Spherical Planets, Moons & Suns

Can someone please explain to me the scientific theory for why all planets, moons and suns in the universe are spherical?

Someone suggested gravity - that makes no sense to me. Supposedly all came about from a massive explosion ”The Big Bang“. If that is so, there should surely be nary a single spherical object in the universe? Everything should have random jaggedness (if that is a word)

Can someone please explain to this poor uneducated lad in simple plain English so I can understand.


mugsy! :smack:

Oops wrong forum - sorry (my first post!)

Can someone please explain to me the scientific theory for why all planets, moons and suns in the universe are spherical?

Someone suggested gravity - that makes no sense to me. Supposedly all came about from a massive explosion ”The Big Bang“. If that is so, there should surely be nary a single spherical object in the universe? Everything should have random jaggedness (if that is a word)

Can someone please explain to this poor uneducated lad in simple plain English so I can understand.


mugsy! :smack:

Gravity makes no sense to you? Think about how much force it takes to break a rock with a hammer; think about how much pressure it takes a hydraulic rock crusher to do its work.

The Earth’s mass is nearly 6 billion trillion tons. That’s enough for gravity to crush it into a sphere. It’s not a perfect sphere – we still have mountains – but significant irregularity in shape only shows up in asteroids about a tenth of that size. Moons smaller than that, like those of Mars, are not, in fact, spherical at all.

Well, to put it in simple terms, gravitation produces an attractive force between bits of matter. All else being equal, if you have a whole bunch of particles trying to get as close to each other as possible, what shape do you think they are going to fall into? The sphere happens to be the shape that gets all the particles as close together as possible, so gravity, being an attractive force, tends to push things towards collecting into spheres.

Of course, gravity is not the only force between particles. The forces that hold solid matter together are also pretty important, and at small scales, they overwhelm the force of gravity. That’s why, for example, everyday objects such as your car, your coffee mug, and your body don’t collapse into spheres under their own gravity. Even some fairly large objects like asteroids are sufficiently rigid to resist gravitational rounding. Gravitational force increases as mass increases, however, and once you get up towards the mass of something like, say, the moon, the force of gravity is strong enough to prevent very large irregular shapes from forming or remaining stable.

That’s the key issue here: gravitational force gets stronger as more mass is collected closer together, but the electromagnetic force that keeps rigid objects rigid stays the same. So if an object were massive enough, the force of its own gravity pulling it into itself would be greater than the forces keeping it rigid, and it would break. Materials with different levels of rigidity (and probably some other properties too) will of course behave differently; a liquid like water, for example, will collapse into a sphere at a much lower mass than, say, iron.

That’s not to say that the big bang spewed out a bunch of huge irregularly shaped objects that then broke down into spheres, though. It’s currently thought that planets form through accretion discs, which are basically big spinning fields of stellar dust that eventually coalesce into dense objects, which very naturally form in a spherical fashion.

Linky here. It’s all about the gravity.

The Big Bang happened quite a few years ago. At first, it only produced elementary particles. Shortly thereafter, some of those particles combined into protons, neutrons, and electrons. Around that point, there was quite a lot of hydrogen, but not much else.

Eventually, gravity caused the hydrogen to form clusters. These clusters became stars. Since then, stars have lived their entire lifetimes–forming, burning, then dying. Some stars die with a whimper–along the way, they generally produce medium-to-light elements such as iron and carbon.

Some stars die spectacularly, in the form of a supernova. The red giant stars that eventually supernova are the source of almost all of the heavier elements such as lead.

So, the Big Bang is really outside the scope of your question. Planetary formation begins inside stars.

Pretty much everything heavier than iron was formed in the heart of a red giant. Iron was formed inside lesser stars. When a red giant dies, it explodes violently–a supernova–and all the material that made up the giant, plus any nearby material formed inside of normal stars, gets blown all over that part of space.

But gravity is still in effect. These bits and pieces draw each other, eventually forming a new smaller star. That star becomes a central point of gravitational focus–every bit of matter is both drawn to the star and drawn to every other bit of matter. A disk of dirty gases forms, all of it revolving around the star. With time, clumps form in the disk.

By now, there are several effects in play: most of the matter is still somewhat affected by the momentum it had when it was ejected by the supernova, the new star is pulling everything toward itself, everything in the disk is pulling on everything else, each clump of matter is pulling all nearby dust and gas toward itself, and the whole mess is spinning arond the new star.

With time, each of those clumps grows larger and begins to rotate on its own axis… again, due to gravity and momentum. Everything in the above paragraph is still occurring (though there is less free dust and gas), but now we can also add in the spin of the individual bodies (planets, moons, whatever the clumps are growing into).

We know that gravity draws heavier elements toward the center of a body, while the lighter ones stay closer to the surface. This causes each planet or moon to have some internal “churn”… don’t forget that the object is also both rotating (in a small circle) and revolving (in a big circle). That’s what causes them to become spheres.
Note, though, that not all bodies in space are spheres. There is a certain mass required for a body to overcome its own tensile strength and take on a spheroidal shape. In fact, one requirement for a body to be classified as a “dwarf planet” or “planet” is that it has been rounded by its own gravity. Many objects haven’t.
You asked for a simple explanation, and this was about as simple as it gets. I haven’t mentioned hydrostatics or various tidal forces or even centrifugal effects, which also play a role in shaping the heavenly bodies. One thing of note, though, is that most stars aren’t spheres… this is due to all the reasons I didn’t mention.

