why dont planets form other shapes?

[MrDudeMan]

my girlfriend got her wisdom teeth out today, so ive been doing a whole lot of sitting. while gazing at the imperfections in the ceiling paint, i wondered why earth formed in an "exact" sphere. i know it is really an oblate spheroid, but at a glance that appears to be almost a sphere.

why didnt it make something crazy? or a cube? or a cube-like structure? basically why cant it form something besides a spheroid since every planet looks like that?

i understand planets that are made of mostly gas will form a sphere since gravity will then take effect and spread it evenly around its gravitational field. but why does that also happen for planets made of solids? i know the liquids would try to be uniformaly dispersed, so im speaking strictly of solids. and no, i dont mean like a mountain...i mean the whole planet looks like a crazy shape.

sorry, i know this reads like a big rambled post full of poo, but i dont have much to do today and im going crazy.

 

[Avilion]

My guess would be that solid planets originally started out as fairly gaseous or molten rock, something malleable, so it was probably formed into a sphere before it was in a solid state.

 

[Matthias99]

i understand planets that are made of mostly gas will form a sphere since gravity will then take effect and spread it evenly around its gravitational field. but why does that also happen for planets made of solids? i know the liquids would try to be uniformaly dispersed, so im speaking strictly of solids. and no, i dont mean like a mountain...i mean the whole planet looks like a crazy shape.

A few points: First, the Earth is not exactly "solid". There's quite a bit of molten rock in there, which behaves more like a very slow-moving liquid. Second, even "solid" materials can flow and be reformed with enough pressure and time (ie, planetary geology timescales, pressure of continental plates -- see, e.g., the Rocky Mountains, or the Grand Canyon).

When you start talking on the scale of something the size of a planet (which generates a substantial gravitational field), over billions of years, even "solids" (unless they are extremely strong) will tend to settle into the shape that has the lowest potential energy -- a sphere. The only reason the Earth isn't even *more* spherical is that it's spinning (and possibly because of the gravitational pull of the moon and sun, to some extent).

Finally, the surface of the earth is hardly flat. There are altitude differences of several tens of thousands of feet above sea level, and if you drained the oceans, the difference between the lowest oceanic trenches and the highest mountains would start to be a nontrivial percentage of the diameter of the Earth. But basically, planets are (mostly) spherical because gravity tends to pull clumps of matter into a spherical shape over time.

 

[filterxg]

It has to do with gravity being applied evenly anywhere within equal distance of a mass (read : Point).

 

[kongs]

Gravity - in a nut shell.

 

[Calin]

The deepest point on Earth is a "hole" in the ocean about 11 km deep, and its highest point is at some 9km high. This is 20 km of vertical variance for a radius of 6400 km - or 0.3%.
The reason for the mostly round form is the gravity - if you take into consideration the cubical shape, imagine a round Earth having on it 8 pyramids that are 3000km high. I really don't think rocks normally found in planets are capable to withstand this kind of effort, so I think they will fracture and massive slides (earth slides, rock slides) will form.

 

[halfpower]

If a planet as large as the earth happened to form in a cube shape, it would, under the force of gravity, immediately crumble,or sink, into a sphere.

 

[Nithin]

also its not a perfect sphere due to rotation. few more pounds at the waist

 

[Battlewaffle]

because with physics, the sphere is the strongest shape with universally equal strength at every point.

For instance, if it were a square, we'd see radically different forces at the corners than on the sides.

A drop of water forms into a circle as it falls because that is the most naturally strong/supported shape in nature. It's equally supported at all infinite angles

Another reason, is gravity. As the planet formed, it's most central point of gravity.....everything is drawn to that point and so it naturally forms a circle because every point is trying to get as close as it can to the center. This naturally forms a sphere.

And then yes, you add in centrifical (spelling is off on that probably) force.

 

[silverpig]

Planets form from the proto-planetary disk of dust and gas surrounding a newborn star. Basically, the earth was a whole ton of little bits a long time ago that came together. It'd have to be pretty contrived to end up with a cubical shaped arrangement, and even then it wouldn't last long.

 

[MrDudeMan]

Originally posted by: Nithin
also its not a perfect sphere due to rotation. few more pounds at the waist

i said that in my OP. its a oblate spheroid.

 

[MrDudeMan]

Originally posted by: Matthias99

i understand planets that are made of mostly gas will form a sphere since gravity will then take effect and spread it evenly around its gravitational field. but why does that also happen for planets made of solids? i know the liquids would try to be uniformaly dispersed, so im speaking strictly of solids. and no, i dont mean like a mountain...i mean the whole planet looks like a crazy shape.

A few points: First, the Earth is not exactly "solid". There's quite a bit of molten rock in there, which behaves more like a very slow-moving liquid. Second, even "solid" materials can flow and be reformed with enough pressure and time (ie, planetary geology timescales, pressure of continental plates -- see, e.g., the Rocky Mountains, or the Grand Canyon).

When you start talking on the scale of something the size of a planet (which generates a substantial gravitational field), over billions of years, even "solids" (unless they are extremely strong) will tend to settle into the shape that has the lowest potential energy -- a sphere. The only reason the Earth isn't even *more* spherical is that it's spinning (and possibly because of the gravitational pull of the moon and sun, to some extent).

Finally, the surface of the earth is hardly flat. There are altitude differences of several tens of thousands of feet above sea level, and if you drained the oceans, the difference between the lowest oceanic trenches and the highest mountains would start to be a nontrivial percentage of the diameter of the Earth. But basically, planets are (mostly) spherical because gravity tends to pull clumps of matter into a spherical shape over time.

not true.

The deepest point on Earth is a "hole" in the ocean about 11 km deep, and its highest point is at some 9km high. This is 20 km of vertical variance for a radius of 6400 km - or 0.3%.

