A very quick and easy way to do this yourself would be as follows.
Two things to note,
First, if your using the Ultimate Edition from Steam, i'm not exactly sure of the instal paths, since i have the game from release, so its perhaps different. I'll post my install path and if it's different i'm sure it would give you an idea at least where to go.
Second, when you find this file, back it up and keep it safe if you ever want to play on the Metaverse, or simply return the game to vanilla.
Go to C:\Program Files (x86)\Stardock Games\GalCiv2\DarkAvatar\Data\English and look for CustomPlanets.xml. You will need an editor, Notepad++ is free and widely used by modders, and Stardock themselves i believe. For TA simply navigate to Twilight\Data\English.
Once you have found the CustomPlanets.xml and opened it in Notepad++ you can start to change the values. You can alter the planets for every starting system if you like, or just the one you use all the time, say the Terrans. I would recomend though changing the custom race settings and playing as your own race, that way all the default AI systems will be normal.
Scroll to the bottom of the file and look for this,
<StarSystem Name="CustomDefault">
<Planet Name="Homeworld">
<Class>10</Class>
<Moon>gfx/planets/m00.png</Moon>
<PercentOfStarSize>0.50</PercentOfStarSize>
</Planet>
<Planet Name="Planet2">
<Class>4</Class>
<PercentOfStarSize>0.40</PercentOfStarSize>
</Planet>
<Planet Name="Planet3">
<Class>0</Class>
<PercentOfStarSize>0.15</PercentOfStarSize>
</Planet>
<Planet Name="Planet4">
<Class>0</Class>
<PercentOfStarSize>0.80</PercentOfStarSize>
</Planet>
<Planet Name="Planet5">
<Class>0</Class>
<PercentOfStarSize>0.60</PercentOfStarSize>
<Rings>gfx/planets/r00.png</Rings>
</Planet>
<AsteroidField>
<Size>3</Size>
</AsteroidField>
</StarSystem>
Thats how it will look as default, but you can change most of the values.
here is one i have made up for my own custom race.
<StarSystem Name="Agnus Cloud">
<DisplayName>Agnus Cloud</DisplayName>
<Planet Name="Agnus Major">
<DisplayName>Agnus Major</DisplayName>
<Class>14</Class>
<Texture>gfx/planets/PlanetQualGreat_1</Texture>
<Moon>gfx/planets/m10.png</Moon>
<PercentOfStarSize>0.80</PercentOfStarSize>
</Planet>
<Planet Name="Rhetik">
<DisplayName>Rhetik</DisplayName>
<Class>26</Class>
<Textures>gfx/planets/PlanetQualGood_4</Textures>
<PercentOfStarSize>0.30</PercentOfStarSize>
</Planet>
<Planet Name="Zyclo">
<DisplayName>Zyclo</DisplayName>
<Class>19</Class>
<PercentOfStarSize>0.25</PercentOfStarSize>
</Planet>
<Planet Name="Remol">
<DisplayName>Remol</DisplayName>
<Class>8</Class>
<PercentOfStarSize>0.20</PercentOfStarSize>
</Planet>
<Planet Name="Agnus Minor">
<DisplayName>Agnus Minor</DisplayName>
<Class>8</Class>
<Texture>gfx/planets/PlanetBarren_1</Texture>
<PlanetType>Barren</PlanetType>
<PercentOfStarSize>0.40</PercentOfStarSize>
<Rings>gfx/planets/r00.png</Rings>
</Planet>
<AsteroidField Name="AtholianAsteroidCloud">
<DisplayName>Argus Cluster</DisplayName>
<Size>4</Size>
</AsteroidField>
</StarSystem>
As you can see i have made many changes, including textures, moons, rings, planet type, you can mess about with a fair amount of stuff, just look through some of the default systems to get an idea of what you can do.
If all you want is for all the planets to be habitable, simple replace the zero in <Class>0</Class> with which number you want. I think 55 is the maximum you can do, but i don't recall for certain.
Hope this helps!
