Why Are Most Large Celestial Objects Spherical?

by Carson

Have you wondered why celestial objects like planets, stars, and moons, all take a spherical shape? Today, we’ll be talking about this in this article.

The Process Where Objects Become Spherical

To convert an irregular clump of materials into an elegant sphere, something needs to be balanced out obviously, and this is precisely how this process leads to massive objects being spherical. The force that can change the shape of those objects is gravity.

Gravity is the force that pushes all materials towards the center of mass. You probably know it as the force that keeps stationary objects on the ground, but it also pulls the object itself inward. That’s why the center of our planet is significantly denser than the crust of the Earth, and that’s why nuclear fusion can occur at the core of our star, providing everyone with warmth and light.

However, when an object’s mass is unevenly distributed, the center of mass will not be in the exact center of the object. In that case, gravity pulls some regions inward more strongly than in other regions. Specifically, the gravitational pull towards the center will be stronger for regions with more mass, which moves towards the object’s less massive regions. Matter starts moving around as the gravitational pull on them is uneven. They move in a way that balances them out, making it a sphere.

When an object becomes a sphere, all regions receive equal gravitational pull. As a result, they will not change their positions around the object significantly, and the object maintains its approximately spherical shape.

Why Aren’t All Celestial Objects Spherical?

Although planets and stars undergo this process during formation, asteroids retain their irregular shape. That’s because the asteroid isn’t very massive compared to other celestial objects. Therefore, their gravitational pull on itself is also weaker and is insufficient to cause the movement of large amounts of materials described above since the force that keeps material from breaking apart is stronger than the gravitational pull. Therefore, asteroids do not become spherical and bear all kinds of shapes you can imagine.

Moreover, some planets and stars are noticeably elongated at the equators. That’s because rotation exerts a centrifugal force around the equator, where the force of inertia pushes the material in a direction different from the direction of the rotation. The force pulls the equator outward such that there are more materials there. Usually, the faster an object rotates around its axis, the more the equator is pulled outward. However, when there are more materials, the gravitational pull on them is also stronger, preventing the equator from bulging outwards aggressively. Consequently, the object’s shape is kept under control in the form of an ellipsoid.

In addition, tidal forces from other objects may affect the movement of materials of an object in subtle ways as well. Since other forces pull the celestial object in different directions, the distribution of material on them could be affected and may be more unevenly distributed than it would have been otherwise.


In this article, we’ve discussed why most large celestial objects and nearly spherical, why some of them noticeably bulge at the equator, and why smaller objects are of irregular shapes. If you want to learn more about related knowledge, please visit the websites in the references below. If we missed any critical points that we should have included, please leave them in the comments below to make this article better.

References and Credits

  1. (n.d.). Why Are Planets Round? Retrieved July 13, 2022, from https://spaceplace.nasa.gov/planets-round/en/
  2. Mark Mancini. (n.d.). Why Are Planets Almost Spherical? Retrieved July 13, 2022, from https://science.howstuffworks.com/why-are-planets-almost-spherical.htm
  3. Fraser Cain. (2014, June 26). Why is Everything Spherical? Retrieved July 13, 2022, from https://www.universetoday.com/112805/why-is-everything-spherical/

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