29
$\begingroup$

In both fiction and non-fiction worlds, that attempt to keep a sense of realism, planets are spherical.

Is it possible for a planet to not be shaped like a sphere?

If such a planet is possible could it sustain life? For example, having a proper magnetic field, atmosphere, etc?

$\endgroup$
  • $\begingroup$ I'm not a meteorologist, and I know nothing about magnetism, but I believe most planets are round because of the way that the gravity pulls together all of the bits that make them up. So I don't think that a cubic planet could exist naturally, but if someone somehow created an Earth-sized cube and flung it into space, I don't see why it couldn't orbit a star. Incidentally, OP, does "Kong" mean anything to you? $\endgroup$ – EFrog Jan 15 '15 at 21:08
  • $\begingroup$ I was just wondering if a donut shaped planet, or something more "Exotic" could exist, just for reference. $\endgroup$ – Shadow Z. Jan 15 '15 at 23:50
  • 5
    $\begingroup$ technically, the definition of planet includes the fact that it is round, so an astronomer would say "no"; what you're really asking is whether a very large asteroid could sustain life. $\endgroup$ – KutuluMike Jan 16 '15 at 1:12
  • 2
    $\begingroup$ I don't think this rises to the quality of an answer, but this Vsauce video shares some interesting analysis of what it would feel like to be on a flat, disc-shaped planet. $\endgroup$ – Michael Grant Jan 16 '15 at 13:39
  • $\begingroup$ By the IAU definition of a planet since 2006: no. (Ducks flying bricks from the "Pluto Is A Planet!!!" crowd). In a more practical sense: you can expect all naturally occurring planets to be spheroids. This is because at those scales, there is no known material that does not behave more or less like a fluid or at least like putty. Gravity will make a spheroid of them. $\endgroup$ – MichaelK Mar 7 '17 at 10:16

11 Answers 11

29
$\begingroup$

It'd be possible for one to exist for a time, but a naturally occurring, non-spheroid planet would be incredibly unlikely. More on that at the end.

It's an easy thing to imagine of course, but that's because we think of things like cube shaped rocks that occur naturally and think 'why not?'. The problem is that objects on a planetary scale don't behave the same as boulder or even small moon sized things. Anything above 200-300km radius squishes into a spherical shape.

Shapes:  dust, potatoes, spheres, disks, halos Source

So, you can have a space potato that's not a sphere, but it's technically not a planet. It could have an atmosphere at any size, though it's less likely to hold on to it if there are other large masses nearby or in high solar winds.

To have something the size of a planet but not a sphere you'd have to find a planet that has had a terrible accident. If you crash a large moon into a planet you might be able to horribly deform the planet and give it some new moons. Assuming the remaining mass is large enough it could still be defined as a planet (massive enough to collapse into a sphere) but non-spherical. The non-spherical shape would be temporary, hundreds of thousands of years perhaps. As it's just been destroyed it's unlikely to have life though.

Edit:

What if Earth were a cube?

There is an interesting article on that here. Basically the corners and edges of the cube would be like massive mountain ranges, as they would be at an angle with respect to the vector of gravity (except in the center of the faces). I wrote a more detailed answer about such a world here.

The oceans would move to the centers of the cube faces. Basically, the Earth would look very different. Likely still inhabitable, assuming whatever magic formed it into a cube kept the the shape, otherwise we'd probably all be killed by the planet shaking earthquakes.

$\endgroup$
  • $\begingroup$ In theory, if one had enough technological or magical power to reshape a planet, (say turn the Earth into a cube), would that have any effect on its atmosphere or orbit? $\endgroup$ – EFrog Jan 15 '15 at 21:28
  • 4
    $\begingroup$ @EFrog Orbit, no it would continue around the sun and retain its moon. The atmosphere would get a bit messed up. All the points on the surface of the cube would not be equal distance from the center of gravity. The corners of the cube would be like massive mountains, likely sticking out of the atmosphere. $\endgroup$ – Samuel Jan 15 '15 at 21:34
  • 1
    $\begingroup$ I necro'd this just so I could express my glee at the turn of phrase "space potato". $\endgroup$ – Sean Boddy Apr 23 '15 at 3:17
  • $\begingroup$ @SeanBoddy I'm sure that entered my mind via Futurama. $\endgroup$ – Samuel Apr 23 '15 at 6:12
27
$\begingroup$

Donut-shaped planets are indeed possible!

