If an external force would give the Earth form of the cube, then at the end of its action, the force of gravity will again gather it into a sphere.
In order to the object has become spherical under its own gravitational force, this force must be large enough. According the Newton’s law of Universal Gravitation, if an object has more mass, then it also has more gravitational force. Hence, the object must have a big mass. In addition, the object has to be plastic. Desirable – liquid or gaseous, as gases and liquids are easier to acquire a spherical shape.
All the stars and planets are spherical form from “birth” and throughout its existence, they are massive and plastic. However, the asteroids (minor planets) are not spherical. Why? Firstly, their mass is much smaller. Secondly, they are not plastic. If, for example, the asteroid Eros had the mass of the Earth, it is also would be round.
Image Credit: NASA