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Let's imagine a roughly (or perfectly?) cylindrical celestial body. Other than it's shape, everything about it astronomically (distance from sun, orbit shape, etc.) is the same as Earth. Is it feasible for it to be habitable by humans, or would something about the shape make that impossible?

The Square-Cube Law
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Mocascoolai
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    celestial body. That would be by definition not a planet. "Gravity causes planets to be pulled into a roughly spherical shape" – Mazura Oct 06 '23 at 00:36
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    The landslides, as the rims slump toward regions nearer the centre, would likely be inconvenient. – Anton Sherwood Oct 06 '23 at 01:41
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    I'm tempted to vote to close because, at best, this is opinion-based. On the one hand, we're one pernicious species when it comes to filling ecological niches. On the other hand, insofar as we understand science, there can be no such thing as a cylindrical planet. Honestly, this is a choice you need to make - a rule that you set - as there cannot be any scientific basis to provie it for you. (Bear in mind that asking if something "feasible" is a weak question here. We deal with fictional worlds. It's always feasible on your world if you want it that way.) – JBH Oct 06 '23 at 01:57
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    @JBH I think we could leave this open: it is a legitimate question, even if the answer is 'no'. It may be obvious to some of us that a cylindrical planet would be unstable. Closing the question would be fair if a trivial search gave a clear reply. I tried. One of the references was question 110478 from this site on the seasons on just such a world. IMHO, keeping this would at least provide the right answer if someone was to ask again. – Richard Kirk Oct 06 '23 at 09:37
  • That's actually answerable, even though that yes, it needs a frame-challenge as stated : "Cylinders don't scale up well, but supposing by the law of high science-fiction it magically keeps its solid shape while retaining gravity, then...". Even though it would have been indeed better the querent offered this assumption to us, effectively showing they have done some research on planet formation. – Tortliena - inactive Oct 06 '23 at 11:33
  • since a cylindrical planet is clearly not natural, you'll have to define its properties for us. eg. how, and in which direction, is the gravity working on it? – ths Oct 06 '23 at 14:43
  • how big is it, do people live on the inside or outside? – John Oct 06 '23 at 20:48
  • Given that the cylinder is same composition as Earth including thin crust and plastic mantel, It would start deforming towards a sphere right quick, As in seconds after the 'play' button is pressed, within 24 hours it would be closer to spherical then cylindrical. – Gault Drakkor Oct 06 '23 at 21:22
  • I watched a video [ https://www.youtube.com/watch?v=fMlGs4X67q8 ] where it's explained the possibility of existing donut-shaped planet. Some exotic features here may give you some thoughts. – Peter.k Oct 20 '23 at 10:26

2 Answers2

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As mentioned, this is not actually a planet, but let's pretend.

One scenario would be a perfectly cylindrical object, created by some ancient race, billions of years ago. Let's presume that the ancients included a mechanism to keep its rotation polar-oriented. Maybe it even has a way to keep the poles pointed towards the north and south of the nearest star.

It gets encrusted with ejecta from supernova over the aeons. It gets trapped by a proto-star and winds up orbiting in the goldilocks zone, with just the right amount of material splattered across it to make a crust across the surface. We can be extra nice and say that water also gets applied.

This scenario leaves us with a layer of basalt rock crowded along the equator. A spherical layer of water would mostly cover the basalt, and the atmosphere would pool on top of that. It would look a lot like a normal planet that someone stuck a pole through.

The experience on the planet would depend on the rock-to-object ratio. If the rock was thick and the cylinder narrow, you wouldn't notice the object until you went far enough north or south. At that point, you would run into a wall on either side that stretched up out of the atmosphere. I'm sure there would be wall-climbing expeditions.

If the rock was thin, then the rock would pile up against an ever-increasing slope until it hit the "maximum angle of repose", which would start around 30 degrees. That zone would undoubtedly be above the gravitationally constrained atmosphere. This geography was explored by Larry Niven, with the planet "Jinx," referred to as "God's own Easter Egg."

Could people live there? Sure. We're tough and clever. Could we evolve there? No. Explaining why not would be a book, but there are a lot of chemicals involved in evolution that are exclusively related to volcanism, and such an object would have no volcanism.

The lack of volcanism would also result in a very flat crust and shallow oceans. Hydrology would wear down rocks, and there would be no tectonic plates to uplift mountains. There would be no granite, because that's chemically created through subduction.

Robert Rapplean
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A 'planet' is generally understood to be a natural body. The IAU distinguishes planets from planetoids and planetesimals by their round shape and that they have swept their orbit clean. This definition excludes Pluto, Ceres and Eris.

A cylindrical planet would not form naturally. If you had one, any irregularity would cause it to collapse into a string of beads under its own gravity. If it is not going to collapse, then that pretty much means it has to be artificial. This is 'Worldbuilding', so we allow artificial planets, even if the IAU does not recognise them yet.

To avoid confusion, let's get the O'Neill cylinder out of the way. That is a practical world where you live on the inside of the cylinder and centripetal force replaces gravity. You can make these and join them into a line of arbitrary length. That gets around the construction site problem, where you have to pay interest on the loan to build the thing for years before you can sell the first apartments.

It might be possible to make a ring (torus) that was really thin. You could make it in empty space, or in orbit around a planet. It would have some gravitational pull. It could even rotate (the flexing as the inside became the outside would be tiny). However, if you are want to retain an atmosphere, it will have roughly an Earth diameter. If you could magically make something like that, it might exist for a while, but it would have to be very rigid to avoid collapsing into beads. It would be very vulnerable to asteroids, both from collisions and tidal effects. You are probably nailed by the construction site problem: it would collapse before you finished building it.

Richard Kirk
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