This is from the Celestial Bodies Sol System map. There are 2 players, one of which is UN or United Nations.
planetCount 25
planet
designName "SolSystem_Terra"
inGameName "Terra"
type "SolSystemTerra"
pos [ 240 , 249 ]
owner "UN"
isHomePlanet TRUE
The "owner" is linked to players at the bottom of the map.
playerCount 2
player
designName "UN"
inGameName "United Nations"
overrideRaceName "Tech"
teamIndex -1
You could try messing with those values to see what will work but I am not a map maker. Try the designName from the SOA2 GalaxyScenarioDef.galaxyScenarioDef in its place and see what happens.
But these planets are different and not just Terran_Home. "SolSystem_Terra" is a separate planet added by celestial bodies.