Largest possible man-made planet-like body

Question

Humanity is an advanced civilization, and has long, long since took to the stars. Their resources are seeming boundless, and their technology is fantastic. For some reason beyond our 21st century understanding, they have decided to perform a very odd experiment. They wish to create a world and strand a few ten thousand brainwashed members of their own race on it, and see how they evolve as a society. The members will have no memory of humanity's past, but will have basic skills (a common language, understanding of simple machines, agriculture, animal husbandry, basic construction, tool making, etc., placing them on par with barbarian Europe during the early medieval ages. They want to watch humanity develop on a planet of the following description:

• Earth like as possible (similar climates, weather patterns, resources, gravity, although it doesn't need earth-like geography, as in the shape, nature, and location of landmasses)
• Physically and structurally stable
• Hidden in its man-made nature enough that it will remain unknown that it was man-made until at least the society reaches early space faring technology (such as what humanity reached in the mid-to-late 20th century)
• And as absolutely large as physically possible

...And they're willing to build this world from scratch.

By no stretch of the imagination does this world need to be a planet in our traditional sense; it's perfectly acceptable for it to be an artificially created structure. The magnetic field this new planet possesses doesn't need to be created by a molten core. In fact there doesn't need to be a core at all! It's even been suggested that making the planet hollow (if such can be engineered, and complications like affected gravity can be dealt with) might allow for a much larger surface area.

In the end the planet just needs to seem like a natural Earth-like planet up until at least the predescribed point in the developing society's future. It's also worth noting that this developing society has no point of comparison as to what a natural planet actually looks like, so as long as it doesn't affect the development of the society they could stumble upon some (hidden) artifacts of the planet being hand-crafted.

Again, humanity has seemingly endless supplies to throw at this project, are in no particular rush to need it completed (a few generations' put towards this effort are perfectly fine), and have extremely advanced technology. However they are still constrained by the laws of physics as we know them.

What would this planet be like to achieve the predescribed goals as closely as possible? What solution might humanity use?

Answer 1

A Shell World.

https://www.youtube.com/watch?v=TfMr_XkWcEs

As Isaac Arthur explains in his video, you can build it as big as you want and the amount of filler material is proportional to the surface area, so its density goes down as the size goes up. You use one megaton of matter per square foot of surface, for Earth-normal gravity.

You could have a thick crust with fake geology and places to mine things.

It's not until they develop advanced technology that they will detect that the world is hollow and has a dynamic support structure under the crust, just as we figured out the insides of Earth. They may be puzzled when developing theories about planet formation and long-term state, though.

On Earth, note that for most of history people thought the planet was “made”, and only started to admit natural processes as distinct from supernatural when they started figuring out the details of what makes it tick. So your postulated people will always assume that the gods made it or something like that.

So I think what you’re asking for is that naturalness is faked, and the equivalent of our geology and scientific inquiry from the 17th through early 20th centuries would point to a natural origin, and only more advanced technology reveal the deception.

I think making it unnaturally large would put a kink in that. The density is not what they would expect as they came to learn these things. The heat budget and thermodynamics would not make sense, and that’s the first science to develop with the industrial revolution (19th century).

Answer 2

Assuming these brainwashed humans "don't know any better," there are plenty of options that wouldn't give away the nature of the experiment. It may even be useful to put them in varying conditions to see the effects of each environment. Here are a few:

The Halo Ring

Anybody who's played Bungie's hit shooter can recognize the Halo rings: artificial ring-shaped structures with a living environment spanning the entire interior. As far as the transplanted humans know, "that's all there is."

The Matrix

Why bother creating a whole new planet if you can just simulate one? This also has the added bonus of being able to change things more-or-less on the fly without any complicated engineering going on behind the scenes.

The Terraformed Planet

The boring option: take a planet that already exists, throw in an atmosphere, some water, and a few other biologic basics and let 'er rip! After a while you should have a biosphere in homeostasis.

The Galaxy Planet

This is a pretty absurd idea, but it would be interesting to see what happens when humans are on an artificial planet the size of a galaxy. Such a scale would eventually mean that humans would evolve quite distinctly, as there could eventually be enormous societies so far apart from each other that traversing between them would take generations. You could even go the Sid Meier's Civilization route and give one of the factions access to superior weapons and technology, and see how long (if ever) it takes to conquer the rest of what used to be humanity.

The Flat World

It would be pretty funny to see people literally afraid of falling off the edge of the world so while you're padding the budget, why not?