInfo item you add.
Hopefully that's not too confusing.
If it is, I'll try to make a somewhat "proper" tutorial, with pictures.