why are planets round?
Because of their gravity. On the surface of the Earth, solid material–say, rock cliffs –can easily stand the pull of gravity without deforming. But go just a few hundred kilometers inside the Earth, and you find everything under enormous pressure, from the weight of the layers heaped up on top. Under such pressure (and helped by the heat down there!), even solid rock deforms like putty. If the Earth were all fluid, gravity would pull it into a symmetric sphere–the same way as it shapes the oceans. The Earth is not fluid, but as mentioned above, it makes no great difference. Actually, a ROTATING fluid Earth would be deformed by the centrifugal force, with the equator bulging out slightly. Gravity is weakened there, by the centrifugal force and by the greater distance from the center. That was observed in Newton’s time, and Newton explained it by essentially using a fluid analogy. Jupiter is much bigger than Earth and rotates much faster: its equator bulges out so much that pictures t
