The ancient Greeks used simple observations of the Sun to prove the Earth’s roundness and size over 2,000 years ago. But did you ever wonder why planets are round or how do they hold their shape?
Gravity causes planets to be round. Gravity pulls equally from all sides of a planet. Like the spokes of a bicycle wheel, gravity pulls from the center to the edges.
All Planets are Round
Our solar system’s eight planets differ in a variety of ways. They are of various sizes. They are at different distances from the sun. Some are small and rocky, while others are large and gaseous. They’re all nice and round, though. Why is this the case? Why aren’t they in the shape of cubes, pyramids, or discs?
Planets form when material in space collides and clumps together. It eventually accumulates enough material to have a reasonable amount of gravity. That is the force that holds everything in the room together. When a planet is large enough, it begins to clear its orbit around the star it orbits. It snags bits of space junk with its gravity.
Gravity pulls equally from all sides of a planet. Like the spokes of a bicycle wheel, gravity pulls from the center to the edges. This results in a planet’s overall shape being a sphere, which is a three-dimensional circle. (Source: Space Place)
Are Planets in a Perfect Circle?
While all of our solar system’s planets are nice and round, some are rounder than others. Mercury and Venus are the most elongated. Like marbles, they are nearly perfect spheres. However, not all planets are perfectly round.
Saturn and Jupiter are slightly thicker in the center. They bulge out along the equator as they spin around. What causes this? When something spins, such as a planet rotating, things on the outside must move faster than things on the inside to keep up. This applies to anything that spins, such as a wheel, a DVD, or a fan. Things closest to the edge must travel the furthest and fastest.
Gravity holds the edges along a planet’s equator, a circle halfway between the north and south poles, but as it spins, stuff wants to spin out like mud off a tire. Saturn and Jupiter are enormous and spin at breakneck speeds, but gravity holds them together. That is why they have a bulge in the middle. The extra width is known as the equatorial bulge.
Saturn has the greatest bulge of any planet in our solar system. The diameters from pole to pole are not the same as the diameters along the equator. Saturn is 10.7% thicker in the center. Jupiter is 6.9% thicker in the center.
Instead of being perfectly round like marbles, they are squished down like basketballs while someone sits on them. (Source: Space Place)
How About Smaller Planets?
Earth and Mars are small and do not rotate as quickly as the gas giants. They are not perfect spheres, but they are more circular than Saturn and Jupiter. The middle of Earth is 0.3% thicker, while the middle of Mars is 0.6% thicker. It’s safe to say they’re very round because they’re not even one whole percentage point thicker in the middle.
Uranus and Neptune are in the middle. The middle of Uranus is 2.3% thicker. Neptune has a 1.7% greater thickness. They aren’t perfectly round, but they’re close. (Source: Space Place)