# why are planets round?
**URL:** https://boards.straightdope.com/t/why-are-planets-round/78565
**Category:** Factual Questions
**Created:** [August 28, 2001, 6:27pm UTC](https://boards.straightdope.com/t/why-are-planets-round/78565 "2001-08-28T18:27:01Z")
**Posts on this page:** 17
**Page:** 1
### Author:  [@chinstroker](https://boards.straightdope.com/u/chinstroker)
#### Post date: [August 28, 2001, 6:27pm UTC](https://boards.straightdope.com/t/why-are-planets-round/78565/1 "2001-08-28T18:27:01Z")
?
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### Author:  [@Wumpus](https://boards.straightdope.com/u/Wumpus)
#### Post date: [August 28, 2001, 6:30pm UTC](https://boards.straightdope.com/t/why-are-planets-round/78565/2 "2001-08-28T18:30:24Z")
Because gravity sucks.
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### Author:  [@Montfort](https://boards.straightdope.com/u/Montfort)
#### Post date: [August 28, 2001, 6:44pm UTC](https://boards.straightdope.com/t/why-are-planets-round/78565/3 "2001-08-28T18:44:50Z")
::Applause::
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### Author:  [@bup](https://boards.straightdope.com/u/bup)
#### Post date: [August 28, 2001, 6:46pm UTC](https://boards.straightdope.com/t/why-are-planets-round/78565/4 "2001-08-28T18:46:01Z")
Spheres are the least surface area to enclose a given
volume.
In a solid spehere, the average distance from a given point
to the center of mass is minimized.
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### Author:  [@kniz](https://boards.straightdope.com/u/kniz)
#### Post date: [August 28, 2001, 7:14pm UTC](https://boards.straightdope.com/t/why-are-planets-round/78565/5 "2001-08-28T19:14:08Z")
When a heavenly body reaches a certain mass **bup’s** explaination takes hold and the body becomes round. That is why our moon is round and Mars two moons are not round.
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### Author:  [@NutWrench](https://boards.straightdope.com/u/NutWrench)
#### Post date: [August 28, 2001, 10:01pm UTC](https://boards.straightdope.com/t/why-are-planets-round/78565/6 "2001-08-28T22:01:20Z")
If the world didn’t suck, we’d all fall off.
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### Author:  [@Nen](https://boards.straightdope.com/u/Nen)
#### Post date: [August 28, 2001, 10:06pm UTC](https://boards.straightdope.com/t/why-are-planets-round/78565/7 "2001-08-28T22:06:07Z")
In general, planets aren’t spherical. Consider the element of obliquity.
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### Author:  [@Doc\_Nickel](https://boards.straightdope.com/u/Doc_Nickel)
#### Post date: [August 29, 2001, 1:02am UTC](https://boards.straightdope.com/t/why-are-planets-round/78565/8 "2001-08-29T01:02:59Z")
I prefer the element of ubiquity myself.
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### Author:  [@RM\_Mentock](https://boards.straightdope.com/u/RM_Mentock)
#### Post date: [August 29, 2001, 5:20am UTC](https://boards.straightdope.com/t/why-are-planets-round/78565/9 "2001-08-29T05:20:50Z")
> [@](#):
>
> \*Originally posted by Nen \*
> \*\*In general, planets aren’t spherical. Consider the element of obliquity. \*\*
Are you using “obliquity” in the sense of “Obscurity in conduct or verbal expression”?
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### Author:  [@Sultan\_Kinkari](https://boards.straightdope.com/u/Sultan_Kinkari)
#### Post date: [August 29, 2001, 5:36am UTC](https://boards.straightdope.com/t/why-are-planets-round/78565/10 "2001-08-29T05:36:12Z")
Wanna know why?
Because a flat surface has a greater gravitational pull than a rounded one(like a magnifying glass, kind of, except the opposite). All of the “stuff”(space debris. etc.) gets pulled onto those flat surfaces, but not necessarily in the portions needed(sometimes more sometimes less) to make the earth perfectly round. So after the debris has been magnetically attracted to those flatter parts, they become rounded and the outskirts of the areas once flat become flat too, and the rounding process continues, over and over again. 😃
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### Author:  [@Dave\_Stewart](https://boards.straightdope.com/u/Dave_Stewart)
#### Post date: [August 29, 2001, 5:37am UTC](https://boards.straightdope.com/t/why-are-planets-round/78565/11 "2001-08-29T05:37:00Z")
I thought the Earth was very slightly pear shaped?
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### Author:  [@Sultan\_Kinkari](https://boards.straightdope.com/u/Sultan_Kinkari)
#### Post date: [August 29, 2001, 5:41am UTC](https://boards.straightdope.com/t/why-are-planets-round/78565/12 "2001-08-29T05:41:45Z")
> [@](#):
>
> \*Originally posted by Dave Stewart \*
> \*\*I thought the Earth was very slightly pear shaped? \*\*
Tha’ cho mama foo!
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### Author:  [@Dave\_Stewart](https://boards.straightdope.com/u/Dave_Stewart)
#### Post date: [August 29, 2001, 5:44am UTC](https://boards.straightdope.com/t/why-are-planets-round/78565/13 "2001-08-29T05:44:05Z")
> [@](#):
>
> \*Originally posted by wishbone \*
> \*\*
>
> > [@](#):
> >
> > \*Originally posted by Dave Stewart \*
> > \*\*I thought the Earth was very slightly pear shaped? \*\*
>
> Tha’ cho mama foo! \*\*
Nope: the Southern hemisphere is supposed to be slightly larger than the North.