The article cites a paper which concludes that certain kinds of toroidal planets are at least internally stable, although not likely and possibly would get ruined by any external interference.

It's true that gravity will make a randomly-shaped blob of matter coalesce into a sphere over time, but there's no reason that it has to do so with a torus. Matter is attracted most strongly to the closest matter.

$\endgroup$
  • 2
    $\begingroup$ I came here to post this. See also this discussion on math.SE regarding tori-worlds. $\endgroup$ – dotancohen Jan 16 '15 at 14:45
  • 3
    $\begingroup$ Also note, though, that the torus shape is not stable over long time periods. Fatter parts of the torus pull material away from thinner parts of the torus, eventually breaking the torus into multiple spheres. The site you linked describes this process in greater detail. $\endgroup$ – ckersch Jan 16 '15 at 14:55
  • $\begingroup$ Stability issues could be fixed if the whole planet is alive or mechanical and capable of redistributing its matter content to prevent the catastrophe. $\endgroup$ – Danijel Mar 7 '17 at 6:31
14
$\begingroup$

Yes, we live on one :-)

Strictly speaking, the Earth is not a sphere, it's an oblate spheroid. Its rotation makes it bulge somewhat, so the equatorial radius is ~30 km greater than the polar. A faster rotation would make a planet even more oblate: Saturn's polar & equatorial radii differ by almost 10%: https://en.wikipedia.org/wiki/Saturn#Physical_characteristics because the centrifugal force from rotation counterbalances the gravity.

Cubes, donuts, &c couldn't exist, though, at least if they're composed of normal matter. It's a matter of strength of materials. The corners of a cube would behave like very tall mountains, and collapse under their own weight.

$\endgroup$
10
$\begingroup$

The sphere shape is entirely based on gravity. That is actually one of the parts of the definition of a planet. That is is large enough that it's gravity will make it a sphere. The objects that are not spheres are much smaller or they would have to be artificially created and maintained. (unless shortly after a large event that distorted the planet, like a moon colliding into a planet, but in time it will round itself out again.

$\endgroup$
7
$\begingroup$

Hard SF writer Hal Clement wrote a delightful book in 1953, Mission of Gravity, which introduced a very physically correct non-spheroidal world, Mesklin. Mesklin, a high-mass planet subject to extreme rotational stress, is shaped like a thin disk with a central bulge. From the Wikipedia article:

"Clement decided, since its mass was 16 times that of Jupiter, Mesklin would have an extremely large angular frequency to partly counter its gravity in order to allow humans to visit part of it. He wanted the equatorial gravity to be 3 g, so he determined the period necessary to make this occur: each Mesklin day is 17.75 minutes long given that the planet rotates approximately 20 degrees a minute.

As a result of this extremely large rate of spin, Mesklin is not even slightly spherical; it has a large equatorial bulge. Mesklin's equatorial diameter is 48,000 miles (77,250 km), while from pole-to-pole along its axis of rotation it is 19,740 miles (31,770 km). Then Clement attempted to calculate the polar gravity, finding it surprisingly difficult. He admits, "To be perfectly frank, I don't know the exact value of the polar gravity; the planet is so oblate that the usual rule of spheres... would not even be a good approximation..." "Whirligig World" reports his initial calculations of the pole gravity to be 655 g; the dust jacket of Heavy Planet reports it as 700 g. A later program created by Clement computed it as 275 g"

This is a much more extreme example of the deformation of Saturn described in jamesqf's answer. Possibly not what you're looking for, but it makes a nice illustration of just how plastic large planets are under the force of gravity and the apparent centrifugal force of spin.

