A perfect sphere is a shape gravity naturally imposes on massive objects, where all surface points are equidistant from the center. However, Jupiter and Saturn deviate from this ideal geometry. These gas giants are visibly flattened at the poles and bulge significantly around their equators, a shape technically known as an oblate spheroid. This non-spherical form results from two factors working in tandem: extremely rapid rotation and a largely fluid internal composition. The dynamic interplay between the force generated by their spin and the lack of a rigid structure allows their massive bodies to be physically deformed.
The Primary Driver: Centrifugal Force
The main reason Jupiter and Saturn possess this flattened shape is the physics of their rotation, which generates a powerful outward push known as centrifugal force. The giant planets spin on their axes with incredible speed, completing a full rotation in under 10.5 hours, which is remarkably fast for objects of their enormous size.
This rapid rotation causes mass within the planet to attempt to move away from the axis of spin. This outward force is strongest at the equator, where the rotational speed is highest. As the planet’s material is flung outward, it creates a substantial bulge around the middle. Near the poles, the rotational speed approaches zero, making the centrifugal force minimal and allowing gravity to keep the material closer to the center.
The centrifugal effect directly counteracts the inward pull of the planet’s gravity. The resulting shape is a dynamic equilibrium between these two forces. For Jupiter, the difference between its equatorial and polar radii is about 4,600 kilometers, a visible distortion caused by this outward push.
The Enabling Factor: Fluid Structure
While Earth also rotates and is an oblate spheroid, the effect is far less noticeable because rocky planets possess a rigid crust that resists deformation. For the centrifugal force on Jupiter and Saturn to be so effective, the planets must have a structure that allows for physical reshaping. Both planets are composed primarily of hydrogen and helium, which gives them a largely fluid, non-rigid internal structure.
These gas giants do not possess a solid surface like terrestrial planets; instead, their atmospheres gradually increase in density inward. Deep inside the planet, immense pressure compresses hydrogen gas into a liquid state, eventually forming liquid metallic hydrogen. This deep layer acts like a massive, swirling ocean, allowing the planet to behave as a single, deformable fluid body.
The lack of a fixed, solid scaffolding means the planet’s material can easily flow and redistribute itself in response to rotational forces. The centrifugal force is able to push the fluid matter outward along the equator, creating the bulge. The 1-bar pressure level, often used to define the planet’s visible “surface,” is merely the boundary of this massive, pliable body.
Why Saturn is Flatter Than Jupiter
Although Jupiter rotates faster than Saturn, the ringed planet is noticeably flatter, meaning its oblateness is more pronounced. The degree of flattening is not determined by rotation speed alone, but by the ratio comparing the centrifugal force to the force of gravity.
Saturn’s lower overall density is the primary reason for its greater oblateness. Saturn has a mean density of 0.687 grams per cubic centimeter, making it less dense than water, compared to Jupiter’s density of 1.33 grams per cubic centimeter. This lower density means Saturn has a significantly smaller total mass and therefore a weaker gravitational force for a planet of its size.
Because Saturn’s gravity is less powerful, it provides less resistance to the centrifugal force generated by its rotation. Even though the rotational speed is slightly slower than Jupiter’s, the outward force is proportionally more dominant against the weaker gravitational pull. The result is a more dramatic equatorial bulge and a greater flattening at the poles.