From: The Big Nowhere
ORIGINAL: Bebop Cola
I'd actually suggest that a surface spaceport of some sort would be built long before any large orbital construct would be. It depends a bit on one's lift technology that gets their ships into space, but if nothing else we have trade that occurs at planets with no "orbital" spaceport as it is.
Perhaps a happy middle ground might be that planets that do not have an orbital spaceport cannot receive trade from medium and large freighters, only small ones. Small freighters, presumably, would be the only ones really capable of landing on the surface to conduct their trades. Medium and Large freighters would probably be more optimized for large hauls in space rather than the ability to land on a planet's surface then take off again.
That said, if spaceports are truly an orbital construct instead of a surface one, I'd like to be able to build them just about anywhere to serve as trade hubs. If my empire is at the ass-end of the galaxy it might make sense to build a commerce center more central to my trade partners. A certain percentage of my private sector would base out of that trade hub in order to ship and retrieve goods from other empires while the remainder of my private sector that's based on my actual planets would work to keep the cargo bays of the trade hub(s) full of the goods I have for trade.
As I imagine it, these remote trade hubs would be treated as a limited function colony:
1. They don't have a population, but they do have a corruption rating. The presence of military ships from the home empire reduces corruption, tying up a defense fleet for the trade hub. More trade means more corruption, requiring a larger fleet.
2. The hub can serve as a home base for a fleet.
3. The hub can be destroyed, but also invaded and captured.
4. If their corruption rating gets too high, they can go independent. They are then somewhat like a pirate faction. If the spaceport has shipyards they will produce escort-class ships for local defense, as well as troops to prevent casual invasion, but they're just an independent merchant guild so don't expect them to be powerful enough to resist a determined empire. They still serve as a trade hub, but the majority of the revenue generated is retained by the trade hub's faction. You only get to keep whatever revenue is generated by selling(dropping off) resources to them.
5. As a private sector facility they don't project a zone of ownership, but they are owned by your empire so they can be blockaded and attacked as part of actions against you.
6. As a spaceport, they serve as a valid drop-off destination for mining ships and freighters from nearby mining sites.
I think the game assumes that each race is sufficiently advanced to build an orbital starport at game start. You do already have a planet side space port, its the reason cargo ships can unload and you can build things from the planet. Basically the game starts you in an era that would be equivalent to our 'space age' or even 'post space age' (even we puny modern humans can build orbitals while they are in orbit).
You actually can build orbitals anywhere as just a trade hub, etc...
You need to make a generic 'star base' and give it commerce parts (IE the cargo pods, docking, and commerce center). The star base will be able to onload/offload cargo, and acts as a refueling depot. I actually use these myself (though give them gas mining ability and build them in gas clouds) with LR sensors to act as fleet depots. And they work just fine. Basically, in my case they are a hybrid star port/mining station/defense platform (as mine are HEAVILY armed).
When I get home from work I'll post you an exmple of the way I set them up. Just keep in mind you have to manually build them.
Distant Worlds Fan
'When in doubt...attack!'