Why are spherical shapes so common in the universe?

Question

I have a simple question. Why are most objects in the observable universe spherical in shape? Why not conical, cubical, cuboidal for instance? I am furnishing a few points to justify this statement:

1. Most of the planets in the universe are spheroids.

2. Other heavenly bodies (e.g.: stars and moons) are also spherical.

3. Even elementary particles like atoms are best described as spheres (I admit that they don't have well-defined boundaries, but we still use terms like atomic radius when describing atoms.)

So, my primary question boils down to this single line:

Why is the spherical shape so ubiquitous in the observable universe (both in the microscopic and the macroscopic world)?

Answer 1

Spherical shapes in the universe are common because the dominant long range forces like gravity and electromagnetism are central (in that they only depend on the distance between objects).

Our planet, the moon, and the sun are all spherical for this reason, gravity pulls every object in towards the center equally.

Answer 2

On a astronomical scale spherical shape is due to gravity and the more massive the body the bigger gravity. Let's start with the stars:

Stars form by collapsing cloud of gas in so called "pre-main-sequence phase", each particle or atom is attracted towards center of mass. The matter in such gas isn't stationary, atoms can move in any direction but on average they move towards center. We could consider pre-main-sequence star to be fully convective, convection meaning exchange matter between deeper layers of star and outer layers (matter from inside goes outside and matter from outside goes inside)

Source: Dina Prialnik, An Introduction to the Theory of Stellar Structure and Evolution

What all of this has to do with the question? Well, gas moving around helps creating spherical star. Let's assume we start with disk of gas, if some atoms move above disk plane they will be pulled towards center of gravity instead their original position making at the same time empty space in their original position for other atoms to move closer.

And these movements of all atoms and radial attraction towards center of mass causes stars to be spherical, in picture above only small portion of disk moved however in more real example every part of disk would be moving in some direction.

Planets are similar, dust around stars is clumping together expanding slowly. That loose material tries to find the local lowest point of potential energy.

There are a lot more mechanisms in play than just gravity that may try to work against "keeping spherical shape" but for stars or planets gravity is dominating factor.

Of course not everything is spherical like some of the asteroids

Answer 3

A planetary body would always want to achieve hydrostatic equilibrium:

In fluid mechanics, hydrostatic equilibrium (hydrostatic balance, hydrostasy) is the condition of a fluid or plastic solid at rest, which occurs when external forces, such as gravity, are balanced by a pressure-gradient force. In the planetary physics of Earth, the pressure-gradient force prevents gravity from collapsing the planetary atmosphere into a thin, dense shell, whereas gravity prevents the pressure-gradient force from diffusing the atmosphere into outer space. In general, it is what causes objects in space to be spherical.

More here: Why are stars, planets and larger moons (approximately) spherical in shape (like, the Sun, the Moon, the Earth, and other planets)?

Answer 4

A classical philosopher like Aristotle would say this is because "sphere is a perfect solid and the heavens are a region of perfection". A more modern version of the essentially same answer would be that the sphericity of heavenly objects reflects the isotropy of space.

Conversely, when isotropy is broken (e.g. by the presence of an attracting/repulsing object nearby or by the body itself, as is the case of rotation), the shape of the body is not a sphere anymore.

Answer 5

Equilibrium states in time-varying processes are local minima of exergy which preserve conserved quantities.

Those things are (or can be reliably predicted to be) roughly sphere-like for which spherical shapes locally minimize exergy. Those things are not roughly sphere-like (or can be reliably predicted to not be) for which non-sphere-like shapes locally minimize exergy, e.g. most molecules, crystals, galaxies, electron clouds of excited-state hydrogen, and stack exchange contributors.

Beside that general fact, the reasons for which one roughly spherical thing and another roughly spherical thing are roughly spherical may be completely unlike. For instance, the reason why a spherical C60 molecule is spherical is pretty much the same as the reason why a rod-shaped CO2 molecule is rod-shaped. Aside from the above paragraph, the reason why a spherical C60 molecule is spherical is almost completely unrelated to the reason why a star is spherical.

If you'd like to know about a more specific category of things (i.e. dense gravitationally bound astronomical bodies, certain molecular arrangements of Carbon, or isolated atoms), I'd suggest one or more follow-up questions.

Answer 6

The surface to volume ratio is the least for a spherical object. This means that a sphere for attractive forces will have the highest binding energy. All objects tend to a state of lowest potential energy and for a spherical object of the same volume potential energy would be minimum.

Answer 7

Although Torus/Donut Shapes are among the most common shapes in the transparent, invisible universe, like the Magnetospheres of various planets and galaxies, what humans can easily discern are dense gravitational and electrostatic accumulations, which in three observable dimensions coalesces into spheres.

If you are in space on a station, you can add drops of a liquid like water and the electrostatic forces, rather than gravitation, will form a sphere of clear water, wobbling from the changes in airflow. Uniform spheres take on cleaner surfaces as their individual components fight for the same distance from the center.

Answer 8

I kept reading and reading and couldn't believe I wasn't finding anything about hydrostatic equilibrium.... Until I did. In space, when an object has enough mass, it will ALWAYS take the shape of a sphere (or an oblate sphereroid) due to hydrostatic equilibrium.

This is actually one of the requirements for a celestial body to be classified as a planet, per the IAU definition.

Stars are a different animal, but the same principle can be applied.

Stars and planets are spherical because they have to be. Mass and gravity won't allow them to take on any other configuration. But that's where the spherical shape stops in my opinion.

Most other objects in nature take on the best configuration that serves their purpose.

Think about a tree - branches and leaves spreading out in every available direction. Why? Because this is the best way to receive the maximum amount of sunlight for photosynthesis.

The double helix ladder of DNA, the manner in which carbon is the easiest molecule to build chains with... The list goes on and on.

From the quantum world to the largest structures in the universe, each and every object takes on whichever configuration best allows it to exist and serve its purpose, and while that will always be the spherical shape for celestial bodies with enough mass, it's rarely the case for anything else in the universe. In fact, I would argue that the spiral is the most common.

Answer 9

In a wider sense: The sphere is the smallest surface that encloses a given volume.

The sphere is the solution to one of the most general optimization problems in three-dimensional geometry.

This makes it, in isotropic nature, the solution to many physical optimization problems, for example finding the minimal potential energy configuration of a blob of matter, the surface of least potential energy of a soap bubble enclosing an amount of gas, the simplest solution for a single electron swinging around a proton etc. If you want, that is a consequence of an isomorphism (or, perhaps, homomorphism) of math and nature. That structural similarity is not surprising; mathematics, practically as well as conceptually — because it is conceived by humans living in the world as it is —, is rooted in nature.

Natural systems, absent external influences, can spontaneously only move in the direction of least potential energy because the path there from any other state releases energy (photons are emitted, heat is generated). Leaving the state of lowest potential energy requires an influx of energy (heating, absorption of photons) or, more generally, outside influence. The result in the latter case is, of course, the lowest potential energy for the overall system.

The sphere is the ideal solution for such problems in the absence of any disturbances, which is why none of the actual existing shapes are spheres — for example, none of the celestial bodies is exactly spherical ;-).