Why do all planets have a spherical shape?
The recognized planets of the solar system are large bodies with a significant mass. All particles of a massive body experience a significant net attraction towards its center. Such a body is, therefore, forced to acquire a spherical shape. If the mass of the body is low, as for example that of a large number that inhabits the asteroidal belt between the Earth and the Mars, these bodies do not have to be spherical. The argument is given above also explains why the Earth cannot support mountains much higher than the Himalayas.