Why Are Planets Round? Exploring the Shapes of Planets
Why are planets round? Despite their differences in size and composition, all eight Solar System planets are nearly spherical. Asteroids, on the other hand, have a greater variety of forms — some even resemble peanuts or dog bones! So why are all planets round, while smaller space objects can take almost any shape? Let’s explore what makes planets round, why some are rounder than others, and how gravity shapes the universe around us.
Also, explore 3D models of the planets with Solar Walk 2 and see just how different these worlds are — yet they all share the same basic shape.
Contents
- Why Are All Planets Round?
- Are Planets Perfect Spheres?
- Why Are Planets Round: Let’s Round Things Off
Why Are All Planets Round?
Planets are round because of the way gravity shapes them. They are born from scattered particles of gas and dust orbiting a star. Due to gravity, the particles attract each other and form clumps that get bigger as they gather more material from their surroundings.
As a forming planet grows, its gravity becomes strong enough to reshape its material. Gravity pulls inward from all directions, producing a nearly spherical body. That’s why planets have a round shape.
Why are planets round and most asteroids not?
The shape of a space object depends on its mass: the bigger it is, the more strongly its own gravity pulls it into a rounder form. Asteroids are generally too small for their gravity to smooth them out.
Many asteroids in the asteroid belt between Mars and Jupiter are remnants of material that never formed a planet. Jupiter’s powerful gravity disrupted the growth of larger bodies, preventing the material from coming together into a planet.
Some smaller bodies also have enough gravity to become nearly round. Those that orbit the Sun, are not moons, and have not cleared their orbital neighborhoods are classified as dwarf planets. Their nearly round shape is one of the key criteria that distinguish them from smaller, irregular asteroids.
Are Planets Perfect Spheres?
Planets are not perfect spheres because they rotate around their axes. As a planet spins, centrifugal force acts outward, opposing gravity. This effect is strongest at the equator, making the planet bulge outward around its middle and flatten slightly at the poles. The extra width is called the equatorial bulge, and the resulting shape is known as an oblate spheroid.
You can even spot Jupiter’s flattened shape through a telescope! Under good viewing conditions, you may notice that its disk looks slightly wider along its cloud bands than across them. A great opportunity is coming on February 11, 2027, when Jupiter reaches opposition and remains visible throughout the night. For a steadier view, wait until Jupiter is high above the horizon.
Which planet is the roundest?
How much a planet is flattened depends not only on its rotation speed but also on its density and internal structure. Venus and Mercury are the most spherical planets because they spin the slowest.
By this logic, you might expect Jupiter, the fastest-spinning planet in the Solar System, to be the most flattened. Surprisingly, that title goes to Saturn! Although it spins a little slower than Jupiter, Saturn is much less dense and has weaker gravity, allowing it to bulge more at the equator.
Do mountains and valleys affect planet shapes?
Mountains, valleys, and other surface features make the surface of the planets uneven, but barely affect their overall shape. Imagine shrinking Earth to the size of a basketball, about 24 cm (9.4 inches) in diameter. At that scale, even Mount Everest would rise only 0.17 mm (0.007 inches) above sea level.
What about Olympus Mons on Mars? Standing about 22 km (13.7 miles) tall, it would rise just 0.8 mm (0.03 inches) above the surface if Mars were the size of a basketball. Even such an enormous mountain barely affects the planet’s overall shape!
Olympus Mons is the largest volcano in the Solar System. But how well do you know the Solar System’s other record-breakers? Test your knowledge of the biggest, smallest, deepest, and more in our quiz!
Why Are Planets Round: Frequently Asked Questions
Is Earth perfectly round?
Earth is round because gravity pulls its matter toward the center from all directions, giving it a nearly spherical shape. However, Earth is not a perfect sphere: its rotation makes it slightly wider at the equator and flatter at the poles. This shape is called an oblate spheroid.
If Earth spins fast enough to bulge at the equator, why don’t we fly off?
Changing a planet’s shape doesn’t require overcoming its gravity. Think of water in a spinning bowl: it rises along the edges but stays inside. Similarly, Earth’s material can slowly shift and deform, making the planet slightly wider at the equator. Even solid rocks deep inside Earth can deform over long periods. Gravity still holds everything together.
Earth spins fast enough to slightly change its shape, but nowhere near fast enough to throw us off its surface. At the equator, Earth’s rotation carries us at about 1,670 km/h (1,040 mph). For someone at the equator to become weightless due to Earth's rotation alone, that speed would have to reach roughly 28,500 km/h (17,700 mph) — about 17 times faster! Assuming Earth’s radius and mass stayed the same, it would complete one rotation in just 84 minutes.
Who discovered Earth is round?
The idea of a spherical Earth dates back to ancient Greece and is often attributed to Pythagoras (c. 570–495 BC). Later, Aristotle (384–322 BC) offered evidence for Earth's round shape, including its curved shadow on the Moon during lunar eclipses. In the 3rd century BC, Eratosthenes (c. 276–194 BC) went a step further: by comparing the Sun's position in two Egyptian cities, he calculated Earth's circumference with remarkable accuracy.
How to prove Earth is round?
When looking out over the open sea, climbing higher lets you see farther. Parts of distant objects previously obscured by Earth’s curvature may become visible. If Earth were flat, those parts would be visible from the ground as well, provided the viewing conditions were good enough. Find more ways to prove the Earth is round in our article.
What do flat earthers think of other planets?
Some flat earthers assume that the planets are really round, but Earth is just not a planet and therefore doesn’t have to be spherical. Different communities of the movement have their own ideas about it. They agree that Earth is flat, but when it comes to the other planets, flat-Earth believers have yet to agree on a single explanation.
Why is the Moon round?
Our natural satellite is spherical because it’s massive and has sufficient gravity to stay round. The Galilean moons – Io, Europa, Ganymede, and Callisto – are also round for the same reason. However, moons can have irregular shapes. For example, the Martian moons Phobos and Deimos resemble asteroids as they are small, and their gravity is insufficient to make them spherical.
Are stars round?
Like planets, stars are nearly spherical because gravity pulls their matter toward the center. However, fast rotation can make them bulge at the equator and flatten at the poles. A striking example is Regulus, the brightest star in Leo. It spins once every 16 hours, making its equatorial diameter about 32% larger than its polar diameter!
Why Are Planets Round: Let’s Round Things Off
Planets are round because gravity pulls their material toward the center as they form. However, they aren’t perfect spheres: rotation makes them slightly wider at the equator and flatter at the poles. The degree of flattening varies from planet to planet, depending on their rotation, density, and internal structure.
Want to explore these differences for yourself? With Solar Walk 2, you can take a virtual tour of the Solar System, examine planets in 3D, and discover what makes each world unique.
