In one of Larry Niven's books about Pak Protectors, one of the Pak creates a toroidal planet using artificial gravity (and no doubt some other Pak techno-magic).

Protector: Toroidal Planet:
Protector: Toroidal Planet

Would such a planet be possible? Is it stable (without outside influence)? Would the surface gravity on the inside be as stable as the outside? Would it spin around it's empty center of gravity?

As much (realistic) info as possible would be appreciated.

  • $\begingroup$ There is a quote commonly attributed to Arthur C. Clarke: "Any sufficiently advanced technology is indistinguishable from magic." How advanced technology are you willing to allow for? We have had some questions about non-spherical planets already and the general answer is that absent outside influence, no matter what materials they are made out of, non-spherical planets do not remain non-spherical for very long. There is simply too much mass involved, leading to too much gravitational attraction pulling it all together. $\endgroup$
    – user
    Commented Apr 19, 2016 at 19:57
  • $\begingroup$ It would take artificial gravity or some other techno-magic. Are you asking if such a planet could form naturally, or if once created (artificially or not) could it maintain its shape without such aids? $\endgroup$ Commented Apr 19, 2016 at 20:55
  • $\begingroup$ Remember, when Brennan switched off the gravity generators the whole thing collapsed. $\endgroup$ Commented Apr 20, 2016 at 0:57
  • $\begingroup$ Nope. This is a duplicate. When I entered it, the original question did not pop up or I would not have submitted it. My apologies for the dupe. $\endgroup$
    – Hirahito
    Commented Apr 20, 2016 at 18:06
  • $\begingroup$ @Michael Richardson - my question was more along the lines of: "While I understand this type of planet would almost certainly never develop in nature, would it be sustainable without outside deal breakers like artificial gravity for any 'decent' amount of time. And by that, I'd say... I dunno... more than a few centuries. $\endgroup$
    – Hirahito
    Commented Apr 20, 2016 at 18:08

1 Answer 1


The stability depends on the construction of the ring, and its mass. The mass is the trickier of the two so I'll start with it first. Gravity has a tendency to make massive things more spherical. On the other hand, if the mass is too light, newtons third law starts coming into play and it become increasing easy to disturb. This all brings us to the construction of the ring- because of how the ring is constructed, it is basically an engineering problem that uses the strength of the materials to counteract gravity's strain, while making it heavy enough so that it has enough inertia that shocks to the system cause little change.

Note here that there would be no gravity forces applied while you were inside the ring (due to the Shell Theorem), however most of these structures were originally conceived to use centrifugal action to hold people to the inner edge of the ring. This means one of two things; either it is spinning and he habitation zone is inside the ring (as your inertial would fling you off the planet if you were on the outside), or that it isn't and the habitation zone is on the outside.

  • $\begingroup$ I see the construction as more of an open-ended bridge, much like the ring world itself, but with far less strain involved. Questions arise from this however: 1) What level of material would be required for this 'bridge' to be somewhat sustainable? Rock? Steel? Something like scrith? 2) Would there be a need for continuous artificial gravity or could you turn it off? $\endgroup$
    – Hirahito
    Commented Apr 20, 2016 at 18:10
  • $\begingroup$ I looked up the Shell theorem. There's a quote here that makes me think the toroidal nature might actually cancel that. Since it's not a sphere, the 'locality' of the gravity for the mass of the ring closest to the person standing on it is not 'cancelled' by the other side (square cube law). Thus, while low, there would be a specific gravity pulling 'down' towards the inside of the toroid even if you were standing on the 'inside' surface. Unless, of course, I've completely missed the math. $\endgroup$
    – Hirahito
    Commented Apr 20, 2016 at 18:18
  • $\begingroup$ On the shell theorem bit, your math is correct, though it doesn't change that the gravitation would be different on the inside compared to the outside. $\endgroup$
    – zoboso
    Commented Apr 21, 2016 at 2:23

Not the answer you're looking for? Browse other questions tagged .