Planets are rotating bodies, and while they often appear as perfect spheres, their shapes are dynamic. The form of a large celestial object balances the inward pull of gravity against the forces generated by its spin. This constant interplay ensures that every planet deviates slightly from a flawless globe. Among the eight major planets, one stands out with a shape so visibly distorted that it holds the record for the most noticeable departure from a true sphere.
The Flattest Planet Revealed
The record for the flattest planet in the solar system belongs to Saturn. Its distinctive, visibly flattened appearance results from its enormous size and incredibly fast rotation. The difference between its equatorial diameter and its polar diameter is substantial. Saturn is approximately 10% wider across its equator than it is from pole to pole, giving the ringed giant a distinct “squashed” or oblate shape.
Defining Planetary Oblateness
The technical term for this deviation from a perfect sphere is oblateness, also known as flattening. Oblateness is a mathematical measure calculated by comparing a planet’s equatorial diameter to its polar diameter. The shape of a massive, rotating body is governed by hydrostatic equilibrium, where the inward force of gravity is balanced by internal pressure and the outward force of rotation. Because material at the equator moves faster, it experiences a greater outward centrifugal force. This force causes the planet’s midsection to swell, leading to an equatorial bulge and polar flattening.
The Physics of Extreme Flattening
Saturn’s extreme shape is due to two major factors: its rapid rotation and its low density. A Saturnian day is only about 10.7 hours long, meaning the material at its equator spins at a tremendous velocity. This high rotational speed generates a powerful centrifugal force that pushes material outward.
The low density is equally important, as Saturn is a gas giant composed primarily of hydrogen and helium. This fluid composition means the planet has less internal rigidity compared to a dense, rocky world, allowing the centrifugal force to more easily overcome the planet’s self-gravity and distort its shape. Although Jupiter rotates slightly faster than Saturn, Saturn’s much lower density results in a more flattened overall shape. The effect of rotation versus density can be quantified by comparing the centrifugal and gravitational forces, a ratio that is significantly higher for Saturn than for other planets. Its equatorial radius is about 6,200 kilometers greater than its polar radius.
How Other Planets Measure Up
Comparing Saturn to other planets highlights the uniqueness of its shape. Jupiter, the largest planet, also rotates quickly, completing a day in just under 10 hours, and is noticeably oblate with a flattening ratio of about 1/15. While Jupiter is the second-flattest planet, its higher overall density reduces the effect compared to Saturn. Earth, a dense, rocky planet, rotates much slower, requiring almost 24 hours for one turn. Consequently, its oblateness is minimal, with a flattening of roughly 1/298. This demonstrates that the combination of rapid spin and low internal density is necessary to achieve the extreme flattening seen in Saturn.