# Flat world with land only between the tropics

I was wondering, how could there be a world with flat poles to such extent that only the tropical area would exist (it would be sort of a wheel).

The weather and nature would be tropical.

How would such a world hold together? How would the ocean behaviour be with such form? Would it be possible to live on the "flat" poles?

• Such a world wouldn't form naturally, it would have to be constructed. May 28, 2015 at 21:33
• @ShemSeger has it right. It'll be interesting to see what people come up with to this. May 28, 2015 at 21:45
• The topic of alternate planet shapes is well understood in physics (it doesn't happen for any reasonably sized planet...spheres win the day). Do you have an alternate model of physics in mind? May 28, 2015 at 22:22
• Yes, we need a lot more clarification here. Are we talking natural? Man-made? In this universe? In a magical universe? How big?
– DA.
May 28, 2015 at 22:56
• My first goal would be as natural as possible. I thought that, if the poles in earth are a bit flat why not make this more extreme. If it's not physically possible, than in an hypothetical situation where this would be possible.
– nsn
May 29, 2015 at 7:38

What you're talking about here is a discworld.

You started with the idea of living on the edge of the disk, so maybe you're not seeing it that way, but the shape is the same. For an artificially constructed (and held together) disk the edges would be far from tropical, they would likely not be habitable at all.

You could work the thickness and density so that the disk would mostly have Earth-normal gravity. Most of the surface of the disk would have close to normal (as in orthogonal) gravity thanks to the infinite plane approximation (as seen on an Alderson disk). As you moved toward the edge of the disc you would feel more and more as if you were walking uphill. This is similar to my answer for the the cube world question, though to less of a degree due to the increased ratio between width and height. Still, the atmosphere would collect toward the center of the disk, making the edges uninhabitable.

If you have your heart set on a tropical edged discworld then you need to make up your own physics. In which case you can make up whatever answer you want for your question.

• If I understand the OP correctly, it's not a Discworl(tm), but a drumworld. People aren't living (only) on the flat upper surface, but on the cylindrical rim. As you rightly point out, you'd also need rimwalls like a Ringworld, in order to keep the air & water from flowing polewards. (So you could conceivably have 3 entirely different habitable zones, separated by vacuum.) Also it has a lot more mass. With a Discworld(tm), most of the gravity comes from the mass of the elephants and turtle :-) May 29, 2015 at 5:22
• @jamesqf yes, exact.
– nsn
May 29, 2015 at 7:42
• @jamesqf A drum, disk, and wheel are all the same sort of shape. The disk I'm describing is not thin, it's nearly as thick as the radius of the Earth, as I described in the linked answer. If the OP wants to add walls to retain the atmosphere then the edges may remain habitable, but then no one could move to the flat poles without going through vacuum. May 29, 2015 at 16:04
• @Samuel: Linguistically, at least, I think disc/disk implies thinness, as e.g. a discus, compact disc, or Discworld(tm). Granted, there's a continuous variation, so you could morph a thin Discworld(tm) world into a very_long_thin_cylinder_world. May 29, 2015 at 17:54
• @jamesqf It's thickness is thinner than its diameter. In this case the thickness is 40% that of the diameter. If the elephants and turtles are included, that's thinner than Discworld™ :) May 29, 2015 at 18:50

If some sort of magic or magical material keeps it in that shape then it would be possible to live on the flat poles, at least close to the edge, but there would be some weird effects.

Gravity pulls thing towards the center of mass (of the planet) so if you are standing on the flat pole it'll pull you a bit sideways, depending on how close to the edge you are. If you drop a ball it'll just keep rolling towards the center of the pole. So for people to live there there would have to be a lot of walls. Also they'll probably have slanted floors so they are essentially at the same angle as the surface. So a town there might look like a giant staircase.

The gravity will be weaker (as was pointed out by the comments) towards the center of the pole. The atmosphere near the center of the pole might be denser and atmospheric pressure higher, since there probably will be more of it above that point. At least if the atmosphere acts as normal. And depending on how the inner workings of the planet look it may be much hotter.

