You can get the celestial object using uget(name):
Uget("Earth"):uPlanetSurfBattle(1);
or also
myEarthVariable := Uget("Earth");
myEarthVariable:uPlanetSurfBattle(1);
Doesn't work in the mission editor, though, because Nexus doesn't recognize the uget und uPlanetSurfBattle functions in the editor (they were intended for campaign-use only, it seems). However, it'll work fine during regular gameplay - even in non-campaign mods.