Why are planets all spheres?

[seriously disabled]

Why are planets all oblate spheroids instead of say cubes?

 

Is it because a sphere is less ordered than a cube? But why is a sphere less ordered than a cube?

Edited December 16, 2011 by seriously disabled

[dimreepr]

Why are planets all oblate spheroids instead of say cubes?

 

Is it because a sphere is less ordered than a cube? But why is a sphere less ordered than a cube?

 

 

Its all in formation of planets. They are all either molten or gassious in formation, so when gravity does its job they are forced into a perfect circle erosion does the the rest. Our planet is only 3% off a perfect circle even now.

[zapatos]

By definition. If it wasn't nearly round it wouldn't be called a planet.

 

The IAU therefore resolves that planets and other bodies in our Solar System, except satellites, be defined into three distinct categories in the following way:

(1) A "planet"1 is a celestial body that: (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and © has cleared the neighbourhood around its orbit.

 

(2) A "dwarf planet" is a celestial body that: (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape2, © has not cleared the neighbourhood around its orbit, and (d) is not a satellite.

 

(3) All other objects3, except satellites, orbiting the Sun shall be referred to collectively as "Small Solar System Bodies".

 

Footnotes:

 

1 The eight planets are: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune.

2 An IAU process will be established to assign borderline objects into either "dwarf planet" and other categories.

3 These currently include most of the Solar System asteroids, most Trans-Neptunian Objects (TNOs), comets, and other small bodies.

 

http://en.wikipedia.org/wiki/Definition_of_planet

[swansont]

Spheres because gravity is a radially symmetric force and in the long-term large masses act like fluids. Oblate if/because they are spinning, for the same reason. The equilibrium surfaces will be equipotentials.

[morgsboi]

Basically, because as it is formed the gravity pulls in objects from all directions. Anyway not everything is a sphere. There is evidence of planets being compressed back and forth into an egg-like shape, like kneading dough.

[DrRocket]

Why are planets all oblate spheroids instead of say cubes?

 

Is it because a sphere is less ordered than a cube? But why is a sphere less ordered than a cube?

 

 

 

A sphere minimizes the potential energy of a body under gravitational forces. It is the shape of minimum surface area for a given volume.

[seriously disabled]

A sphere minimizes the potential energy of a body under gravitational forces. It is the shape of minimum surface area for a given volume.

 

But why does nature find it necessary to minimize potential energy? I mean what law of classical mechanics explains this?

 

I found this link but it is not completely helpful:

 

http://www.physicscentral.com/experiment/askaphysicist/physics-answer.cfm?uid=20080512095443

Edited December 17, 2011 by seriously disabled

[swansont]

But why does nature find it necessary to minimize potential energy? I mean what law of classical mechanics explains this?

 

I found this link but it is not completely helpful:

 

http://www.physicscentral.com/experiment/askaphysicist/physics-answer.cfm?uid=20080512095443

 

If there is a difference (gradient) in the potential energy there is a force.

 

[math]F=-\nabla{U}[/math]

 

So the tendency is to be a sphere until the gravitational and internal forces are of comparable size.

[DrRocket]

If there is a difference (gradient) in the potential energy there is a force.

 

[math]F=-\nabla{U}[/math]

 

So the tendency is to be a sphere until the gravitational and internal forces are of comparable size.

 

One tends to fall downhill rather than uphill.

[morgsboi]

Anyway not everything is a sphere. There is evidence of planets being compressed back and forth into an egg-like shape, like kneading dough.

 

Actually, come to think of it those are moons, sorry. Such as Europa. Have a look at this: http://en.wikipedia.org/wiki/Europa_%28moon%29

[MigL]

I believe the principle Seriously Disabled is referring to is the principle of least action developed by Fermat and deMauripitius ( spelling ?? ) and later by LaGrange and Hamilton. The action is basically the sum of the differences between kinetic and potential energy ( Ek-Ep ) and the action is minimised when potential is reduced by increasing kinetic energy. Or as previously stated, things fall down instead of up.

Edited December 22, 2011 by MigL

[JohnStu]

Earth is not exactly a sphere. It is wider than its height.

[D H]

This thread is starting to verge into wronger than wrong territory. In the words of Isaac Asimov,

“

When people thought the earth was flat, they were wrong. When people thought the earth was spherical, they were wrong. But if you think that thinking the earth is spherical is just as wrong as thinking the earth is flat, then your view is wronger than both of them put together.

So as to avoid being "wronger than wrong" myself, I'll first say that what Dr Rocket said in post #6 is basically correct:

 

 

A sphere minimizes the potential energy of a body under gravitational forces. It is the shape of minimum surface area for a given volume.

A slight refinement of this statement is needed to explain the equatorial bulges of the planets: An oblate spheroid minimizes the potential energy of a rotating, self-gravitating body.

 

This explains the slight flattening of the Earth (flattening = 1/298.257) and the more extreme flattening of Saturn (1/10.2). The deviations from this oblate spheroid shape are quite small, even for Mars. Of the eight planets, it is Mars that is most "out of round" (i.e., exhibits the greatest deviation from an oblate spheroid shape). Something, or some things, quite powerful happened to Mars in the distant past that created features such as the Borealis basin, the Tharsis bulge, and the Hellas Planitia. But even Mars is more or less spheroidal.

[khaled]

Earth is not a perfect sphere, but rather a spheroid .. where lands at northern & southern pole are semi-flat

[morgsboi]

Earth is not a perfect sphere, but rather a spheroid .. where lands at northern & southern pole are semi-flat

Yeah, the name for it is an oblate spheroid.

 

Europa shape changes shape changes due to the gravity of Jupiter.

http://en.wikipedia.org/wiki/Europa_(moon)

[D H]

Earth is not a perfect sphere, but rather a spheroid .. where lands at northern & southern pole are semi-flat

Yeah, the name for it is an oblate spheroid.

I already mentioned this in post #13.

 

 

 

Europa shape changes shape changes due to the gravity of Jupiter.

By a small amount. The tidal flex is somewhere between 1 meter and 30 meters. The smaller value results if Europa is completely solid (solid ice over a solid core); the larger if Europa has a significant liquid ocean under its ice surface. Note that the Earth, too, changes shape due to the gravity of the Moon and the Sun. There are tides in both the oceans and in the Earth itself.

 

However, Europa is still quite "round", as is the Earth. The Earth's equatorial bulge, the Earth tides, and the tidal flexing of Europa are tiny compared to the overall size of the Earth and Europa. Saying that these features mean that the Earth (or Europa) isn't "round" is, in the words of Isaac Asimov, "wronger than wrong."

[morgsboi]

I already mentioned this in post #13.

 

 

 

 

By a small amount. The tidal flex is somewhere between 1 meter and 30 meters. The smaller value results if Europa is completely solid (solid ice over a solid core); the larger if Europa has a significant liquid ocean under its ice surface. Note that the Earth, too, changes shape due to the gravity of the Moon and the Sun. There are tides in both the oceans and in the Earth itself.

 

However, Europa is still quite "round", as is the Earth. The Earth's equatorial bulge, the Earth tides, and the tidal flexing of Europa are tiny compared to the overall size of the Earth and Europa. Saying that these features mean that the Earth (or Europa) isn't "round" is, in the words of Isaac Asimov, "wronger than wrong."

 

I know the details but it is compressed enough for there to be water under the surface due to the friction......... or so scientists say.