Why are planets round when asteroids and Deimos and Phobos are not?

Question

Why are planets round when asteroids and Deimos and Phobos are not?

Answer 1

Is the earth round? When viewed from space yes. However, viewed from a few thousand feet up and at an angle, it is far from round.

Our planet is no more round than these asteroids when you view them from equivalent distances.  

Answer 2

the asteroids are too small, their mass is not big enough for gravity to pull them together into a sphere shape

Answer 3

Smaller (less massive) objects do not have enough mass to "pull" themselves into spheres.  Most asteroids are too small to do this but not the biggest ones like Ceres and Vesta.  Deimos and Phobos are not very massive.  It has been said that an average person on the surface, could, in theory, throw a baseball into orbit around one, I forget which one, but the point is they are small and not massive enough.

Answer 4

Because a planet has a greater mass/surface area and it orbits the sun. whereas asteroids are a separate entity and Phobos and Deimos are the moons of Mars.

Answer 5

As many have already said, there's a limit on an object's mass where it will either be a sphere or a spheroid.  Obviously the Martian moons don't measure up.

Check out the asteroid Ceres.  It's the largest in the belt, and very spherical considering most asteroids are not seen as such.

Answer 6

their mass did not allow their gravity to overcome the phisical fores of the material they are composed of and could not make the objects round

Answer 7

What poor ole xyzpdqfoo is trying his best to say is "hydrostatic equilibrium":

Answer 8

Earth and the other planets are DEFINITELY more round than small asteroids like Phobos and Deimos.

The reason, like the others have said, is because of their size.  The planets are massive enough that the physical weight of all the rock in the upper layers heats the lower layers to a plastic-like consistency almost like play-doh, and then gravity molds it into a sphere.  The small objects like asteroids aren't massive enough, so they are still solid, hard rock all the way through.