Official rule text
15.2 STEP 2— The player who controls the planet that is being bombarded chooses and destroys one of their ground forces on that planet for each hit result the bombardment roll produced. a If a player has to assign more hits than that player has ground forces, the excess hits have no effect. RELATED TOPICS: Invasion, Planetary Shield
View official source
Source: Living Rules Reference and FAQ - Prophecy of Kings
Expansion: Prophecy of Kings
Version: 2.0
Reference: Rule 15.2