$\endgroup$
  • $\begingroup$ The church could like this one! The escape velocity could be surpassed for the outer portions of the disk, would be interesting. Another interesting question would be if this kind of object could be the result of a natural phenomena. $\endgroup$ – WalyKu Oct 24 '16 at 16:29
4
$\begingroup$

It's not possible unless the planet is made of unobtainum.

If you had, for example, a cube-shaped planet with a liquid center, you would have a huge amount of pressure on the liquid interior of the planet applied by the corners of the cube, which would effectively function like gigantic mountains. This pressure wouldn't exist on the faces of the cube, so the internal pressure would push them outwards, likely resulting in the formation of some absurdly huge volcanoes as the liquid interior of the planet, under the pressure applied by the corner mountains, burst through cracks in the surface of the low-pressure faces of the cube. The corners would sink down and the flat portions would fill up with lava, which would slowly cool into the shape of a sphere.

$\endgroup$
  • 1
    $\begingroup$ By the time you reach the mass of a large asteroid, gravity overrides the strength of any reasonable material and the planet is spherical (subject to rotational deviations). $\endgroup$ – Oldcat Jan 16 '15 at 0:02
  • $\begingroup$ Someone should calculate cube with volume of the Moon. Should be cold enough, and gravity low enough for it be possible. $\endgroup$ – Peter M. Jan 16 '15 at 0:33
  • 4
    $\begingroup$ @PeterMasiar, Won't work. A back-of-the-envelope calculation for a cubical Moon made of granite gives a pressure at the inscribed spherical surface (the point at which the difference between a cube and a sphere becomes irrelevant) of 3 GPa, far beyond granite's compressive strength of 0.2 GPa. In short, at the pressures involved, solid rock acts like a liquid. $\endgroup$ – Mark Jan 16 '15 at 8:58
  • $\begingroup$ @Peter Masiar: People have calculated the maximum possible height of mountains, e.g. skeptics.stackexchange.com/questions/5848/… You could get somewhat higher using things like carbon nanotubes, but even that has limits. For instance, in the Christian Bible's Book of Revelations (21:14), Heaven is described as a cube (of gold!) about 1500 miles on a side. $\endgroup$ – jamesqf Jan 16 '15 at 18:28
  • $\begingroup$ But see Eli Rose’s answer below which has a citation that contradicts this. $\endgroup$ – JDługosz Mar 7 '17 at 6:28
0
$\begingroup$

What about some planet in orbit around a very heavy black hole?

There could be some situations where the stable state of such a system included a non-spherical planet.

Also, you could get some interesting relativistic time effects, like in Interstellar.

$\endgroup$
  • $\begingroup$ "There could be some situations where the stable state of such a system included a non-spherical planet" - could you elaborate? if the planet is being deformed by the gravity of the black hole, it should also be torn apart and (at least partially) swallowed, i believe. The glorious term for this is "spaghettification". No, really. $\endgroup$ – katzenhut Mar 26 '16 at 17:18
  • $\begingroup$ Well the sphere is a perfect symmetry of forces. What if the forces are not perfectly symmetric around the centre of mass, and yet still they balance? It seems that other shapes are possible. If you look at a binary system, one star can eat another for millions of years, producing a long jet. So you can definitely get non spherical shapes in systems that are stable but in motion. What if aliens reinforced with a skeleton the basic asymmetry of such a system for some purpose, freezing the spaghetti planet in place ( bonsai planet ? ) Also the "station" in Interstellar is like a hollow drum. $\endgroup$ – populacecris Mar 27 '16 at 8:13
0
$\begingroup$

One of the defining traits of a planet according to the IAU is that it be in hydrostatic equilibrium. That means it can't be holding its shape against its own gravity which results in an oblate spheroid with at most fairly minor lumps.

