Why are planets spherical?

[moonunit100]

I was wondering why planets are spherical, or at least very nearly so. Even most moons seem to be round, with some exceptions such as Saturn's moon Epimetheus. Epimetheus is an odd shape probably due to the intense cratering on it's surface, so why aren't other moons/planets a similar shape.

 I can understand how a planet such as earth may have avoided getting hit by lots of meteors by luck and because it's atmosphere protects it, but what about other planets without such an atmosphere?

[bernardo]

Gravity.

The bigger a planet (or moon) gets, the stronger its gravity is.  Therefore mountains and other high land get squashed under their own weight, and crumble down to a certain limit in height.  Similarly, depressions and craters get filled in.  Imagine what would happen if the planet was made of treacle: high lumpy bits would gradually collapse and fill the dimply bits.

On Earth, the highest mountain is 5 miles high, and the deepest depression is 7 miles down.  Mount Everest could not have been 10 miles high because if it were, it would have collapsed anyway.  On Mars, the highest mountain is much bigger than Everest, because the gravity is weaker.  Smaller moons can be lumpy because their gravity is much weaker.

[Gef]

Sorry, but I don't agree with bernardo. The fact that Mars has higher mountains than Earth has nothing to do with gravity but more to do with the erosive forces of the Earth's atmosphere. At some point during their formation the planets were balls of gas (indeed the bigger palnets still are) and if you spin a ball of gas then it becomes almost spherical. Gravity has a part to play in pulling the material together in the first place and it is the reason why Earth is slightly 'flatenned' at it's poles.

[Clanad]

Ummm... As the Earth rotates, points on the equator are traveling at roughly 1000 miles per hour to complete one revolution in a day, while points near the poles are barely moving at all. This creates a tremendous amount of centrifugal force (the same apparent force that tries to pull you up out of your seat when a roller coaster goes over the top of a hill and starts back down the other side) along the equator and very little near the poles. The net result is to stretch the Earth out along the equator and leave it relatively unchanged or flattened near the poles.
And... The weight of rock causes enormous pressure deep below a mountain. This can cause the rocks to flow, which will lead to the mountain "sagging". Because the Earth is more massive than mars surface gravity is higher on Earth. This means mountains on Earth will get to this critical pressure before those on mars. Hence we expect to see smaller mountains on earth than on Mars. So... have to agree with the idupitable bernardo...

[brachiopod]

My God, bernardo, Gef and Clanad all on one post. Ok, deep breath and here goes.....

Not criticism, merely clarification.....

I think we can all agree that gravity alone will give any accreted fluid mass (ie. gas or liquid {or even a solid/liquid mixture}) a spherical shape, since it acts equally in all directions.
Even the oblate spheroid of a rotaing body is, to all intents and purposes, spherical.
The extremes of altitudinal height and bathymetric depth are not exactly discernable from space either
These are merely surface or near surface processes that, although very significant, don't really affect the 'spherical index' of the Earth.

I would love to develop the treacle analogy, and how lithostatic 'sagging' in the absence of orogenic forces has a far greater effect than any weathering / erosive force in reducing the height of the Earth's highest mountain ranges but tht's largely irrelevant to the original question.

As for answering moonunit's question, it is the smaller satellites and asteroids (with respect to even your smallest rocky planet) that have the irregular shapes, as they lack the fluidity of larger planet's cores. By their definition, they will not have formed directly from your average gaseous agglomeration, but will have formed later from bits that have been knocked off the main molten planetal mass (such as the Earth's moon) or even much later, and be the result of a collision between solid bodies. As such, th

[brachiopod]

What happened there? Half my answer has disapperaed.
I'm b�ggered if I can be bothered to type it out agin, but I hope you get the gist of it.

[moonunit100]

Thanks guys, excellent stuff. But I'm still not sure why the planets are spherical (as near as dammit) and why some don't have a flat edge for instance.

Brachiopod starts to talk about the planets as NOT being formed by bits being knocked off main planetal masses, but I thought Pluto was just that.

I'd love to hear some more information on this subject as it's all so interesting. Keep it coming everyone!

[-=BigD=-]

planets are spherical because they'd look silly if they we're cubes.

Thats what my ever informative physics teacher told me when i asked the same question back at school. I think he may have been joking.

[Gef]

Good answer brachi - time for another question and another drink :-)

[moonunit100]

lol BigD!

Nice one, everyone needs a teacher like that. :)

[conewood]

Small amounts of material is held in shapes by electrostatic forces. This is true of small bits of rock, your body, a cup - anything. When you add more and more material to your starting lump of rock in planet formation, the electrostatic forces holding lumpy bits in their shape does not change. However, as you add more material, the gravitational attraction between the material increases in proportion. Once you reach a level where the gravitational force exceeds the electrostatic forces, it will become a basic sphere. Because some electrostatic forces are greater for some materials and less for others, you will be able to have big rock mountains defying the gravity to some extent - a few kilometers high on Earth - but not to a massive extent. The strength to hold up the corners of a cube shape would massively exceed the strength of natural rocks, so all planetary bodies over a certain size (well, mass actually) would be pretty well spherical. Vesta, now being orbited and photographed is on the borderline so it is almost a sphere but a bit 'lumpy'. Ceres is more circular. Smaller asteroids are more varied in shape.

[darylimjz]

I think, among the long explanations here (not that they're bad), I find one particular point most appealing and answers the question with straightforwardness. Brachiopod said : "... gravity... acts equally in all directions." Therefore all and any substance, with a pulling force within, forms a spherical force. (Of course assuming the force acts equally in all directions as mentioned, in which case gravity does.) If it still doesn't answer you, think radius of circle.