There are six different planetary improvements that an empire can use to upgrade its colonies.
Purchasing the same type of upgrade on the same planet grows progressively more expensive.
With the exception of designating your capital, all planet upgrades have the same costs. Different types of planets have different maximum levels for each type of upgrade.
This upgrade increases the maximum population a colony can sustain. It also eliminates the planet underdevelopment penalty associated with new colonies. This upgrade is important because the larger the population of a planet, the greater the tax income it contributes to an empire. Terran Planets usually support the largest infrastructure and Asteroids the least (Dead Asteroids only stop siphoning money away with the upgrade), but it's still advisable to max this out quickly, as it will lead to increased credit production and prevent you from losing income to under-development.
Population by Planet and Infrastructure Level
|Upgrade Level||Terran||Desert||Volcanic||Ice||Asteroid||Dead Asteroid|
(*) Indicates planet underdevelopment is suffered at this level.
This upgrade increases the planet's health by allowing the populace of the planet to "build" various shelters and emergency services to protect them from a multitude of disasters. Thus allowing it to survive extended planetary bombardment. This upgrade is important because it gives defending fleets more time to arrive and to destroy the attacking fleet before a colony is destroyed.
Hit Points at Each Upgrade Level
|Upgrade Level||Terran/Desert/Ice/Volcanic||Asteroid/Dead Asteroid||Pirate Base|
This upgrade explores the planet for any special Planet Bonuses or Artifacts a planet may possess. This upgrade is important because the special bonuses can make the planet more efficient and artifacts provide empire-wide bonuses. You can search any planet up to two times, but no planet can ever have more than two planet bonuses and one artifact. Artifacts or planet bonuses can never be found on player homeworlds. If you conquer an enemy homeworld, be sure not to purchase this upgrade!
Designate Capital Planet
This upgrade makes the planet the new capital of an empire. This upgrade can be useful if an empire's current capital is in a bad location, since allegiance is based upon a planet's distance in relation to the capital. The upgrade costs 2000 credits, 300 metal, and 200 crystal.
This upgrade increases the number of logistic slots available on a planet. Logistics slots are used for constructing productive Orbital Structures such as Shipyards, Refineries, and Trade Ports. This upgrade can be useful if an empires wants to build additional research or productive structures. Upgrading Logistical Capacity will also produce more constructor vessels, which enables faster assembly when you are building a number of structures.
All Logistics Structures other than Extractors cost 4 slots (extractors cost 0).
In most cases, it's actually cheaper to colonize a new planet for its "free" logistics space rather than paying for additional space at an existing planet. That's if there is still unclaimed real estate around.
Logistic Slots Supported by Planets at each Upgrade Level
|Upgrade Level||Terran||Desert||Ice/Volcanic||Asteroid||Dead Asteroid/Pirate Base|
This upgrade increases the number of tactical slots available on a planet. Tactical slots are used for constructing defensive Orbital Structures such as Point Defense turrets, Hangar Bays, and Repair Platforms. This upgrade can be useful if an empire wants to permanently augment a colony's defense. Upgrading Tactical Capacity will also produce more constructor vessels, and in larger quantities than Logistics upgrades, which enables faster assembly when you are building a number of structures. Which might matter a lot if you're hastily replacing destroyed defenses.
The first two levels of this upgrade add one extra construction frigate to the planet. Different tactical structures require different numbers of tactical slots.
Tactical Slots Supported by Planets at each Upgrade Level
|Upgrade Level||Homeworld||Terran/Desert/Ice||Volcanic||Asteroid/Dead Asteroid/Pirate Base|