PBS Space Time explains why the Earth is a round sphere while the Milky Way galaxy is a flat disc. Those two shapes are the most common for large bodies in the Universe, and the reason the shapes are so often repeated comes down to the principles of symmetry and equilibrium.
Our universe is not a very diverse place when it comes to shapes. Large celestial bodies become spheres, galaxies become discs, and there is little room for variation. Why is this? Well it turns out physics has some pretty strict rules when it comes to creating new planetary bodies. On this week’s Space Time, we explore equilibrium and symmetry, the two defining factors that shape our universe.