Planet Technology
Each planet type, with the exception of the Terrestrial type, has a corasponding technology to research. Planet's offer no benift to your organization untill the inital technology has been researched, additionally you are unable to improve a planet untill you have researched the related technology upgrades for it. It is wise to focuse early research efforts on planet technologies favoring the planet improvement upgrades that you intend to use for each planet type.
Terrestrial worlds are the exception to the planet research rule and have no available technologies. You are always able to build any improvement you wish on any terrestrial world. For all other planet type's you will have five (5) upgrade oppotunities, the first four (4) can be used to improve the Population, Trade, Industry or Research potential for the world type while the fifth (5th) grants the ability to terraform the world ... that is to convert it into a Terrestrial world type.
Note that some Gar have the ability to convert worlds into other world types such as the Fallen Gar's Scorch ability which converts worlds into Molten type planets. The Planet Technology Terraform ability however can only be used to convert a world into a Terrestrial world.