why are planets all round

[pretender]

Why is this so?????

[Sisyphus]

Do you mean why are they everywhere? Or why are they circular?

[ydoaPs]

it's part of the definition of a planet.

[pretender]

the reason they are all circular...

[Klaynos]

Due to the spherical symetry of gravity.

 

This has already been discussed on the forums:

 

http://www.scienceforums.net/forums/showthread.php?t=14902

 

[noz92]

Most planets begin as a bunch of asteroids that collide and form molten material that is held together by gravity. Because gravity pulls in every direction, the molten planet just forms in a sphere around its center of gravity.

 

At least that's my guess.

[pretender]

Thanks for all the replies...

[devilboy8243]

so how does it make a sphere? the mass is not all the same so it will pull some areas out fartehr than other. your theory is not all there. you have to help me believe that or that you have explain yourself more so i can understand where you are coming from

[noz92]

The most massive parts would become the center of gravity. The rest would flow around the massive parts and the most dense parts would sink to the center while the least dense parts would stay at the top. The pressure from the rest of the planet holds the center in the center and keeps the most massive part of the planet in the center.

 

Sorry if I didn't explain my theory well enough.

[sunspot]

Actually the earth is not spherical. It is wider at the equator than from pole to pole. Using existing theory the rotation of the earth may have something to do with the bulge at the equator. Maybe the magnetic field has something to do with the contraction from pole to pole.

 

One observation by NASA determined that seismic wave travel faster N-S than E-W, but at a greater margin that is expected by the distance differences. This data implies the earth is denser N-S than E-W. This interior density difference may imply the earth is bulged at the equator due to its lower mass density in the E-W direction.

[Klaynos]

Actually the earth is not spherical. It is wider at the equator than from pole to pole. Using existing theory the rotation of the earth may have something to do with the bulge at the equator. Maybe the magnetic field has something to do with the contraction from pole to pole.

 

One observation by NASA determined that seismic wave travel faster N-S than E-W' date=' but at a greater margin that is expected by the distance differences. This data implies the earth is denser N-S than E-W. This interior density difference may imply the earth is bulged at the equator due to its lower mass density in the E-W direction.[/quote']

 

Please see my above posted link, which comments on the bulge and suggests by someone I tend to belive that it's accounted for by rotation...

[Sisyphus]

The planets are sherical because any other shape would collapse under its own weight once a certain threshhold of mass is reached. A sphere, however, is fully collapsed, with no part of it able to get closer to the center of gravity without pushing something else farther away. Hence, gravity tries to form every object into a sphere. For the same reason, the bulk of the denser materials is always in the center of the larger sphere, because it is pulled with greater force towards the center than the less dense materials, which it naturally displaces.

 

EDIT: The earth bulges slightly at the equator because of the inertia of rotation, not the magnetic field, which is far, far, too weak.

[Edtharan]

Saturn is another planet that bulges at its equator. It is spinning faster than earth. This produces quite a noticeable bulge.

[softdragonz]

Of all shapes, sphere is shape that has the least Potential Energy

 

Reason: Sphere has the largest surface area for a given volume

 

And it is tendancy of a body to attain the least possible Potential Energy State ( That is why you are attracted to earth ... mgh )

 

So, ultimately the shape of planets come about to be spherical....

[softdragonz]

The earth bulges slightly at the equator because of the inertia of rotation, not the magnetic field, which is far, far, too weak.

 

Why should earth face inertia of rotation? I mean inertia comes into picture only when there is some opposing force right? What opposes the rotation of earth .... ( Is is sun's gravity?? Just a guess )

[sabbath]

Is it safe to conclude then that all planets are spherical or oblate spheroid because that is the most stable shape/form that they can assume?

[lethalfang]

Why should earth face inertia of rotation? I mean inertia comes into picture only when there is some opposing force right? What opposes the rotation of earth .... ( Is is sun's gravity?? Just a guess )

 

The opposing force comes from rotation.

The inertia comes from the fact that a instantaneous velocity vector points outward for a rotation object.

[swansont]

The opposing force comes from rotation.

The inertia comes from the fact that a instantaneous velocity vector points outward for a rotation object.

 

Surely you mean tangentially (or perhaps I am misunderstanding your point)

[Sisyphus]

Why should earth face inertia of rotation? I mean inertia comes into picture only when there is some opposing force right? What opposes the rotation of earth .... ( Is is sun's gravity?? Just a guess )

 

It's the rotation that opposes gravity. Or rather, the inertia of each part of the Earth. If gravity "turned off" suddenly, the Earth would fly apart tangentially, with each individual part of the Earth continuuing in a straight line.

 

I do appreciate the rolleyes though.

[[Tycho?]]

It's the rotation that opposes gravity. Or rather' date=' the inertia of each part of the Earth. If gravity "turned off" suddenly, the Earth would fly apart tangentially, with each individual part of the Earth continuuing in a straight line.

 

I do appreciate the rolleyes though. [/quote']

 

Wait, would the earth actually fly appart without gravity? I would think that it would hold together at its current rotation speed.

[Klaynos]

There would be nothing holding it in rotation...

[[Tycho?]]

There would be nothing holding it in rotation...

 

Something like rock is not held together by gravity, its held together... because its rock, chemical bonds and stuff. Is the earth's rotation actually able to overcome the forces holding the crust in place?

[insane_alien]

yes but there are massive pressures inside the earth. suddenly theres going to be nothing holding them in. and what happens with high pressures, thats right an explosion(not necessarily a quick and dramatic bomb style one but i couldn't think of a better word.

[Royston]

Wouldn't it start with all the oceans and sedimentary layers floating off into space...that'd look pretty neat, if that's correct.

 

EDIT: oh, the centre would expand as well as the pressure is released...still, would be very impressive to watch.

[Sisyphus]

The bulk of the earth is molten rock, and so would have very weak cohesion. The core is solid, but only from extreme pressure. Take away gravity, and you take away the force holding it together, and all you're left with is something very very hot and very very compressed, which would explode with unfathomable force and vaporize itself from the heat and sudden absence of pressure.