Planets are an important part of Sins of a Solar Empire.
Main article: Planet Types
There are different planet types which provide different types of income, bonuses, and some uncolonizable planets have planetary abilities. Terran Planets are good for their large population and tax income to sustain an economy. Volcanic Planets are useful for their Metal asteroids in orbit. Should you be playing the Vasari Loyalist you can gain an ability to change Planet Types to asteroids or dead asteroids by stripping them of all natural resources.
Main article: Planetary Improvements
Planets can be upgraded in multiple ways to help improve your empire. A few examples, civilian Infrastructure will erect space elevators on your planets which will increase tax from your planets. Or you can scout out your planets to find hidden resources or artifacts.
Main article: Allegiance
Allegiance is a percentage measure of how loyal your colonies are to you. It is individual for each colony.
Effects of Allegiance
Allegiance directly affects the income rates of planets. At an allegiance of 70%, credit, metal, and crystal income rates are cut to 70%. Low allegiance can tremendously affect your economy, so it is wise to pay attention to it. If it drops to 0%, your colony will revolt and be lost.
The maximum allegiance for a planet is based on its distance to your homeworld, as shown in the table below. The base value, however, starts out 10% below the maximum.
Allegiance is affected by culture. If friendly culture (yours or an ally's) is dominant at a planet, the allegiance will increase up to its maximum. If enemy culture is, it will decrease down to 0.
Constructing Culture Centers (Broadcast Centers, Media Hubs, and Temples of Communion) at the edge of your empire near an enemy will spread your culture into his empire, decreasing his allegiance and crippling his economy.
Constructing Culture Centers within your empire will increase your allegiance by 10% up to the maximum, granting you 10% increased income, as well as making it more difficult for your enemies to sabotage it. This can definitely be worth it.
If your capital is to one side of your empire (common because homeworlds are usually at one edge of the system), it may be wise to move it near the center, so that the overall allegiance is increased. Compare these situations(Homeworld is represented by the X):
110 100 90 80 65 X-------O-------O-------O------O Sum=445 90 100 110 100 90 O-------O-------X-------O------O Sum=490
In the second, your allegiance has increased 45%, which represents a 10% increase in your overall income. That quickly pays for the expense of moving your capital.