Non-spherical planets

Thomas_Anderson · May 29, 2010

Is it possible for one to exist, or are there scientific reasons why such a thing couldn't be?

As for how it could be non-spherical, I was thinking something like a very large, irregularly shaped asteroid. Or perhaps sculting via futurisitic tech. Could there ever be a cube planet?

Speaking of cubed planets, how would the gravity work? Would you fall into space if you walked off one end? Or would it be like Mario Galaxy, where as you leave one edge, the other edge grabs you?

Xelebes · May 29, 2010

Spheroids are most common on planets because of the distortion caused by gravity pulling it together. Asteroids, if being small enough, could possibly be cuboid. Toroids are best approximated by a body of bodies, not so much a single body.

With cuboid planets, if you so wanted to have such a planet, would have the gravity centred around the middle. That is, the closer you walk to the edge, the more gravity will want to pull you in on an angle, thus making you more likely to topple if you were standing erect and not perpendicular to the gravity.

benbradley · May 29, 2010

The smaller something is, the easier it is (gravitationally) to make it whatever shape you want.

"Self-gravity" will tend to bring a non-rotating mass into the shape of a sphere. The larger the mass, the more strongly it will tend to become a sphere, and the harder it will be to make into another shape. This will happen with a relatively small mass of liquid, but at the scale of planets, their solid mass acts much like a liquid as well.

Virtually all planets rotate, which changes their "natural" shape into an oblate spheroid:
http://en.wikipedia.org/wiki/Oblate_spheroid

Most planets look very much like spheres, but they are generally oblate spheroids, being flattened at the poles, so their circumference around the equator is longer than the circumference measured going through the poles. A planet rotating very fast would be easily noticable as having its poles flattened relative to its circumference.

Xelebes is correct, but I don't like that wording.

On a cube planet you will be standing on flat ground only at the center of a face. As you walk toward an edge, the face will become an incline, until at the edge the face will be inclined 45 degrees in relation to gravity. It will be like walking uphill, because it IS walking uphill. The other face will be inclined 45 degrees the other way, and it will be just like being at the top of a huge roof. The top is a line going (say) straight away from you north and south, while the two east and west planes go down.

Sarpedon · May 29, 2010

only the smallest of objects in our solar system are not spherical:

Look at the wikipaedia entry for Jupiter's moons:http://en.wikipedia.org/wiki/Jupiter's_moons

You'll see that the non spherical ones are tiny: 250 km in length for the biggest irregular one.

Chris P · May 29, 2010

Spheres minimize the surface area to volume ratio, which is why drops of liquid tend to spherical shapes. I imagine the same thing happens to a planet, especially those with molten cores. If one solidified compeletely and then got blasted by a major collision or something I don't see why it couldn't be non-spherical, but I'm a bit out of my league here.

Albedo · May 29, 2010

benbradley said:
The smaller something is, the easier it is (gravitationally) to make it whatever shape you want.

"Self-gravity" will tend to bring a non-rotating mass into the shape of a sphere. The larger the mass, the more strongly it will tend to become a sphere, and the harder it will be to make into another shape. This will happen with a relatively small mass of liquid, but at the scale of planets, their solid mass acts much like a liquid as well.

Virtually all planets rotate, which changes their "natural" shape into an oblate spheroid:
http://en.wikipedia.org/wiki/Oblate_spheroid

Most planets look very much like spheres, but they are generally oblate spheroids, being flattened at the poles, so their circumference around the equator is longer than the circumference measured going through the poles. A planet rotating very fast would be easily noticable as having its poles flattened relative to its circumference.

Xelebes is correct, but I don't like that wording. On a cube planet you will be standing on flat ground only at the center of a face. As you walk toward an edge, the face will become an incline, until at the edge the face will be inclined 45 degrees in relation to gravity. It will be like walking uphill, because it IS walking uphill. The other face will be inclined 45 degrees the other way, and it will be just like being at the top of a huge roof. The top is a line going (say) straight away from you north and south, while the two east and west planes go down.

It'd be like walking up an unfathomably big hill. So big that gravity would be considerably lower at the summit compared to the base, (by a factor of [FONT=Times New Roman, serif](√[/FONT][FONT=Times New Roman, serif]2)[/FONT][FONT=Times New Roman, serif]^2[/FONT] i.e. one half, I think) actually making it easier to climb the higher you went. If it wasn't for the total lack of an atmosphere along the edges, that is.

Of course, gravity is why an Earth-sized cubic planet couldn't exist under the laws of this universe. Unless the whole thing is made of absurdlystrongium those "mountains" at the corners would collapse under their own weight, forming a (you guessed it) sphere.

scottVee · May 30, 2010

Comically, the current definition of a planet includes the suggestion that it must be large enough form gravity to make it (roughly) spherical. So no. But little chunks can be any shape you want.

Lhun · May 30, 2010

Chris P said:
Spheres minimize the surface area to volume ratio, which is why drops of liquid tend to spherical shapes. I imagine the same thing happens to a planet, especially those with molten cores. If one solidified compeletely and then got blasted by a major collision or something I don't see why it couldn't be non-spherical, but I'm a bit out of my league here.

At the energy levels involved, rock is essentially a liquid. There is no known material which could maintain a non-spherical shape when present in planet-sized chunks. So the only way for a planet to be non-spherical is a constant force acting upon it, and holding it in (or out of) a certain shape. Centrifugal force keeps planets pear-shaped.

ColoradoMom · May 30, 2010

scottVee said:
Comically, the current definition of a planet includes the suggestion that it must be large enough form gravity to make it (roughly) spherical. So no. But little chunks can be any shape you want.

This was what I was thinking - if they aren't spherical - they aren't planets.

C.bronco · May 30, 2010

ColoradoMom said:
This was what I was thinking - if they aren't spherical - they aren't planets.

