Ever since people started looking up at the night sky, they’ve been amazed by how round our planets and moons are. But why is that? Well, it’s not that simple. In the last 50 years, we’ve been exploring space and visiting all sorts of objects in our solar system. We’ve seen things as small as asteroids, which are just a few hundred metres big, and as big as the gas giants, which are over 140,000 kilometres wide.
And what we’ve found is that, across this huge range of sizes and compositions, there’s a pattern that emerges: when a celestial body gets big enough, its own gravity pulls it into a rounded shape that’s almost a ball. This transition seems to happen around 200-400 km in radius for most types of material. The physics behind this is all about the tug-of-war between forces of attraction and forces of repulsion. On one side, you’ve got the short-range forces, like chemical bonds between atoms, and on the other, you’ve got the long-range force of gravity. When a celestial body gets really big, its gravity pulls it into a rounded shape that looks a bit like a ball.
The Physics of Sphericity
When things are really small, there are lots of short-range forces at play, and it’s these that determine how things are shaped and structured. The electromagnetic force is really important here, as it’s what holds atoms together to make molecules and the stuff that makes up our lattice structures. Chemical bonds can have a bit of a mind of their own when it comes to direction, and that’s what gives us those cool and intricate symmetries we see in crystals and other solids. The strong nuclear force is like the glue that holds our nuclei together, and it’s this that then lets them interact with electromagnetism to share electrons.
It’s all about how these particles work together, and it’s this that gives things their shape. Even if you start with really tiny stuff, like dust particles, or bigger things, like asteroids, gravity won’t be the main thing that decides their shape. It’s more to do with how the particles bump into each other and stick together as they’re forming.
But, and this is where it gets interesting, gravity has two special features that help it take over from these short-range forces when there’s enough material together. First, gravity is always attractive, pulling any two masses together. And second, gravitational attraction follows an inverse square law, meaning the force gets weaker the further away the objects are. This makes gravity pull material towards the centre of mass.
The pressure induced by an object’s gravity acting to compact itself ultimately overpowers any ability for mechanical strength to retain more irregular shapes when objects exceed the hundreds of kilometers in radius scale for primary rocky/metallic compositions, or the hundred or so kilometer scale for weaker icy objects rich in volatiles. Gravity pulls from all directions equally towards the center of mass, exerting an isotropic pressure that favors adopting a highly symmetric spherical shape for stability. This transition towards dominance of gravity depending on size helps explain key differences observed between tiny asteroids, dwarf planets like Ceres, terrestrial planets, and gas/ice giants.
Roundness Versus Hydrostatic Equilibrium
When we’re checking out how round something is, it’s important to remember that roundness and hydrostatic equilibrium are two different things. Roundness is a good sign that an object is pulled into shape equilibrium by its own gravity, but it’s not the only thing we need to think about.
Roundness just means that something looks a bit round, but it doesn’t tell us anything about what’s going on inside. Lots of examples like Saturn’s moon Mimas and some small asteroids are round, but they’re not in hydrostatic equilibrium. Meanwhile, Saturn’s largest moon Titan and most dwarf planets meet the full criteria for hydrostatic equilibrium. This basically means that their shape is determined by balanced self-gravitation for a spinning body rather than inherent material strength.
It’s still being investigated whether borderline cases (like Vesta and Enceladus) are in true equilibrium. This is being studied using space mission data. It seems that developing hydrostatic equilibrium generally needs more mass than just having a round shape. The estimates for this range from needing at least 250-400 km radius for icy dwarf planets to ~600 km or more for rocky bodies without lower density ices.
Conclusions: Why Are Planets Round?
All planets and larger moons overwhelmingly demonstrate highly spherical shapes thanks to sufficient mass to be in hydrostatic equilibrium. You can see this happening in all types of planets once they’re more than 800-1000 km wide. The gas giants are a great example of this, with a diameter of over 140,000 km! Their gravity is so strong that it pulls the fluid around them into perfect spheres. Rocky worlds smaller than 500 km tend to be irregularly shaped, or rounded but not in equilibrium because they are less affected by gravity. Instead, their shapes are controlled more by how they were formed and random collisions between smaller particles.
While rotation and collisions can distort otherwise balanced spherical shapes, basic roundness is a pretty common gravitational outcome for larger, complex, multi-material solar system bodies. This has been confirmed by recent spacecraft surveys over half a century of spaceflight.
