What minimal magic is needed to maintain a huge torus planet?

Question

I'm creating a fantasy world (dragons, giants, generally medieval, generally rare magic, potential inter planar travel), and I would like the world shaped like a donut. Obviously I could just say "The world is magically sustained as a torus" and be done, but I want to know what magic needs to be there before getting too far into it.

Since I'm allowing inter-planar travel, I'm starting with the concept that the torus is held together by a ring in its center that contains millions of nodes that can transport to different planes/planets. The gravity from these planes leaks through causing the torus to maintain cohesion, and not form into a sphere.

Other problems I know need solved:

1. What prevents the atmosphere from being ripped away?
2. How do orbits of stars/moons prevent collisions?

So the question is: what handwavium/magic is needed to sustain a torus big enough that stars (could be singular if that makes more sense) and moons (several) orbit from the inside to the outside of the torus? Bonus: What other physical characteristics are reasonable for such a planet? (Tides, diameter of the donut, diameter of a slice of the donut, temperatures, weather, etc.) Double bonus: reasonable solutions to the physical issues. :)

Notes:

The torus big D diameter is light minutes to light hours across.

The torus little d Diameter is probably significantly smaller than the earth because gravity is "leaking" through the magical nodes or portals.

This question is similar, but calls for a much smaller planet.

This question has some great thoughts about physical characteristics of a normal spherical planet.

Answer 1

You're going to have an enormous gravitational problem. The inner part of the torus, for example, is going to have virtually no gravity except the illusion of it produced by the torus' rotation - effectively, residents of that inner part would seem to be inside a shell, and gravity from different parts of the torus would cancel each other out.

If you spin the torus fast enough to create the illusion of normal gravity on the inner side, and make the whole planet about two or three times as massive as Earth, you'll get normal gravity on the other side, too. Now, in between, the gravity will just be bizarre, pulling a little askew depending on where you are.

I don't know what you mean about stars orbiting - do you mean actually the stars in the night sky? Because if so you're talking about things light-years away passing through the center of the torus, which makes the torus bigger than any contiguous structure in existence by a factor of like thirty orders of magnitude. If you just want the sun to orbit, you've still got a problem - either the planet is so big that it would take billions of years to circumnavigate, or the Sun is so close that the atmosphere boils away. So you'll have to have some sort of alternate version of a star.

Moons are easier, by a long shot - a moon will orbit the center of mass, but it can be in a wildly elliptical orbit. Make the orbit so elliptical that it nearly passes through the center of mass, and you've got your orbiting moon. Problem is, at the other end of its orbit it'll probably be so far away as to be invisible.

To be honest, if you want a pseudo-scientific approach to this, I'd suggest delving into old cosmology. For example, a modification of the classical "crystal spheres" model of the cosmos would work nicely - your torus could have a crystalline medium flowing around it, carrying celestial objects through the center or wherever the heck you want. Otherwise, the best advice I can give is to say "okay, gravity works completely differently in every way".

Answer 2

I'm assuming you want a relatively constant gravity throughout the whole of the world...not constantly changing gravity as you traverse the planet. So I'm going to chuck out the reasonable physics and give the magic route. The key thing here is the planet itself isn't a torus, magic just gives the image that it is and we are going to rely on planar travel to do so.

The setup is going to be two overlapping planes. From 60 degrees north to 60 degrees south, the planet behaves as normal (travelling east for long enough will get you back to where you started). At 60 degrees north or south however, you exit this plane (can be anything from obvious planar travel to a completely seamless transition where you don't even know you crossed planes) and enter a second plane. This second plane exists as it's own planet (you can travel east until you reach the same spot you started at), just smaller as to mimic the inside of the torus vs the larger area of the outside. Using planes...the second plane can easily be the inside of a ring (rings rotation causes consistent gravity). Travelling east on the second plane will eventually get you back to your starting point.

This setup will give the image and feel (from the surface anyway) that this world is a torus. Starting from the outside world at the equator...travel north to 60 degrees and find yourself entering the north side of the second plane. Travel south across this plane and end up back on the first plane at the far south and continue travelling north until you hit the equator for a full round trip.

Of course this world won't look like a torus from space, but from the ground it will feel like it.

Answer 3

Little to None... There is a way to make Toroidal planets in reality. The problem is that they're unstable and easily disrupted. Just throw up a force field, magic or otherwise, of some kind and you're good.

I don't like the site, but here's a link to an article on the subject:
http://io9.gizmodo.com/what-would-the-earth-be-like-if-it-was-the-shape-of-a-d-1515700296