I've seen Mods of Hollow Worlds, by the mad modding genius Jack Dole on earlier versions of SE. Simply a Selena with a hole on one side and a tiny sun visible on the inside. Im sure you weren't able to enter the hole, but the interior was a full sized plant landscape only convex. I imagine it wouldn't be hard to enlarge the hole, add another on the other side and enlarge it too so you end up with a Terra looking ring in scale with a normal sized star in the center. Again you couldn't land. Maybe his magic is using transparent textures, maybe it is the clipping created by adding an invisible sphere partially inside a visible one. Who knows.
CPU: AMD FX-8350 8 core processor 4GHz / GPU: GeForce GT 730 @ 1920x1080, 60Hz with 1GB adapter RAM / RAM: Patriot Signature 4GB 1600MHz 240-Pin DDR3 (only 2GB work, don't buy it) / Motherboard: MSI 970 Gaming MS-7693