Have you ever asked yourself why nearly all familiar planets have a round shape and not, say, the shape of a cylinder or a cube?
Interesting fact: planets appear perfectly round to the eye, but in reality, they are not at all. In this article, we'll explain why.
Image Credit: ONYXprj via Shutterstock / HDR tune by Universal-Sci
First things first: in essence, two main factors contribute to the round(ish) form of planets: gravity and the characteristics of a sphere.
Gravity
Gravity is a fundamental force that acts between two objects with mass. The more mass an object has, the stronger its gravity. Gravity plays a vital role during the formation of a planet.
Planetary formation begins with the aggregation of dust particles into clumps. These clumps gradually grow larger, evolving into pebbles, then into larger rocks. The existence of gas in the protoplanetary disk helps solid material particles to cling together, leading to the formation of even larger structures, known as planetesimals. Although some may break apart, others persist and serve as the fundamental building blocks of planets.
Over time, these planetesimals grow larger and attract more and more matter until they become a single, massive object. As the planet continues to grow, its gravity becomes stronger and pulls even more matter toward its center. This goes on up to a point where the planet has become large enough to clear its own path around its host star.
When a planet is forming, its matter is in a fluid state (not necessarily liquid - 'fluid' here just means it can flow). This allows the matter to easily move and arrange itself into the shape that gravity is pulling it into, which is a sphere.
The force of gravity on a planet acts equally in all directions, drawing material toward its center and increasing the density of its core. This pull of gravity results in the planet taking on a spherical shape, which is essentially a three-dimensional representation of a circle.
Why spherical?
A sphere is the most symmetrical shape that can be formed from the accretion of matter. As mentioned, the force of gravity acts evenly in all directions, making a sphere the perfect shape to resist deformation. If (for example) a planet were shaped like a cube, its corners would be way more vulnerable to collision and deformation.
In addition, a sphere has the largest volume for a given surface area. This means that a planet with a spherical shape will have 'more room' for its materials than a hypothetical planet with a different shape.
Why planets are not perfectly round
Although all the planets in our solar system may appear perfectly spherical, none of them are. However, some are more spherical than others. Out of all of them, Mercury and Venus are the most spherical, while on the other end of the spectrum, Saturn and Jupiter are the least spherical due to a bulge at their equator.
Saturn: the leas spherical planet of our solar system - Image Credit: NASA / JPL / Space Science Institute via Wikimedia Commons
Why do planets have a bulge at their equator? It is due to the fact that they rotate around their axis. When an object rotates, the outer edge must travel faster than the inner parts to keep up. This holds true for any rotating object, be it a large Ferris wheel, a table fan, or something smaller like a blue ray disc. The outer edge simply has to cover more distance in the same amount of time.
At a planet's equator, the largest distance has to be traversed for a single rotation. Gravity is doing its best to keep everything contained. However, the planet's rotation produces a centrifugal force (matter wants to 'fly away' like mud flinging off a tire), resulting in a 'bulge' at their equator where the effect is strongest. This excess width is referred to as the equatorial bulge.
Equatorial Bulge visualized - (Image Credit: Universal-Sci/ CC BY 4.0)
Like all planets in our solar system, Earth has an equatorial bulge as well, but it is rather small: Earth is about 43 kilometers (27 miles) wider at its equator than measured from pole to pole. In other words, the difference is slightly more than 1/300th of its equatorial diameter. (If the Earth were reduced to a globe with a diameter of 1 meter at the equator, the equatorial bulge would only measure a mere 3 millimeters).
Mars has a much larger bulge than Earth: about 1/150th of its equatorial diameter, but this is still relatively minute compared to Saturn, which has the largest bulge of all the planets in our solar system. Saturn is about 1/10th thicker at its equator compared to its diameter from pole to pole.
So now you know: planets are round due to the effects of gravity and the properties of a sphere, and they are not perfectly spherical due to the centrifugal effects of their rotation.
Sources and further reading:
How do planets form? (NASA exoplanet exploration)
What is a rogue planet? (Universal-Sci)
Planets don't get bigger than this! (Universal-Sci)
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