# Why are Planets and Moons Round?

Discussion in 'After Hours Lounge (Off Topic)' started by Scott Strang, Jul 11, 2003.

1. ### Scott Strang Screenwriter

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Are they round because of the motion of the gases and other chemicals and just happen to dry that way?

2. ### Greg_R Screenwriter

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I'd guess that the force of gravity would act on the molten body & gasses (thus pulling with the same force in all directions). This would result in a sphere which would cool in that shape.

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Equal force (or absence thereof) from all directions ==> round shape.

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4. ### Scott Strang Screenwriter

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5. ### BrianW Cinematographer

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Equal force in all directions? I’m not so sure that describes it well enough. Certainly, if the forces were drastically unequal as a function of direction, there may be bulges, but the overall shape would still be basically spherical.

And an absence of force certainly doesn’t lead to round shapes, as there are many oddly-shaped objects in the solar system small enough to be regarded as “not held together by gravity.” In these objects, the absence of gravitationally-assisted cohesion makes almost any shape possible. And, as you might guess, everything in between is a matter of degree. The greater the mass, and therefore the greater the force of gravity, the greater the tendency will be for a body to assume a spherical shape.

In simplest terms, spherical shapes, at least in this context, simply have the lowest state of energy. If you get a giant bulldozer and try to reshape an Earth-sized planet into, say, a cube, then the corners of the cube wouldn’t be able to maintain their high state of energy. (The farther a mass is from the center of the planet/moon, the greater its potential energy, so the corners of the cube would be the points at which the potential energy is greatest.) Consequently, the corners would crumble back down, and the planet would resume its round shape (and release a lot of heat in the process).

It is also worth noting that looking at this question in terms of energy states also explains why soap bubbles are round, as well. For whatever reason, many things in this Universe just don’t like being very energetic if they don’t have to be.

6. ### BrianW Cinematographer

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P.S. The force at play keeping soap bubbles round is surface tension, not gravity. I didn't mean to imply that soap bubble's shapes are maintained by force of gravity, though that's what it looks like I was saying.

7. ### Bill_D Supporting Actor

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Anything that spins will eventually be round regardless of it starting state. Besides, with all the movement, bumping and scraping the corners would get knocked off leaving the middle.

How's that for an edumacated country boy?

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What I wrote is so wrong, I can't believe I typed that :b.

The energy angle sounds good.

9. ### Bill_D Supporting Actor

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10. ### BrianW Cinematographer

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I don't think that's quite what Holadem was getting at.

11. ### Andrew Testa Second Unit

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So far BrianW is closest to the truth.

Planets and moons (and stars) are spherical due to a combination of gravity and erosion. Obviously erosion is only a component for solid bodies.

During formation, planets accrete material through very violent means: huge planetesimals smashing into each other and re-coalescing. The constant bombardment releases enough heat to keep the surfaces molten for some time. Since a liquid seeks its level (or sphere of constant gravitational potential) they flow into rounder shapes as they grow. The larger the planet, the more gravity pulls in the planetesimals and the hotter the body, the more spherical it gets.

If the planet gets large enough its gravity is able to hold on to the gasses in the accretion disk. As it grows more and more gas gets swept up and you wind up with a gas giant: exotic solid core surrounded by whatever molecular gases had precipitated out of the accretion disk at that distance. Like a liquid, gasses will seek a sphere of constant gravitational potential.

Once the planet has a solid crust then gravity and erosion take over. Large protrusions (like mountains) are built like icebergs. They have a large root that extends deep into the mantle so their center of mass floats at the same level as the surrounding flatland. Mountains eventually sink into the mantle as the root liquefies and the mountain loses mass on the bottom. As long as the mantle remains molten, mountains will very slowly melt down from the bottom. The larger the planet, the smaller mountains can be since their buoyancy on the mantle is reduced and they sink faster. When the mantle is no longer molten, the mountain will be frozen in place and only erosion will be working to whittle it down. This is why mountains on Mars can be so much larger than mountains on Earth.

Also at work is erosion. As BrianW mentioned about potential energy, anything loose on the surface wants to roll downhill. The higher the gravity, the shallower a slope has to be for rubble to roll downhill. Sources of erosion are surface liquids, wind, and impacts, all of which make solid rock into small pieces of rock or dust that collect at the bottom levels.

With very small bodies there is not enough gravity to form a sphere. That's how we wind up with potato and dumbbell shaped asteroids.

Rotation of the bodies doesn't have any impact so far as making them spherical to begin with. It does cause an oblateness due to the equatorial mass wanting to accelerate away from the center. Most bodies are almost spherical, but fast rotating gas planets like Jupiter are visibly oblate.

Andy

12. ### Ron-P Producer

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Didn't any of you see the end of Men in Black?

Peace Out~

13. ### Steve_Tk Cinematographer

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They are only round because we have not found some square ones yet.

14. ### Jeff Kleist Executive Producer

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Steve, I don;t know if you're kidding or not. But they just explained why they are round. If we were to find an oblong planet or something, there would likely be something like a black hole nearby

15. ### James E Stunt Coordinator

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Holadem, Bill, not to take a side, but i wanted to add "Welcome to Diskworld"

james

16. ### John Watson Screenwriter

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Most of them have been around for a long time

17. ### John Watson Screenwriter

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Maybe the Gods used to play a lot of baseball

18. ### John Watson Screenwriter

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A square planet - that would be one where they still buy Lawrence Welk records

19. ### James E Stunt Coordinator

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20. ### Kirk Gunn Screenwriter

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Thanks for the explanation Andrew. You've even added a new word to my vocabulary: Planetisimal. I'll have to use this in my next Staff Meeting...

However, even with your wonderful articulation and common-sense theory, I still don't buy it: