The planets are round bodies. In fact, even the larger asteroids are rounded. But if we find asteroids smaller in diameter than about 500 kilometers, then those bodies are usually irregular in size.
(Now I want you to tell me how many miles 500 kilometers corresponds to.)

If Earth, moon, and other planets are almost exactly spherical, why are most asteroids highly irregular in shape? The asteroids are in a sense large chunks of rock; obviously rocks can take on any shape. But we notice that the larger bodies, of diameter greater than about 500 kilometers, are spherical or rounded.
Why?

It is a question of STRENGTH OF MATERIALS. The larger an asteroid or planet, the greater the PRESSURE at the center. This is because the stuff below gets compressed or squashed by the stuff above. If all of your friends brought in all of their beenie babies and piled them up, the ones at the bottom would get squashed, right ?

When the central pressure at depth in a planet or large asteroid EXCEEDS the strength of the rocky material, the material will DEFORM and flow like a very sticky viscous liquid. It is also hotter near the center and that also causes the rocky material to behave like a very, very viscous (sticky) liquid. Quite simply, as the temperatute and pressure increases the strength of the material decreases and finally, at a pressure that corresponds to a depth of several hundred kilometers, the rocky stuff is able to flow in response to the forces of gravity. Gravity pulls everything down or in and if you think about it a sphere is the idealized shape that a body will tend towards because in a sphere material is brought as close to the center as it can be without bumping into another piece of material!!!!

I hope this answers your question !!

Maybe I can help explain why the planets are round.It basically has to do with gravity. Gravity is pulling all parts of the Earth toward the center as close as they can get, and the best way for this to happen is for the Earth to be a sphere because all points on the surface of a sphere are the same distance from the center. In other words, on the surface of a sphere there are no points that are winning over other points on the Earth's surface in getting close to the center. If the Earth were a cube, then the corners would be sticking far out compared to the centers of the sides and the Earth's gravity would pull on the corners and flatten them out so that the corners were no farther away from the center than the sides.

Even mountains that stick up a few miles higher than the surrounding surface of the Earth are constantly feeling the pull of Earth's gravity, and this keeps them from getting too high. Parts of the Himalayas are still growing because of tremendous forces acting to push them up, but other parts of the Himalayas are collapsing under their own weight because the Earth's gravity is pulling them down. This may be hard to imagine when mountains are made of solid rock, but solid rock is not very strong compared to the force of gravity when you are talking about very heavy things like a whole mountain range.

OK, now here is something to think about. In fact, the Earth is not a sphere (although it is close). This is because the Earth rotates around an axis that goes through the north and south poles. Think of a merry-go-round and the force you feel when you are near the outer edge of the merry-go-round. You tend to get thrown off the merry-go-round. In the same way, the part of the Earth that is at the equater and that is farthest from its center axis feels a greater outward pull than parts that are closer to the poles. This force counteracts gravity, and since this counteracting force (centrifugal force) is greater at the equator than at the poles, the Earth is not a perfect sphere. Can you figure out whether the centrifugal force would make the Earth more like a bagel or more like a hotdog?

Planets are round because of the nature of the forces that act on the matter than makes up a planet. What force do you think caused a cloud of matter to collapse into the Earth? What keeps the Earth from collapsing into a much smaller object?