I agree. None I have heard of qualify as planets.

lpetrich · Jul 5, 2010

One can even estimate how much the nonsphericity will be.

The sizes of mountains and other such irregularities will be limited by the yield strength of their material. This is the maximum shear pressure that they can tolerate. In general:

P ~ rho*g*h

rho = density of material
g = acceleration of gravity ~ G*rho[sub]bulk[/sub]*r
h = height of mountain
r = avg. radius of object
G = gravitational constant

so h[sub]max[/sub] ~ P[sub]yield[/sub]/(rho[sub]mtn[/sub]*g)

The relative maximum size of a mountain is thus

h[sub]max[/sub]/r ~ (P[sub]yield[/sub]/P[sub]central[/sub])*(rho[sub]bulk[/sub]/rho[sub]mtn[/sub])

where the central pressure is approx. G*rho[sub]bulk[/sub][sup]2[/sup]*r[sup]2[/sup]

One can estimate the amount of flattening by finding (centrifugal acceleration) / (gravitational acceleration)

When one works that out, one finds

flattening ~ w[sup]2[/sup]/(G*rho)

where w is the angular frequency of rotation, which is 2*pi/period

That's why most planets and large moons look nearly spherical.

geardrops · Jul 6, 2010

Xelebes said:
Spheroids are most common on planets because of the distortion caused by gravity pulling it together.

I'm just contributing to say ^ this.

That is all.

Pthom · Jul 6, 2010

Despite the flap over the demotion of Pluto and the resulting refinement (in August, 2006) of the definition, the following still applies, especially in light of this discussion.

"A planet is a celestial body orbiting a star or stellar remnant that is massive enough to be rounded[sup]*[/sup] by its own gravity, is not massive enough to cause thermonuclear fusion, and has cleared its neighbouring region of planetesimals."

"IAU 2006 General Assembly: Result of the IAU Resolution votes". International Astronomical Union. 2006. http://www.iau.org/public_press/news/detail/iau0603/. Retrieved 2009-12-30.

and

"Working Group on Extrasolar Planets (WGESP) of the International Astronomical Union". IAU. 2001. http://www.dtm.ciw.edu/boss/definition.html. Retrieved 2008-08-23.

[sup]*[/sup]... has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape

pdknz · Jul 7, 2010

Just to present a contrarian view here. Larry Niven has pushed the limits of the non-spherical planet concept in several directions. Ringworld is probably the most noteable non-spherical construct, although it's not really a planet. There was a constructed planetoid in Protector, although I don't remember the shape; and a non-planetary gaseous toroid called the smoke ring in The Integral Trees.

Liosse de Velishaf · Jul 7, 2010

pdknz said:
Just to present a contrarian view here. Larry Niven has pushed the limits of the non-spherical planet concept in several directions. Ringworld is probably the most noteable non-spherical construct, although it's not really a planet. There was a constructed planetoid in Protector, although I don't remember the shape; and a non-planetary gaseous toroid called the smoke ring in The Integral Trees.

It's not about engineering possibilites. Ringworld is a construct connected by chemical bonds. Natural planets are held together only by the force of gravity. You could certainly build cubic objects of various sizes, but they aren't going to appear naturally.

mscelina · Jul 7, 2010

Ack. You guys are too smart for me. Lemme get my husband to explain a couple of posts to me and I'll get back to you.

Speaking as an (apparently) uneducated on the possibilities of planet shapes kind of editor, if a cuboid planet showed up in a manuscript in my slush pile, the writer would have to work their butt off to get my disbelief to willingly suspend. Just my two cents.

thothguard51 · Jul 7, 2010

mscelina said:
Speaking as an (apparently) uneducated on the possibilities of planet shapes kind of editor, if a cuboid planet showed up in a manuscript in my slush pile, the writer would have to work their butt off to get my disbelief to willingly suspend. Just my two cents.

What she said....

Lhun · Jul 8, 2010

Liosse de Velishaf said:
It's not about engineering possibilites. Ringworld is a construct connected by chemical bonds. Natural planets are held together only by the force of gravity. You could certainly build cubic objects of various sizes, but they aren't going to appear naturally.

Not really chemical bonds, more like magic. And of course, given similarly magical materials, planets made from them wouldn't need to be spherical.

Liosse de Velishaf · Jul 8, 2010

Lhun said:
Not really chemical bonds, more like magic. And of course, given similarly magical materials, planets made from them wouldn't need to be spherical.

Haha, good point. But they still wouldn't be natural planets.

Bartholomew · Jul 10, 2010

If you had two gas giants with an identical, but life-sustaining gas on their outer layers orbiting eachother, could you have an irregular object more or less suspended between their gravity, but also caught within their atmospheres?

Asteroid monkeys!

Lhun · Jul 10, 2010

No. Gravity doesn't work like rubber bands, having something suspended between several attractors is not stable.
if it's actually inside an atmosphere even less, since you get friction.

Liosse de Velishaf · Jul 10, 2010

Inside the atmospheres sounds like a Roche limit issue...

Non-spherical planets

Thomas_Anderson

Xelebes

Delerium ex Ennui

benbradley

It's a doggy dog world

Sarpedon

Chris P

Likes metaphors mixed, not stirred

Albedo

Alex

scottVee

Lhun

New kid, be gentle!

ColoradoMom

C.bronco

I have plans...

lpetrich

geardrops

Good thing I like my day job

Pthom

Word butcher

pdknz

Having way too much fun

Liosse de Velishaf

mscelina

Teh doommobile, drivin' rite by you

thothguard51

A Gentleman of a refined age...

Lhun

New kid, be gentle!

Liosse de Velishaf

Bartholomew

Comic guy

Lhun

New kid, be gentle!

Liosse de Velishaf