Official rule text
62.1 Players who do not have units on either side of a combat are resource value, influence value, and trait, if it has one. If a player not opponents. Those players cannot use abilities or have controls a planet, they keep that planet’s card in their play area. abilities used against them that are used against an opponent.
View official source
Source: Twilight Imperium: Fourth Edition - Living Rules Reference: Prophecy of Kings
Expansion: Prophecy of Kings
Version: 2.0
Reference: Rule 62.1