When we look at the Solar System, we can see countless objects of various shapes and sizes, from tiny specks of dust to large planets and stars. However, one common feature among large objects such as planets, stars, moons, and the Sun is their spherical shape, while smaller objects like dwarf planets and comets seem to defy any specific shape.
Gravity plays a key role in unraveling the mystery of why large celestial bodies assume a spherical form. The larger an object is, the greater its mass, resulting in a stronger gravitational force. Gravity always pulls towards the center of mass of an object. When these celestial bodies are formed, they initially lack a defined shape, but gravity pulls the matter towards their centers over millions or billions of years, eventually giving them a fixed spherical shape.
However, a celestial object needs to be sufficiently massive to generate a strong gravitational force that can overcome the structural strength of the material forming it.
Static Equilibrium
When a planet generates enough gravitational force to overcome the material’s strength, it tends to pull all the matter towards a spherical shape. Areas that are too high will be pulled downwards, compressing the underlying layers of material, while areas that are too low cannot be pushed outward.
Once a planet reaches this spherical shape, it is said to be in a state of “static equilibrium.” However, not all planets achieve static equilibrium, as it depends on their composition.
An object composed primarily of liquid water, for example, is easily moldable because water molecules are highly mobile and lack the strength to resist gravitational forces.
On the other hand, an object composed of solid iron would require much greater mass to have its gravity overcome the strength of the iron. In the Solar System, the minimum diameter required for an icy object to become spherical is at least 400 kilometers, while solid objects need even larger diameters.
Moon Mimas, a moon of Saturn resembling the Death Star, has a spherical shape and a diameter of 396 kilometers. It is currently the smallest celestial body that meets this criterion.
Continuous Motion
Things become more complex when we realize that all celestial objects have a tendency to rotate or spin in space. If an object is spinning, positions along its equator (points equidistant from the two poles) will experience a slightly weaker gravitational force compared to positions near the poles.
The result is that the perfect spherical shape in static equilibrium transitions to what is called an “oblate spheroid” when objects along the equator appear somewhat bulged compared to their poles. This is true for our Earth, with an equatorial diameter of 12,756 kilometers and a diameter from pole to pole of 12,712 kilometers.
The effect becomes more pronounced as a celestial object spins faster. The planet Saturn, which consists mainly of gas but also has a solid core, rotates on its axis in approximately 10 hours and 30 minutes, shorter than Earth’s 24-hour cycle. As a result, Saturn appears significantly more oblate than Earth.
Some stars even exhibit greater asymmetry. The bright star Altair, observable in the northern sky during winter months, serves as an example. It rotates around its axis in just 9 hours, causing its equatorial diameter to be over 25% larger than the distance between its poles.