Why can't you test a mod?

http://soase.weebly.com/sins-mod-set.html

Are you going to use the same texture sheets, if not start there. Copy the gas giant textures and rename and edit them. Copy the gas giant mesh, rename it and tell it to use the new texture sheets.

Add the new planet entity to the entity manifest and update the count

Add the new planet entity to the GalaxyScenarioDef.galaxyScenarioDef and update the counts. The planet needs to be added under planetType and update Count

planetType
    designName "Swamp"
    designStringId "IDS_PLANETTYPE_SWAMP" <---Add to String\English.string file, update count. name used in game
    orbitBodyTypeCount 1
    orbitBodyType "Swamp"

and orbit body and update count

orbitBodyType 

     typeName "Swamp"
     entityDefName "PlanetSwamp"
     defaultTemplateName "Template:DefaultStart_Asteroid"

then add to the planet classes for map making, here is 1 secion of the classes "RandomTrueNonIce"

planetType
designName "RandomTrueNonIce"
designStringId "IDSGalaxyScenarioTypePlanetRandomTrueNonIce"
 orbitBodyTypeCount 22
 orbitBodyType "Terran"
 orbitBodyType "Terran"
 orbitBodyType "Desert"
 orbitBodyType "Desert"
 orbitBodyType "Volcanic"
 orbitBodyType "Volcanic"
 orbitBodyType "Rock"
 orbitBodyType "Rock"
 orbitBodyType "Forest"
 orbitBodyType "Forest"
 orbitBodyType "Toxic"
 orbitBodyType "Toxic"
 orbitBodyType "GasGiant"
 orbitBodyType "City"
 orbitBodyType "Industrial"
 orbitBodyType "Military"
 orbitBodyType "Paradise"
 orbitBodyType "Relic"
 orbitBodyType "Grass"
 orbitBodyType "Grass"
 orbitBodyType "Swamp"
 orbitBodyType "Swamp"

Add the new icons to the Icons_planets texture and tell the brush files in the Windows folder the location of the icons.

Here is all the brushes for 1 planet added. Use notepad++ find in files to find each file

brush
 name "HUDICON_PLANET_SWAMP0"   <--- entity file points here
 content "Simple"
 fileName "Icons_planets_m0d"   <--- Texture sheet used (I made another)
 pixelBox [ 205 , 644 , 50 , 50 ]   <--- 205 , 644 location on texture sheet. 50 , 50 size of icon
 brush
 name "HUDICONSMALL_PLANET_SWAMP0"
 content "Simple"
 fileName "Icons_planets_m0d"
 pixelBox [ 175 , 644 , 28 , 23 ]
 brush
 name "INFOCARDICON_PLANET_SWAMP0"
 content "Simple"
 fileName "Icons_planets_m0d"
 pixelBox [ 178 , 670 , 22 , 22 ]
 brush
 name "MAINVIEWICON_PLANET_SWAMP0"
 content "Simple"
 fileName "Icons_planets_m0d"
 pixelBox [ 205 , 644 , 50 , 50 ]
 brush
 name "MAINVIEWUNDETECTEDICON_PLANET_SWAMP0"
 content "Simple"
 fileName "Icons_planets_m0d"
 pixelBox [ 260 , 644 , 50 , 50 ]
 brush
 name "PICTURE_PLANET_SWAMP0"
 content "Simple"
 fileName "Icons_planets_m0d"
 pixelBox [ 1 , 641 , 167 , 75 ]

Lots of work and I am sure I missed something

Put modding question in the MODDing forum section