All planets in the Solar System are spherical. In fact, any significantly massive objects are. This is because the larger an object is, the greater its gravitational force will be, and so as mass builds up the pressure of this weight causes the majority of the material to become liquid, and thus a sphere is formed.
However, let’s say the Earth could be made into a cube. Gravity would be strongest at the centre of each face (since the force of gravity increases the closer to the centre of gravity). As a result, all water, and indeed our own atmosphere would be drawn towards the centre of the faces. So the edges of the Earth would be barren rock with no atmosphere, and the centre of each face would play host to giant oceans and a very thick atmosphere, each region potentially with its own distinct ecosystems. It would be a very different planet to our own.
Answered by Sophie Allan, National Space Academy Education Officer for the National Space Centre