Why are all the planets round; why aren't there any square or oddly shaped one?

Question

If it's because they all rotate, so the presure from the ratation smoothens the surface, then wouldn't this planet some day become a pebble unless the rotation stops?

Answer 1

There are a lot of body's in space that are oddly shaped
there just small.
Quote
[If it's because they all rotate, so the pressure from the rotation smoothens the surface, then wouldn't this planet some day become a pebble unless the rotation stops?]
That's not how it works
This is how it works
When matter in space clumps together its irregularly shaped.
When enough matter clumps together gravity begins to pull equally towards the center of mass of the clump.
As the gravitational pull towards the center increases,
"because of additional matter being pulled to the clump"
The surface will smooth out.
The clump is also getting bigger not smaller.
Over time the clump can become a planet if enough matter is present near by.
The rotation cant take matter away from the planet.
The earth has mountains that reach a level of 6miles above sea level.
But in the case of a neutron star which may be smaller but millions of times more massive, i.e. more dense,more material
Its "mountains" may only be able to reach a height of 2 or 3 inches.
Mere ripples on the surface of the star.
This because the neutrons stars gravitational pull is millions of times stronger then the earths gravitational pull at the surface.
There doesn't have to be any rotation at all
its this pull towards the center of the body that smooths out the surface which forms a sphere, aka "round planet".
If the surface rotation is fast enough it can cause a bulge around the equator that would cause the sphere to flatten like a pancake.
Galaxies do this over time
hopes this answers your question

Answer 2

Why don't you ask why raindrops are round?

Although it is a different force with raindrops (surface tension of water), it is simply that if a force is acting evenly in a piece of matter, it must make the most effecient shape - a sphere has every part of its surface the same distance from the center.

A cube's corners are further from the center of the cube, aren't they? That is not a state of equilibrium for an internal force trying to draw the matter together.

In space, with no interfering forces, a body will to assume the perfect sphere. That is never entirely achieved because of the spin of the body trying to throw out the material at the fast moving equator, and often some gravitational interaction from nearby bodies - the Earth is distorted by the moon and vice versa.

Answer 3

It has nothing to do with rotation/centrifugal force. It has everything to do with gravity. In essence, planets are round for the same reason why rocks roll downhill.

Small moons and asteroids can be very irregularly shaped, since their gravity isn't strong enough to pull their mass into a spherical shape.

Answer 4

it depends on how it was created.
planets usually have enough mass, which on formation piled up on each other, which is basically enough to create a round object, due to gravity.

Now rotation make things somewhat different.
actually planets aren't perfectly round.
Due to rotation (getting that intertial momentum on formation) planets are usually flattend.
Earth for example is not perfectly round, its diameter from pole to pole is smaller than the diameter measured from one side of the equator to the other.

As long there is rotation and the core is not cooled down to a solid this flattened shape is how a planet looks like.
just if a planet would not rotate and its core would cool down later it would be nearly round.

Answer 5

Hi. Gravity pulls any high points (corners) toward the center.