If you made a planetary scale structure out of super string unobtanium (Rather than something as squishy and flexible as rock) that was not in hydrostatic equilibrium, it technically wouldn't be a planet. Ring worlds, orbitals, and the like are not planets.

Unlike the "clear its neighbourhood" requirement (the thing that got Pluto reclassified), no one really disputes this part of the definition of "planet".

The spin rate required to get enough oblateness to be significant to anyone other than scientists, engineers, and the like just isn't going to occur in something that fits the definition of a planet naturally. The collapsing dust cloud forming a star and its planets is only going to have so much angular momentum. It's also going to be subject to some crazy tidal forces (which will slow it down)

In theory a torus (doughnut shape) can be in hydrostatic equilibrium if it's spinning fast enough. Even if it could get spinning that fast, and get into the torus shape, it wouldn't be stable in the long term. If it got even slightly out of balance it would tear itself apart.

The rough version of this is: No, a planet is (to the tolerances of a ordinary person) spherical by definition. Anything not spherical is not a planet.

$\endgroup$
  • $\begingroup$ I asked about that earlier. $\endgroup$ – JDługosz Mar 8 '17 at 10:16
0
$\begingroup$

I think that a planet is definitely able to be misshapen and still be a planet! Our very own Earth is not a perfect sphere! It is a bumpy oblate spheroid, though it looks like a marble from space. Earth's poles are squashed, and it is swollen at the equator. Because of the bulge, the distance from Earth's center to sea level is roughly 21 kilometers (13 miles) greater at the equator than at the poles. This was all discovered by Issac Newton. Earth has a bit of plasticity that allows the shape to deform slightly. An oblate spheroid is not perfectly correct either, because of the fact that mass is distributed unevenly within Earth, also causing more deforms.

$\endgroup$
-1
$\begingroup$

It is not possible because of gravity, with the spherical shape of our Earth's core, the earth is shaped into a sphere. If there were a donut shaped planet, there would be no place for the core, therefore it wouldn't be classified as a planet. With the hole in the center, the center of gravity wouldn't exist. To be classified as a planet, it has to have a core, and atmosphere, and has to be shaped as a sphere.

$\endgroup$
  • 1
    $\begingroup$ Hello Axel and welcome to Worldbuilding! You may wish to revise and expand your answer a little! This will help you get more upvotes and build reputation more quickly. $\endgroup$ – Serban Tanasa Apr 20 '16 at 20:44
-3
$\begingroup$

If there was a solar system with two sun's and a planet in-between, if the sun's rotated while the planet was, it might keep the planet in a disklike shape. Or split the planet in half. But that chance is still there. It would look like that one planet from Netflixs Voltron.

$\endgroup$
  • 2
    $\begingroup$ Welcome to WB! As you explore the site some more, you’ll notice that the idea of a planet balanced between two stars has been discussed in more detail (it's not stable). However, why would that cause a disk shape? Out own planet’s tides from the sun are not large enough to noticably make it non-sperical, and putting a sun on each side would actually reduce the tide bulge significantly, since the bulge is caused by the gradient change of gravitational force and you’re actually smoothing it out. $\endgroup$ – JDługosz Mar 7 '17 at 6:36
  • $\begingroup$ Do you mean that the planet is at the center of mass of a trinary system formed out of two stars and a planet? Either way, I'm pretty sure that the stars' gravity at that point would be far, far lower than what would be required for an equilibrium elongenated formation of mass, especially one that is still able to hold together. Not my downvote, but I can easily imagine that this is how whoever downvoted this was reasoning. $\endgroup$ – a CVn Mar 7 '17 at 6:42
  • $\begingroup$ Adding to what @JDługosz wrote, planets do form oblique spheroids ("flattened balls"), but not because of gravity; rather in spite of gravity. That is caused by the planet spinning around its own axis of rotation. $\endgroup$ – a CVn Mar 7 '17 at 6:44

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.