Official rule text
33.6 When a player becomes eliminated, their strategy cards are returned to the common play area whether those cards have 34.3 A player exhausts their planet cards to spend either the been exhausted or not. resources or influence on that card.
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Source: Twilight Imperium: Fourth Edition - Living Rules Reference: Prophecy of Kings
Expansion: Prophecy of Kings
Version: 2.0
Reference: Rule 33.6