Why Are Planets Round?


A load of old balls.

Kicking off the science portion of this blog, I’m going to start with an easy question I used to get asked a lot when I did Outreach for the university: why are the various planets, moons etc. round? It’s a fairly simple answer with some wide-reaching ramifications.

Baldly (or possibly badly) put, it’s because gravity. Gravity is constantly pulling every piece of matter that makes up a planet like the Earth towards its centre of mass – in this case the Earth’s core. The Earth is probably a bad example to use here because of its complicated internal structure, but it doesn’t matter that planets start out being made of what we might think are strong, non-malleable materials like rock; thanks to internal heating provided both by the compressive effect of the gravity itself and the slow decay of radioactive elements inside the planet in question, the material inside it is made just “soft” enough that it will deform under pressure and slowly flow inwards over millions of years.

But if gravity is constantly pulling every bit of a planet towards its centre of mass, what’s stopping the Earth from shrinking and shrinking until it collapses into a black hole? The answer is — again–  pressure. If you have a chunk of stuff, and you exert some external force to reduce the volume of that stuff, you consequently increase the pressure inside the stuff that pushes outwards and resists your external force. Think about compressing air in a bicycle pump: at first it is easy since the internal pressure inside the pump is low, but as the air is compressed into a smaller and smaller space working the pump becomes harder and harder until you eventually reach the point where the pump pedal won’t work even if you put all your weight on it. The pressure of the air inside the pump is now equal to the force exerted by your body weight – in other words, it has reached a state of equilibrium with your body.

The same principle applies to planets. As the planet is compressed under the force of its own self-gravity, the internal pressure pushing outwards and resisting this compression will start to rise. The smaller the planet gets, the greater the internal pressure. Eventually the outwards force provided by the internal pressure will equal the inwards force exerted by the planet’s gravity, and the planet will stop shrinking. The planet has reached what is called hydrostatic equilibrium, and since the two forces balance each other out at a fixed distance from the planet’s centre of mass the effect is kind of like taking a pair of compasses, setting them to a fixed distance, and drawing a big circle. Except in 3D, obviously.

Now, a few notes and corollaries. When the IAU introduced the new classification of dwarf planets a few years ago, one of the criteria a body needed to have in order to qualify was that it had to be in hydrostatic equilibrium – i.e. round. This was a handy way of delineating the boundary between planetoids (planet-like bodies such as Pluto) and regular bog-standard asteroids and comets, since the state of hydrostatic equilibrium requires a certain level of gravity in order to start the compression process, which in turn requires a minimum amount of mass. You won’t find any bodies below about 400 km diameter which have compressed themselves into this round shape; they’re not big enough. This is why even the larger asteroids retain their rocky, irregular shape, and it’s also why the one asteroid that was large enough to achieve hydrostatic equilibrium (Ceres) got reclassified as a dwarf planet.  However, even with a few of the smaller planetoids we find that they’re not perfectly round. Other forces besides their own self-gravity act on them and distort their shape. Tidally locked satellites like the Moon will always be facing the same way relative to their parent body and thus the parent body will always exert gravitational tidal forces on the same “side” of the satellite; this can warp the shape of the satellite if it is particularly small and the parent body is particularly large. Rotation also has an effect – even the Earth is 20 km fatter at the equator than it is at the poles thanks to its rotation, and in the further reaches of the Solar System you can find weird things like Haumea, which has a rotational period we scientists refer to as batshit insane and which has severely deformed it into an ellipsoid.

Hydrostatic equilibrium applies to stars too, except in their case they’re so big that the internal pressure due to compression is nowhere near enough to stop a star’s mass from collapsing in on itself. Instead it’s the fusion fire burning at the heart of every star which provides the outwards pressure necessary counterbalance the star’s immense gravity. But what happens when a star runs out of fuel and that fire winks out? That’s a process that can be described by several phrases. Interesting. Lethal to anyone standing within about a hundred light years. And very definitely a post for another day.

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One thought on “Why Are Planets Round?

  1. Yann says:

    “pressure necessary counterbalance”
    Forgot a “to” there. ;-)

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