Why are the planets round?
The sun and all eight planets in the solar system are round. Why? The gravitational force of the planet's mass pulls all of its material towards the center, smoothing out any annoying irregularities. Many of the smaller bodies in the solar system are not round, because their gravity is not enough, to smooth their shape.
We can see it from the escape velocities of different bodies. To escape Earth's gravity, you have to travel at a speed of approx 11 km / second or approx 40 000 km / hour. These speeds require the largest rockets. Earth has mass 6 x 10 ^ 24 kg and is quite round. To escape the gravity of comet 67P, visited by the European probes Rosetta and Philae, you have to travel at a speed of approx 1 meters per second. Comet 67P isn't round at all; has a mass 10 ^ 13 Kg, almost a trillion times lighter than Earth and is shaped like a rubber duck.
When the body reaches a diameter of more than a few hundred kilometers, it becomes rounder. In our example, the diameter of the Earth is approx 12700 Km; Comet 67P is approx 4 km in diameter.
Despite its improbability, some scientists wondered, what a cubic planet would look like. Assuming, that the rocky part of the planet would somehow retain its volume, air and water would not have any such magical properties and would accumulate in the center of each side of the cube. Life would be confined to the shores of the central lakes, and the edges and corners of the cube would be gigantic impassable mountains.