• Actually, gravity gets weaker as you approach the poles. The most likely effect on atmosphere is that the equator is quite close to vacuum. May 28, 2015 at 23:19
• @WhatRoughBeast Approach them from where? If you sliced a piece from a planet, and it magically didn't fall apart then the center of that newly created flat area would be closer then the surface, so gravity would feel stronger (F = G(mass1*mass2)/D^2), at the very edge it'd be same as the surface. May 28, 2015 at 23:29
• @WhatRoughBeast is correct. Think about standing on the edge, all the mass is below you. Now, as you move toward the center of the flat area more and more mass is now above you, canceling a component of the remaining mass below you. As you say, the tangential gravity in the center is zero, that's because its been canceled. This means the total gravity magnitude is smaller. That is, lowest gravity in the center. All the vectors still point there, but it's still the minimum point. May 28, 2015 at 23:41
• @Maxim - Sorry, you need to review your physics. That formula only holds for point masses or spherically symmetric masses with radius less than the distance to the center. At a pole, being near the center causes some or most of the gravitational force to cancel (depending on how thick the remaining disc is), since every unit mass in one direction will have its radial force component cancelled by a another unit mass in the other direction. The normal components will add, but they drop off with distance. May 28, 2015 at 23:42
• @WhatRoughBeast You're right, I was thinking of it too abstractly, like if you are above the surface there is less gravity then on it, but going "below" surface means that there is a lot more mass around you so that formula doesn't apply May 28, 2015 at 23:48

Presumably you could have a universe where the spin of an object distorts its gravitational signature in such a way that any spinning large body will approximate a cylindrical shape (basically, spin changes the center of gravity from a point to a line).

Then most celestial objects would turn into cylinders. They wouldn't be perfect - they would still bulge around the core - but you could get close. The exception would be tidally locked moons, which would spin until they locked in place, at which point they'd start reforming into a sphere.

Your planet will end up having pole-spanning oceans (probably largely frozen over), so that water doesn't hit the edge and just fall off.

Note: This is very handwavy, for a couple of reasons. First, when you get down to the gritty details messing with physics like this tends to screw up other things. So it's hard to say that you could just change gravity and not screw up anything else. Second, I'm not sure that a planet like this would develop similarly to ours, with plate tectonics and such, so it might not be habitable. Or you know, if stars would still function with this kind of gravity. But you could probably get away with it if you're not aiming for hard sci-fi.

There is maybe one condition under which such a planet might work: It would need to have a very rapid rotation, as in one hour long days, probably even a lot shorter than that, someone else can do the math, but it would depend on how flat you want your planet to be. And it would need to have a lot of atmosphere.

If it did not have a rapid rotation, then the atmosphere at the equator would most likely be thinner than than at the summit of everest, as all the water and atmosphere would flow off into the poles and create a spherical gas planet that has a large disc of crust in it's centre, basically leaving the equator in the upper atmosphere.

Earth is wider at it's equator because of its rotation, it's not a perfect sphere, and is actually 64km wider around the equator than it is from pole to pole, and as a result of it's rotation, the atmosphere is thicker over the equator and the oceans are deeper because they bulge out due to planetary centrifugal forces. If a planet was rotating rapidly enough as it formed, it might flatten out a bit as well as throw some of that atmosphere out over the equator, and make it livable.

At the poles - the gravity would at least feel much greater because those centrifugal forces wouldn't be present. I don't imagine that the environment would be very hospitable at the poles, I'm guessing that despite the shape of the core and crust, that the planet would still be spherical, and the flat poles would have an immense amount of dense gases over them. The poles would be stormy, dark, and probably toxic.

I'm picturing something like this:

It's basically a gas planet with a squashed earth inside it, and it rotates really fast. I call it "Earth Sandwich".

The real question is, what would make a planet rotate so rapidly?

• @ArtOfCode - Here's a math problem for you, how fast would my planet have to rotate for it to stay flat? May 29, 2015 at 1:49
• Just an idea, could a comet hit the planet with such force that the planet would be spun faster the same way you speed up a en.wikipedia.org/wiki/Roundabout_%28play%29 roundabout or would the force needed just destroy the planet ? May 29, 2015 at 5:28
• @Magic-Mouse It'd probably take a really big chunk off of the planet. I was thinking more along the lines of the planet's orbit is near a belt of asteroids that have some kind of a gravitational or magnetic effect on it's rotation, and every time the planet passes by it, it's rotation speeds up. Like the opposite of what's happening to the earths rotation, the moon is actually weighing the earth down–in a matter of speaking–and slowing it's rotation. You know leap years? Those weren't a thing a few million years ago-as the moon gets further away from the earth, the days get longer. May 29, 2015 at 5:36