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It's a highly unlikely (probably impossible) planetary system with a supermega earth and an ittybitty gas "giant" there is also a bit of lens / perspective.
The reality is that if the moon was a gas giant then then it would make the supermassive rocky type (earth like) so massive that the gravity would be sucking the moon in at such a rate that it would crashing very soon. I'd also expect to see the atmosphere being sucked down already.
You are right, a planetary system like this would be impossible according to our current knowledge of physics. Good thing Everspace Classic isn't a sim, right? :D
The main issue here is that the ingame FOV isn't ideal for taking screenshots.