Why Are Planets Round?
Earth and the other solid planets were liquid when they formed. Surface tension and gravity force fluids including planets, stars, and raindrops into a round sphere.
To understand why Earth and other planets all have a round spherical shape, think about raindrops. Seeing falling raindrops is difficult, however small raindrops have a spherical shape.
Why are Rain Drops Round?
The popular conception that raindrops are teardrop-shaped is incorrect. Small raindrops are spherical. Larger raindrops are distorted by air pressure as they fall. In space floating, water drops would all be spherical.
Blobs of liquid with no other significant forces acting on them will take on a spherical shape. Liquids will be round spheres whether they are the size of raindrops, planets, or stars. Earth and other solid planets are spherical because they were once liquids. Scientists call both liquids and gasses fluids. Stars and the large gas giant planets are mostly spherical because gas flows easily like liquid and the same forces are at work.
Why Liquids Have a Spherical Shape
If no other significant forces act on them, liquid blobs become spherical. The molecules in a liquid blob, whether it is the size of a raindrop or planet, pull on each other. Depending on the scale and mass of the liquid the pulling results from a combination of electromagnetic and gravitational forces.
Atoms and molecules near the center of the liquid blob have forces from the other molecules pulling in all directions, so the forces cancel. They don’t move. Atoms and molecules near the surface are pulled towards the center because there are no molecules outside to pull them outside. The forces do however attract molecules near the surface to each other. This is the same effect as if the surface of the liquid blob were a rubber sheet or balloon. Scientists call these forces acting between molecules on the surface of liquid surface tension.
The surface tension on a liquid blob pulls the liquid into a shape that has the least amount of surface area. This shape is a sphere. So the surface tension forces liquid blobs into a spherical shape.
The spherical shape happens at any scale. It also occurs in fluids that are either liquid or gas. So, raindrops, liquid or gas planets, the Sun, and stars are all-around spheres. They are not all perfect spheres because they spin and bulge at the equator. Other forces are at work.
Earth & Solid Planets Were Once Liquid
Surface tension will pull liquids and gasses into a spherical shape but not solids. The fact that Earth, Moon, Mercury, Venus, Mars, and the larger moons of Jupiter or Saturn are all spherical tells astronomers that they were once liquid. These worlds are all solid now but were liquid when they first formed. The smaller moons of Jupiter or Saturn and asteroids are not spherical because they were never liquid.
The planets formed from leftover material when the Sun formed. In a process called accretion, small pieces of rocky and metallic material collided and stuck together until they grew to the size of planets. These repeated collisions released enough heat energy to melt the metallic and rocky materials. So Earth and other solid planets were liquid when they first formed. They solidified as they gradually cooled.
Asteroids and small moons never became hot enough to be liquid, so they did not form into round spherical shapes.
The same principles of physics work to explain why liquids are spherical on size scales from raindrops to planets and stars. The elegance and beauty of physics is using the same physical principles to explain a variety of phenomena that seem unrelated.
Walker, JS, Physics, Prentice-Hall, 2002.