Calin beat me to that...

 

[Matthias99]

Originally posted by: Bigsm00th

Originally posted by: Matthias99

i understand planets that are made of mostly gas will form a sphere since gravity will then take effect and spread it evenly around its gravitational field. but why does that also happen for planets made of solids? i know the liquids would try to be uniformaly dispersed, so im speaking strictly of solids. and no, i dont mean like a mountain...i mean the whole planet looks like a crazy shape.

A few points: First, the Earth is not exactly "solid". There's quite a bit of molten rock in there, which behaves more like a very slow-moving liquid. Second, even "solid" materials can flow and be reformed with enough pressure and time (ie, planetary geology timescales, pressure of continental plates -- see, e.g., the Rocky Mountains, or the Grand Canyon).

When you start talking on the scale of something the size of a planet (which generates a substantial gravitational field), over billions of years, even "solids" (unless they are extremely strong) will tend to settle into the shape that has the lowest potential energy -- a sphere. The only reason the Earth isn't even *more* spherical is that it's spinning (and possibly because of the gravitational pull of the moon and sun, to some extent).

Finally, the surface of the earth is hardly flat. There are altitude differences of several tens of thousands of feet above sea level, and if you drained the oceans, the difference between the lowest oceanic trenches and the highest mountains would start to be a nontrivial percentage of the diameter of the Earth. But basically, planets are (mostly) spherical because gravity tends to pull clumps of matter into a spherical shape over time.

not true.

The deepest point on Earth is a "hole" in the ocean about 11 km deep, and its highest point is at some 9km high. This is 20 km of vertical variance for a radius of 6400 km - or 0.3%.

Calin beat me to that...

I guess it depends on your definition of "nontrivial".

 

[MrDudeMan]

Originally posted by: Matthias99

I guess it depends on your definition of "nontrivial".

i was thinking .1-.5% is trivial

thanks for your reply btw. it was informative.

 

[gbuskirk]

Why isn't a balloon square? When you inflate it, the tension in the latex equalizes over its surface. Inequalities in tension produce movement/stretching until equilibrium is reached. Likewise, a distribution of matter in a shape such as a cube produces imbalances in gravitational forces, which over time tend to redistribute the matter toward a lower energy state. Non-spherical shapes may also be in equilibrium, but have a higher energy state. The increase of entropy will result in a movement to a lower energy state.

 

[MrDudeMan]

Originally posted by: gbuskirk
Why isn't a balloon square? When you inflate it, the tension in the latex equalizes over its surface. Inequalities in tension produce movement/stretching until equilibrium is reached. Likewise, a distribution of matter in a shape such as a cube produces imbalances in gravitational forces, which over time tend to redistribute the matter toward a lower energy state. Non-spherical shapes may also be in equilibrium, but have a higher energy state. The increase of entropy will result in a movement to a lower energy state.

excellent. thank you.

 

[Chode Messiah]

the higher the gravitational pull,the more objects tend to form into spheres. Low Gravity(the moons of mars) are uneven and misshappened, High gravity(neutron stars) are almost perfect spheres.

 

[kumanchu]

Originally posted by: Chode Messiah
the higher the gravitational pull,the more objects tend to form into spheres. Low Gravity(the moons of mars) are uneven and misshappened, High gravity(neutron stars) are almost perfect spheres.

it isn't the "gravitational pull" that shapes matter into spheres, rather it is the mass itself and the gravitational field that mass creates. in theory, those "low gravity" or objects of lower mass will still, due to flow, form spheres. however, the time it would take to observe something like this is absurd (relative to a human lifespan) and other environmental factors such as debris, and gravitational fields of other nearby matter will also affect the shape.

when you talk about neuron stars, the gravitational field is greater due to the increased density of mass. the density of mass creates a gravitational field that will increase the flow rate of the matter.

in anycase, the reason why things tend to form spheres (according to modern physics) is directly related (as pointed out by many) to entropy.

*edit for misplaced word

 

[Calin]

Now, let's imagine a planet made of iron fillings, having at the center a huge magnet. This hypothetical planet would have two caps over its poles (magnetic poles), and little iron fillings in between. It might resemble a dumbbell more than anything else, and its shape would be so that is has the lowest potential energy.

 

[Mday]

gravity, symmetry, and surface tension. the spinning makes it oblong.

 

[jattardi]

As other posters have said, the spherical shape is due to the force of Earth's own gravity "crushing" it into such a shape. However, as also has been noted, there is a slight bulge at the equator. This is because the Earth's fast rotation causes the slight bulge.

 

[Cruise51]

1-not a perfect sphere
2-not solid
3-gravity
4-centrifical force

 

[trevinom]

Originally posted by: gbuskirk
Why isn't a balloon square? When you inflate it, the tension in the latex equalizes over its surface. Inequalities in tension produce movement/stretching until equilibrium is reached. Likewise, a distribution of matter in a shape such as a cube produces imbalances in gravitational forces, which over time tend to redistribute the matter toward a lower energy state. Non-spherical shapes may also be in equilibrium, but have a higher energy state. The increase of entropy will result in a movement to a lower energy state.

This is like comparing apples and oranges.
When you're talking about planitary gravity, it's the inward force that is pulling everything to a central point. In a ballon, you're talking about equal distribution of air molecules under pressure trying to find equilibirium. The two have nothing to do with each other, IMHO. (How about the elongated balloons, the ones they use to make shapes,they aren't round)


And for the record, there is no such thing as centrifical force,this is an eggcorn: a spell-as-you-speak error. I think what you were trying to say is centrifugal force and technically, The correct physics term is Centripetal Force. Centrifugal force is what makes an object released from a spinning platform be perceived as moving out in a straight line away from the platform as it continues spinning. I think coriolis forces are also used to explain this perceived motion.