Why Are Planets Round?

Article written: 12 Jul , 2017
Updated: 12 Jul , 2017

The Solar System is a beautiful thing to behold. Between its four terrestrial planets, four gas giants, multiple minor planets composed of ice and rock, and countless moons and smaller objects, there is simply no shortage of things to study and be captivated by. Add to that our Sun, an Asteroid Belt, the Kuiper Belt, and many comets, and you’ve got enough to keep your busy for the rest of your life.

But why exactly is it that the larger bodies in the Solar System are round? Whether we are talking about moon like Titan, or the largest planet in the Solar System (Jupiter), large astronomical bodies seem to favor the shape of a sphere (though not a perfect one). The answer to this question has to do with how gravity works, not to mention how the Solar System came to be.


According to the most widely-accepted model of star and planet formation – aka. Nebular Hypothesis – our Solar System began as a cloud of swirling dust and gas (i.e. a nebula). According to this theory, about 4.57 billion years ago, something happened that caused the cloud to collapse. This could have been the result of a passing star, or shock waves from a supernova, but the end result was a gravitational collapse at the center of the cloud.

Due to this collapse, pockets of dust and gas began to collect into denser regions. As the denser regions pulled in more matter, conservation of momentum caused them to begin rotating while increasing pressure caused them to heat up. Most of the material ended up in a ball at the center to form the Sun while the rest of the matter flattened out into disk that circled around it – i.e. a protoplanetary disc.

The planets formed by accretion from this disc, in which dust and gas gravitated together and coalesced to form ever larger bodies. Due to their higher boiling points, only metals and silicates could exist in solid form closer to the Sun, and these would eventually form the terrestrial planets of Mercury, Venus, Earth, and Mars. Because metallic elements only comprised a very small fraction of the solar nebula, the terrestrial planets could not grow very large.

In contrast, the giant planets (Jupiter, Saturn, Uranus, and Neptune) formed beyond the point between the orbits of Mars and Jupiter where material is cool enough for volatile icy compounds to remain solid (i.e. the Frost Line). The ices that formed these planets were more plentiful than the metals and silicates that formed the terrestrial inner planets, allowing them to grow massive enough to capture large atmospheres of hydrogen and helium.

The leftover debris that never became planets congregated in regions such as the Asteroid Belt, the Kuiper Belt, and the Oort Cloud. So this is how and why the Solar System formed in the first place. Why is it that the larger objects formed as spheres instead of say, squares? The answer to this has to do with a concept known as hydrostatic equilibrium.

Hydrostatic Equilibrium:

In astrophysical terms, hydrostatic equilibrium refers to the state where there is a balance between the outward thermal pressure from inside a planet and the weight of the material pressing inward. This state occurs once an object (a star, planet, or planetoid) becomes so massive that the force of gravity they exert causes them to collapse into the most efficient shape – a sphere.

Typically, objects reach this point once they exceed a diameter of 1,000 km (621 mi), though this depends on their density as well. This concept has also become an important factor in determining whether an astronomical object will be designated as a planet. This was based on the resolution adopted in 2006 by the 26th General Assembly for the International Astronomical Union.

In accordance with Resolution 5A, the definition of a planet is:

  1. A “planet” is a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighborhood around its orbit.
  2. A “dwarf planet” is a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape [2], (c) has not cleared the neighborhood around its orbit, and (d) is not a satellite.
  3. All other objects, except satellites, orbiting the Sun shall be referred to collectively as “Small Solar-System Bodies”.

Montage of every round object in the solar system under 10,000 kilometers in diameter, to scale. Credit: Emily Lakdawalla/data from NASA /JPL/JHUAPL/SwRI/SSI/UCLA/MPS/DLR/IDA/Gordan Ugarkovic/Ted Stryk, Bjorn Jonsson/Roman Tkachenko

So why are planets round? Well, part of it is because when objects get particularly massive, nature favors that they assume the most efficient shape. On the other hand, we could say that planets are round because that is how we choose to define the word “planet”. But then again, “a rose by any other name”, right?

We have written many articles about the Solar planets for Universe Today. Here’s Why is the Earth Round?, Why is Everything Spherical?, How was the Solar System Formed?, and here’s Some Interesting Facts About the Planets.

If you’d like more info on the planets, check out NASA’s Solar System exploration page, and here’s a link to NASA’s Solar System Simulator.

We’ve also recorded a series of episodes of Astronomy Cast about every planet in the Solar System. Start here, Episode 49: Mercury.



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4 Responses

  1. How are celestial bodies formed at all?
    Under the influence of high vibrations of ether, which fills the universe, strings are formed in three spatial directions and in their cross-sections the matter forms in the form of quarks, enucleons, and positrons. In particular, the formation of gluons by annihilation of electron-positron pairs. In the chain of thermodynamic processes, quarks and gluons form stars of magnets, pulsars, a super-new, neutron star, composed of quark gluon plasma. When the supernova explodes (a very magnetic field), subatomic particles are formed (proton: 3kg of particles and positron and neutron: 3kg particles and gluon), and especially from gluons, electrons, positrons, photons are formed. The ratio of ether and matter (solid state) causes gravity, whose task is to assemble the mass in the crowds until critical mass and critical gravity is obtained, when matter returns to the form of ether from which black holes are formed). Gravity acts similar to the cohesion force of the liquid droplet, and therefore the body assumes a spherical shape. (Let these scientists understand and accept – it’s free). Magnetism is the relation between ether and gluon and it is the cause of all phenomena related to the formation of energies of various waves. Et cetera .
    This is the right way of understanding the structure of the universe (my copyright).

  2. Sammy says

    What Matt described here is the explanation that is generally found to Agee most with observation, aka science. Nicola, nice try to explain the universe, but I would advise you to not overestimate your own views and read some science books on why ether is no longer part of science for about a century.

  3. Sammy, it is certain that there are many books and theories about the comprehension of universe , but few are presented in these books, and they agree with natural laws.
    This conclusion by some scientists and experimenters that ETHER does not exist is to ensure that their contaminating theories are at the top of science, no matter how accurate they are, since it is important to become popular, even in a false way.
    Here I will briefly give you some facts that confirm that ether exists.
     -1. Nikola Tesla had an ether support in his experiments, because he made a motor (say electro), which he built into the car and with him achieved a speed of about 150 km / h without any fuel.
     – 2. My insight, how does magnetism and gravity arise? It is important here to know how a current flows through a conductor if it is in a changeable magnetic field? , Or why there is a magnetic field around the conductor through which the current flows. Who does it and how it supports. ?
    Science does not know that without ether, photons can not move, because photons and ether are of electromagnetic character, it is only necessary to connect it in the head and understand in a simple way. Science has imposed a rule that nothing can be recognized if it is not mathematically proved or, sometimes, experimental.
    The proof of a lot can be seen, when it is explained, why our Moon always has one side facing the Earth.? She did not explain it in a logical way.

  4. fantazas says

    Can we be so sure they all are that nice and round? So far we have only solid observations of their shape from this solar system? So astronomers could still find some of odd shape out somewhere.

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