Planetary Interaction (PI) can seem somewhat complex but once colonies are setup it can be a lucrative income for players. Setting up colonies on planets is well covered in the series of 'Flight Academy' tutorials.
If players find that they can't create any colonies then it's possible they may have reached their colony limit. This would be reflected in the message 'Unable to Build - Maximum Number of Colonies Reached.' which is displayed when they attempt to deploy another Command Center. If this is the case, then players can check their current colonies by looking under the 'Planetary Colonies' section of the Business menu via the NEOCOM. Should players already have colonies in place but would like to get rid of them there is no need to go to the system your colonies are located in. Colonies can be destroyed remotely by viewing the planet in 'Planet Mode', then clicking on the Command Center and selecting 'Decommission'.
Building colonies is only the first step as materials still need to be moved from the planet's surface into an accessible location in space. For most players materials would be exported to the orbiting Customs Office. Should the colony be located on a planet in high security space, then exporting will incur a tax to both the owners of the Customs Office as well as to CONCORD. This is reflected in a double entry for the export costs in players' wallets. Training the 'Customs Code Expertise' skill will reduce the CONCORD portion of the tax.
If no friendly Customs Office is in orbit than players will see this reflected when they try to launch materials from the launch pad.
The lack of available Customs Offices does not necessarily mean players cannot retrieve materials from the surface. It is possible to launch materials stored in your Command Center. Materials launched in such a manner would remain in a container for five days in space before the container degrades fully and is removed. Such Planetary Launches end up nearby the planet they are launched from and are not directly able to be warped to unless players are the owner of the launch. In which case it is possible to warp to the container via your 'Planetary Launches' tab of the journal.
Please be aware that while Alpha Clones can perform any actions on a planetary colony, they will not be able or export anything to or from the colony.