That's how pirate planets are created:
<Trigger>
<OnEvent>OnStartMegaEvent</OnEvent>
<PerformAction>
<Action>SpawnFaction</Action>
<ValueParam>0</ValueParam>
<ValueParam>6</ValueParam>
<StringParam>FACTION_PIRATES</StringParam>
</PerformAction>
<PerformAction>
<Action>DoScenarioConversation</Action>
<StringParam>PirateColony_MegaEvent_Message_Conversation</StringParam>
</PerformAction>
</Trigger>
Has to be an event or at start, though. Triggers cannot be used in tech or improvements.