That looks pretty normal to me. Lone gas giants do exist, and it's quite possible (albeit a bad idea) to fly a behemoth over there and turn it into a shield.

Sublustris wrote:

I thought it suppose to be impossible to build Citadels around systems without a sun, but I guess I remember it wrong and it only applies to shields.

Is it a single player game? If so, can you please check what happens if you obliterate it. Does it create a sun? 

Did they maybe change that with the citadel improvements?

Citadels provide a couple other benefits, so I can't really see why it'd outright disallow you to set one up there. It's kind of like some of the conditional Dust improvements with high upkeep costs- it'll allow you to build them because there's still some use, it just won't necessarily be a good idea.

Citadel's primary role is system protection though, and I thought they were solar powered.

Sublustris wrote:

Citadel's primary role is system protection though, and I thought they were solar powered.

After you create a citadel, floating structures appear in the system view. I always thought those generate the field.

I mean 

And those superstructures supposed to be system batteries or battle stations