I've seen references a handful of times to the fact that a toroidal world is theoretically possible but realistically won't form naturally. That gives me some thought: What objects are possible, but wouldn't naturally exist? Something that, if we came across it, we'd be able to immediately go "Someone built that."
To be complete, I am excluding megastructures such as Dyson Spheres, Ringworlds, and the like. I want weird things, like a planet set up in an orbit that's too "Perfect" or the above torodial world. To be more complete, the objects in question should be easily identifiable as artificial by simply existing, not by what's on them
Additionally I am similarly excluding the how of the object. Putting the planet together, or moving stars to a specific orbit, is not in the scope of this question. Presume that whoever built it had an unlimited budget of time, energy, and resources.
Further Clarification: Said object must be easily visible from the edge of the system. Assume that the object is being detected by a ship just entering the system via a hyperlane or grav point or some similar specific-natural-point to specific-natural-point FTL system.
I am looking for the most unquestionably artificial object that can scientifically exist. Something that one look from a star system's width away makes you go "Someone made that." The object shouldn't be larger than a planet and should be made of "natural" objects - No refined metals or other things. There also shouldn't be any scanning or exploration required to determine its nature.