You can have pasta-shaped moons.
Once you allow pasta-shapes moons, or worlds, you open up a whole lot of possibilities!
Is that an actual photo or an artists rendering?
Photo taken by the Cassini orbiter.
There are a few different questions here:
Can solid toroidal object exist? Sure - a small one can exist in space, with no problems.
Can one naturally occur? Harder, but for a sufficiently small object, if you’re not committed to a “perfect” donut shape.
Can a toroidal object circle a sun? No way. Like @Darren_Garrison says, it would be unstable - it would crash into the Sun unless active countermeasures were taken
Could a toroidal planet exist large enough to have appreciable gravity (the picture below is fiction) http://www.isfdb.org/wiki/images/1/1c/IAS_1981_05_Angelini.jpg
Probably not - gravity would cause it to become spherical pretty quickly (and at planetary scales, as @Chronos says, every plausible material becomes effectively liquid).
@eburacum45: Did you get that picture from larryniven-l.org?
It could have its axis of rotation 90 degrees to the axis of revolution, though. That is, like a rolling donut.
You’d still have some tidal forces unless it precessed at once per year (i.e., the axis of rotation always pointed toward the star). I don’t quite have the intuition for what’s needed to make that happen.
The tidal forces would try to force the planet to precess, which would probably interfere substantially with planet’s rotation around its axis of symmetry.
If the host star were spinning rapidly enough, the equatorial bulge might be enough to cause a natural precession of 1/yr.
My first thought was that it was just conservation of angular momentum. A massive star is spinning slowly, with a radius of millions of kilometers, then suddenly collapses in on itself and now has a radius of 16 km, and as a result is spinning absurdly fast. After a brief google, most people point to that explanation, but some others seem to suggest that the spin is a result of the explosion itself.
Saturn is Losing Its Rings at “Worst-Case-Scenario” Rate
A mere blink of a galactic eye
No, from Wikipedia. Here is the CC page.
Here is my own version; I’ve tried to render the ring-shaped ice-caps on such a world.
from here
The big problem with a donut world is the tendency of the planet to bunch up into beads. Perhaps this could be resisted somehow, by adding some sort of constricting sleeve around the short radius, which senses when the surface starts to swell and applies pressure to counteract this tendency. A kind of artificial peristalsis effect, one might say.
You might be able to do it with an electromagnetic coil of some sort; surely the ‘beading effect’ must start as a relatively small effect, which could be caught early and resisted.
I meant to picture of the Smoke Ring
The properties of gravity mitigate against it because, as material moves together because of gravity, they naturally form a sphere. However, as we’ve observed, rings can form. You could consider the asteroid belt a giant ring and, of course, you have the rings of Saturn.
At a quick glance, no one seems to have mentioned the original cover of Larry Niven’s 1973 paperback Protector, which clearly shows a probably-too-small toroidal world with an unlikely “donut hole” world in the center:
#/media/File:Protector(1stEd).jpg)
I don’t recall the context, but it was probably an artificial creation of the Brennan-pak, which would explain its unlikely small size, atmosphere, and that certainly unstable center filling.
Yes, he used some kind of artificial gravity to keep it from collapsing, and later he destroyed it by turning off the gravity generator.
The copy of protector that I read did not have that cover, I had a much later edition. I never really would have thought that Kobold looked quite like that, though.
Anyway, certainly not natural, and if we are starting to get into planets created and maintained by highly advanced technology, donuts are actually quite feasible, if you care enough.
I believe it was a large lump of neutronium kept stable by a stasis field.
‘the’ not ‘to’
I would probably expect the planet to be spinning inline with the Sun so the inner part of the ring would be permanently fairly dark? The ice ring should probably be carried through the whole inner section.
In your average Solar System, there’s only one light source and not a lot of extra light from reflective surfaces, nearby.
The outmost parts of the inner section might catch a little light and reflect some light into the hole so that it’s rarely a full pitch black, but it is probably in permanent twilight at least (and possibly full black - the donut would block out a lot of stars and any other from any moons).
Let’s look at the mathematics of this, but first pick a number between 1 and 10. Got it?
Was it a rational number? Or better yet an integer? That’s impossible (not really). You could have picked any real number between 1 and 10 and since there are countably infinite rational numbers but uncountably infinite real numbers then p(choosing a rational/integer) = 0 and p(choosing an irrational) = 1 BUT that does not mean choosing a rational/integer is impossible and choosing an irrational is guarantied like it would with a finite set.
Point being that you need to be careful talking about the implications of probability and expectation in an infinite world.