I have always wondered why planets and moons are always in the shape of a sphere. Why are there any square or triangle planets? At the very least, why are they not uneven?
Most planets ARE uneven. The earth is not a perfect sphere: it is “flat” at the poles.
Planets are generally spherical or, rather, eliptical because they spin. When a huge chunk of non-solid matter (gas, magma, whatever) turns around repeatedly, it is very unlikely to form a triangular shape.
Welcome to the SDMB!
The answer is gravity. Remember that the amount of gravity is based on the amount of mass an object has. Large planets and moons (say, over 1,000 km in diameter) have enough gravity to flatten out any outstanding features. The net effect of a planet’s gravity is to pull toward its center. Mountains collapse under their own weight, debris from erosion rolls downhill, etc.
Smaller moons/asteroids may not have enough gravity to do this…and we can see many strange shapes. Many are irregular like a potato and there is even one example of a dog-bone shaped asteroid. http://antwrp.gsfc.nasa.gov/apod/ap000510.html
That’s the quick & dirty answer.
You can get more detailed explanations about slight variations in spherical shapes…like mountain build-up (plate techtonics) and erosion, slight flattening out along the equator due to rotation, etc. There is also more detail to be found in regards to the formation of planets, moons, etc.
Small things can have any shape because its gravity is much weaker than the internal forces that bind the rock together. That is, a small rock is rigid enough to withstand its own gravity.
But what happens if you have a rock hundreds or thousands of miles long? It has a huge amount of gravity. But the binding force doesn’t get any stronger - it’s no more rigid than that little piece of stone in your backyard. So eventually the bit that is farthest from the center of the big rock will crumble and fall towards the center of the rock. After a while, you end up with a rock with no parts sticking out very far. A sphere, in other words.
Just think about this: if you add a thousand-mile high mountain to Earth to make it non-spherical, what happens? The mountain would just crumble down.
Darn! I came across a web page that supposed a cube-shaped planet. It basically calculated how quickly the forces of gravity would pull the eight big mountains and 12 ridges into the six lowlands.
Although the Earth isn’t a perfect sphere (because of general bulging or surface irregularities), it is rounder than a highly-polished cue ball.
The process of gravity smoothing out any corners a protoplanet may have is also assisted by the fact that most planets form through multiple impacts of large objects. This has the effect of releasing lots of heat, so the rock the planet is made of melts. It is hard to have a liquid planet with any other shape but a smooth sphere (or at least, spheroid). As the crust forms over millions of years of radiative cooling, the planet remains rounded.
This should also answer why the non-rocky planets, called gas giants, are also round.
Well, It would make sailing uphill a bitch besides God hates juggling square objects 
I like Saltire’s answer. The planets were essential liquid or dense gas when forming. While it appears they orbit, they are really falling under a gravitational force (sun). Just like rain drops falling in a vacuum on Earth, they would be spherical due to their surface tension. Thus planets were falling in a vacuum, with surface tension, when born, thus they are spherical.
No it’s not surface tension! I’m sure Saltire didn’t mean that too. Surface tension is not significant on such a large scale. Gravity is.
Scr4 … When starting out as a small planetoid, gravitational is not important, i.e if the new gases planet is say 1 mile in diameter, the main force is surface tension. Certainly you are not going to associate its spherical shape due to internal gravity forces at that point are you? From that point forward the planetiod will maintain it’s spherical shape, internal gravity or not. The smallest of the moons circling other planets where internal gravitational forces are is essential zero, are still spherical!
DeutschFox
Small plantoids are not spherical. Have you seen an asteroid? Asteroids are plantesimals that never formed into a planet because of the disrupting effects of Jupiter’s gravity. They have all kinds of strange shapes, most of which are not the least bit spherical.
Minor quibble: They do orbit. An orbit is just a special case of falling.
“Small plantoids are not spherical. Have you seen an asteroid?” Correct, but they never reached a liquid or gaseous state, they are just fragments … if they had reached either state they would have be spherical.
Re: Minor Quibble
“There are only two things that are infinite: The Universe, and human stupidity-- and I’m not sure about the Universe.”
–A. Einstein
Very appropriate … thanks … a planetary orbit is the falling of an object under the influence of gravity … it just happens to have a tangential velocity … Keppler 101
The point is, there are no planetoids that became spherical by surface tension. There are no spherical 1-mile asteroids that we have seen. And if you carefully melted a 1-mile asteroid and it got spherical, I’d think that gravity would contribute more than surface tension. They are both very very weak, however, and a slight external perturbation would prevent it from becoming spherical.
“And if you carefully melted a 1-mile asteroid and it got spherical, I’d think that gravity would contribute more than surface tension.”
If an ounce of a liquid substance such as mercury, but with an infinitely low vapor pressure, could be in “orbit” around the sun, what shape would it have? …