With so much of Stellaris focusing on exploring the cosmos, expanding territory, building up stations and fleets, it can be easy to forget about the planets in which the vast majority of your populations inhabit. Planetary colonies are, without a doubt, one of the most important aspects of an empire’s economy and infrastructure, essentially acting as the foundation from which the rest of your Empire stands, so you should probably take care of them.

Each Empire starts with one planet as their homeworld and capital, with an underlying goal of finding new habitable planets to colonize. If managed properly, planets could provide both the needs of its populace and its Empire's demands. When finding a new world to colonize, it’s important to make sure it's habitable for your Empire. This will naturally increase population happiness and stability and cost fewer resources to keep the colony running like food and consumer goods.

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Districts represent large areas of development on the planet dedicated towards a particular purpose, whether that be housing or resource gathering. The total number of districts that can be built on a planet is equal to the planet's size. The deposits on the planet further constrain the number of mining, generator and agriculture districts. Each district built can increase the overall sprawl of your Empire. To produce manufactured goods like consumer goods and alloys, you’ll need to build special buildings. Buildings can only be built depending on the population size of a planet. But if you claim, restore or build an Ecumenopolis, certain districts are fully dedicated to manufacturing these special resources. Planets can also be designated for certain roles that can help the rest of your Empire. Planets with many city districts can be labeled as Urban Worlds, for example, or planets with lots of stronghold buildings designated as Fortress Worlds.

Stability is a measure of a world's overall stability, either socio-political or in terms of drone functionality if playing a machine or hive-mind Empire. This is influenced by many factors such as happiness, housing, amenities and crime or deviance. Maintaining stability on your planets is highly advised; each point of stability slightly increases resources from jobs, planet trade value and immigration pull. At low stability of 25% or under, this can cause terror attacks, protests and more. Empires with a significant slave population that isn’t nerve stapled may see hunger strikes and riots.

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If a planet’s stability decreases below 10%, this can lead to rebellion. The number of planetary rebel armies depends on the planet’s population, with one assault army for every population and two slave armies for every enslaved population. If the rebel armies are defeated, the planet will get a 20% increase to stability for 10 in-game years. If the rebels win, they'll take control of the colony, the system’s starbase and any other colonies present in the system, creating a new, independent Empire. This rebel Empire will sometimes be annexed by a hostile Empire nearby.

Crime and Deviancy are measures of the overall level of non-compliance in the population of a world. While Unemployment does not increase Crime or Deviancy, it can add modifiers that cause them if measures aren't taken. Crime and Deviancy are reduced by certain Jobs, as well as Governor leaders. If steps aren’t taken to address crime, a criminal underworld can be established on the planet. This underworld can only be removed by reducing criminal activity to 0%.

If you have the Megacorp DLC, Empires with the Corporate Authority can establish a Branch Office on another Empire's worlds as long as the world owner has a Commercial Pact or Federation with the Corporation Empire or is their Subject. These Offices can provide bonuses to both the Empire that built them and the planet they are built upon. Empires with the Criminal Heritage Civic can establish their own Branch Offices without a Commercial Pact. These buildings also provide bonuses to both the Criminal Empire and the planet, but they will also increase the amount of crime on the planet. The planet owner will be allowed the Expropriation Casus Belli if any criminal offices are built.

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When at war, rival Empires might be successful in invading your territory and reaching your planets, sending ground invasions to capture them. In this case, your defense armies are your last line of defense. The number of defense armies can be increased by building and upgrading certain buildings like strongholds and precincts. You can also enact a decision to declare the planet be placed under martial law, increasing stability and soldier jobs for your populations to take. If subject to orbital bombardment, a planetary shield can be built, decreasing bombardment damage by 50%. However, this is a rare technology that likely won’t appear till mid to late in the game.

A planetary population should always be carefully monitored. If there are not enough amenities, job openings and living space, this can decrease population happiness and stability. If a planet’s population is growing too rapidly, you can enact a Decision to Declare Population Controls or Discourage Planetary Growth, depending on what laws and policies your Empire follows. Decisions represent planet-wide projects that can offer short-term solutions to maintaining your worlds. Depending on your Empire policy, you can also resettle populations to live on other planets with openings in jobs and living space. This can be useful for dealing with unemployed or homeless populations to make them useful. Resettlement can also act as a sort of evacuation of your populations if threatened by a rival Empire or Crisis threat.

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