Maybe gravity is pulling it down 🙂
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### Author:  [@MonkeyMensch](https://boards.straightdope.com/u/MonkeyMensch)
#### Post date: [August 29, 2001, 6:36am UTC](https://boards.straightdope.com/t/why-are-planets-round/78565/14 "2001-08-29T06:36:57Z")
> [@](#):
>
> \*Originally posted by wishbone \*
> \*\*
>
> Because a flat surface has a greater gravitational pull than a rounded one\*\*
>
> and here I thought gravitation had to do with mass and distance
>
> **(like a magnifying glass, kind of, except the opposite)**.
>
> I think you might want to get a better metaphor
>
> **All of the “stuff”(space debris. etc.) gets pulled onto those flat surfaces, but not necessarily in the portions needed(sometimes more sometimes less) to make the earth perfectly round. So after the debris has been magnetically
> \*\*
> magnetically?**
>
> attracted to those flatter parts, they become rounded and the outskirts of the areas once flat become flat too, and the rounding process continues, over and over again. 😃 \*\*
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### Author:  [@RM\_Mentock](https://boards.straightdope.com/u/RM_Mentock)
#### Post date: [August 29, 2001, 11:57am UTC](https://boards.straightdope.com/t/why-are-planets-round/78565/15 "2001-08-29T11:57:30Z")
> [@](#):
>
> \*Originally posted by Dave Stewart \*
> **Nope: the Southern hemisphere is supposed to be slightly larger than the North.**
The earth is pear shape only in the sense a watermelon could be considered a cube. No, not even that close.
[Is the Earth Pear-shaped?](http://mentock.home.mindspring.com/agu1992.htm)
When we talk about the shape of the earth, we are talking about its gravity field–the shape of the water surface if all the continents were criss-crossed with canals, essentially. That shape is called the _geoid_, and it is that shape that influences the orbits of satellites. By analyzing the orbits, we can determine the shape.
To first approximation, the earth is a sphere. Even its twenty kilometer bulge at the equator, due to rotation, would allow it to pass muster as a [billiard ball](http://mentock.home.mindspring.com/ifaq.htm#IFAQ007) on some tables. That is hardly pear-shaped.
The centrifugal bulge is considered a second-degree shape. Imagine a cross-section going through the poles–a low at one pole, high at the equator, low at the other pole, and high again at the equator. That’s like two periods of a sine wave, hence degree-two. If you do the same thing with three periods, you get the degree three pear-shape. The earliest satellite analysis determined that there was some slight effect from such a degree-three gravity field, and that was known as the pear-shape.
Subsequent satellite analysis has shown that other degree three shapes (there are six others) have even much more power. They all resolve into a single degree-three shape which is most accurately described as a rounded-off tetrahedron, which is not a pear-shape but could be called one if you _squinted_–and its four points are at most 100 _meters_, which is swamped by the twenty kilometers of the degree two. Also, one of the points would be on the equator.
As to the OP, another way to answer it is that the gravity field of any mass tends to be spherical–the farther away, the more spherical. That is equivalent to the fact that mass can be treated as a single point at its center of mass, to first approximation. Then, matter tends to form equipotential surfaces, which is another way of saying “water flows down hill”. What you end up with, after the matter has “flowed”, is a pretty good sphere, for large bodies.
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### Author:  [@bughunter](https://boards.straightdope.com/u/bughunter)
#### Post date: [August 29, 2001, 8:15pm UTC](https://boards.straightdope.com/t/why-are-planets-round/78565/16 "2001-08-29T20:15:33Z")
Well, **bumpus** and **RM Mentock** are correct about how gravity makes things round, but there are two forces operating towards that end: gravity and surface tension.
Gravity, the force pulling the surface _inward_ has been exhaustively explained.
Surface tension is a sideways force that pulls the surface _together_, seeking to minimize the surface area. It’s why when you blow soap bubbles, they’re round. Or when an astronaut plays with his tang, it floats around in spheres. [sub]God I just reread that sentance in preview, and it sounds like a dirty joke![/sub]
But wait. Planets aren’t soap bubbles or blobs of orange juice! Well, no, but they were once close. During their formation, they were hot and molten, and surface tension and gravity worked together to shape them into spheres.
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### Author:  [@RM\_Mentock](https://boards.straightdope.com/u/RM_Mentock)
#### Post date: [August 29, 2001, 10:09pm UTC](https://boards.straightdope.com/t/why-are-planets-round/78565/17 "2001-08-29T22:09:02Z")
> [@](#):
>
> \*Originally posted by bughunter \*
> \*\*But wait. Planets aren’t soap bubbles or blobs of orange juice! Well, no, but they were once close. During their formation, they were hot and molten, and surface tension and gravity worked together to shape them into spheres. \*\*
I don’t know about that. Once you get up to planetesimal size, doesn’t gravity pretty much overwhelm surface tension? I could be wrong. What are the numbers here?
If you throw a tang bauble into the air, it pretty much splats, on earth. The difference between the size of the sphere, and the size of the puddle, represents the difference between the strength of gravity and the strength of surface tension. Even for a three-inch water balloon, when it breaks, it’s going to create a pretty flat wet spot. (you’re right, this thread should be in the cecilingus forum).