Why are planets round?

Answer 1

Forget all the answers above that do not mention gravity. people make things up.

A sphere is merely a body where every part of the surface is equi-distant from the center. Every piece of matter in the universe will try to form a sphere, but with solid masses, it requires a body of a certain size before the weak force of gravity can pull it all into a sphere (don't get confused about raindrops, it is surface tension that attempts to make fluid drops into spheres)

Have a look at pictures of asteroids. In general, they are too small for their gravity to exert enough force to squeeze them into a sphere, so they are all sorts of shapes, often like giant potatoes.

Generally, above about 1000 km wide (need to check that), there is enough gravity for solid matter to form a sphere.

There are further consequences of gravity the bigger your blob gets. About 10 times bigger than Jupiter, and gravity is enough to squeeze the center of the object so much that its atoms collapse and nuclear reactions are fired up - it becomes a star, and will begin to shine with its own light.

So, at the end of the day, all astro bodies are much the same, from the tiniest meteoroid to the largest star. Their difference, apart from size, is the form that they take due to their mass, and hence the effect of their own gravity on them.

Answer 2

Spherically distributing the matter in a large body (or ellipsoidally for spinning bodies) results in the smallest gravitational potential energy. That is, any other distribution of matter would have a higher gravitational potential energy and would spontaneously rearrange itself (with a release of energy) until it became a sphere.

This rearrangement depends on the matter's ability to move around within the body. A highly rigid body would not become a sphere until its mass was very large. However, planetary sized bodies made of rocks/ice end up being spherical for this reason.

Answer 3

Gravity pulls the material together. A sphere has the least possible surface area for a given volume, and is thus the most compact.

Mountains and valleys are minute features, relative to the size of the earth. If you could shrink the earth to the size of a standard classroom globe, it would be the smoothest globe in existence.

Answer 4

Gravity. The center of a planet pulls every outer part of the planet in with the same strength, so you have an even surface.

Answer 5

When a planet is formed gravity is pulling all particles together with the same force in all directions, similar to how surface tension of water forms a perfect sphere in rain drops.

Answer 6

Mass. If the body has sufficient mass, in time It will evolve into a sphere. Many body's of low mass remain irregular, for example Phoebe, moon of Saturn and my favourite friends character by the way. Probably a kuiper belt object, but is and never will be round. Insufficient mass.

Answer 7

centrifugal force and surface tension. because the earth spins the way it does, it spreads out in the roundish shape that it is. but the earth isn't perfectly round, it's kinda doughnut shaped, it's wider than it is tall.

the thing with surface tension is the natural stickyness that all elements have, like water. if you've ever seen water as it's mid-air, it's round. that's because the molecules are sticking together. same logic goes to liquid rock, such as magma.

Answer 8

Coz Everyone Likes a Ball Game Including God

Answer 9

Gravity pulls at the planet from all angles creating a sphere.

But the planet isnt actually a sphere or we wouldnt have mountains or trenches.

Answer 10

Matter always tries to be as stress-less as possible. Spherical shapes are the most stress-free.

Any drop is a sphere it is the other influences like drag that give it the drop-shape we all know.

Soap bubbles are also a good example. They will always be a spherical as circumstances will let them be.