Hope that helped some.
ETA: I see that I was beaten to the punch by the pros. :slight_smile:

Thankyou all. That was most helpful. I knew there must be some explanation.

I must say some parts still seem a bit confusing for me but I am satisfied there is a logical scientific explanation. It’s not necessary that I understand everything about all things, even if I could. But to be told it has ”something to do with gravity“ was most unsatisfactory.

Thanks again.

Gravity’s the correct answer, though – picture it this way: the big bang spread out things just about evenly, except for very small variations in density (that we today still can observe in the cosmic microwave background). Those variations meant differing matter density, which lead to gravitational accretion – roughly speaking, areas with more matter attracted more strongly than those with less matter, thus accumulating yet more matter etc. Gravity seeks to accumulate the most matter within the smallest possible boundary, thus it tends to form spheres, since the sphere is the lowest area within which to include a given volume. Another way to see this is by using the notion of potential energy: the potential energy of an object in a gravitational field increases with the distance from the center of gravity, thus a spherical configuration has the lowest possible potential energy (in the static case), since all the points on its surface are the same distance from the center; lower energy configurations are always preferred.

Oh, and welcome, mugsy!

I thought gravity was supposed to be a weak force? How does it have the power to pull ever larger pieces of solid matter into a uniformly spherical shape then? I’ve gotta be honest, the whole ‘outer space’ thing mystifies me! There’s just too much going on! :confused:

You’re overthinking it.

Look at this from an intuitive perspective: Gravity cetainly is strong enough to keep you planted in your chair, right? And it’s strong enough to roll rocks down hills, and make buildings collapse, and bridges fall down.

Suppose you wanted to make a tall building. Really tall. One that you could see from space, it sticks up so far. The Earth is 8000 miles in diameter, so you want to build one that’s a substantial fraction of that–let’s say a 40 mile tall building (that’s 1% of the radius).

Now, what do you think would be a major issue with construction a building that tall? Lots of issues of course, but the one I’m thinking of is the weight of the building itself. How do you build foundations that are strong rnough to support the enormous weight of the building? Answer: you can’t. If you try to build such a building, at some height the weight of the building itself will crush the foundations. What causes weight? Gravity.

The same argument can be extended to the shape of planets: Any non-spherical part of a planet is effectively a large mountain. If the mountain is large enough, the base of the mountain can’t support the weight above, and the mountain itself will collapse.

Just FTR, this has absolutely nothing to do with gravity. Gravity is, by far, the weakest force in the universe. It takes the entire mass of the Earth to exert a force on you of a couple hundred pounds (give or take - don’t know your weight).

FTR, this has everything to do with how much work gravity has to do to even out the crust. Crushing rocks, eh?

Welcome to the SDMB, mugsy.

Since this doesn’t appear to be a comment on one of Cecil’s columns, I’ll move this thread to our forum for factual queries, General Questions. I’ve also edited your thread title to make it more descriptive, which is always appreciated.

moderator CCC

Also, the planets formed when they were hot enough to be liquid, so it doesn’t take much force to form them into a sphere.
Remember - there are objects in space that are not spheres - small ones like asteroids and comets, for example.

I’ve merged the two threads. For future reference, it’s better to ask that a misplaced thread be moved than to start a new one. Not a huge deal, but it makes life easier for us.

moderator CCC

It’s weak compared to the other three basic forces, but unlike magnetism is always attractive, and unlike the strong and weak nuclear forces it works at a distance.

The strong and weak nuclear forces are much stronger in the range they work, but they are nano scale forces. They work at the atomic and subatomic level. Once you start getting several atoms together, those forces are spread too far apart and begin to average out. Gravity is very weak at the small scales of atoms and molecules, but it is cummulative. While electro-magnetic and subatomic forces have polarity (positive and negative), gravity does not. So gravity is cummulative, and over large scales (the size of mountains) the addition makes gravity much stronger.

Your gravitational attraction to the Earth is not strong enough to overcome the forces holding your atoms and molecules together. That is why you sit on your chair, walk on the ground or on buildings, etc. You don’t fall through them.

Even mountains can be fairly strong enough to fight gravity. If the rock is solid, it can reach up in jagged peaks. What breaks down mountains is weathering that weakens the mountains, allowing gravity to act.

But try to stack a couple of continental plates on top of each other and see what happens. Things start to get messy. Eventually, get enough matter is a big enough pile, and the gravitational force of all the bits on all the other bits tries to pull all the matter into the smallest surface area for a given volume it can fit. Mathematically that shape is a sphere. That is because each bit wants to be as close as it possibly can to every other bit. If some bits are sticking out in corners, they are farther away from some of the other bits than they need to be.

At that point, the cummulative force of gravity of all the bits exceeds the forces of the atomic and molecular forces holding the matter together in rigid shapes. Ergo, the rock breaks apart and reshapes.

Gravity is weaker than other forces. Think of the size of one of those car lifting junkyard magnets compared to the size of the mass needed to counter it (ie The Earth).

But we are talking planet sized masses here. At that scale, the Earth is essentially a ball of liquid surrounded by a thin film (the crust). So like any liquid, it will flow down to the lowest point. Since there is no “lowest point” in space, it flows towards the center of gravity creating a more or less perfect sphere.

And actually, even compared to just about anything you’d find on Earth - billard balls, bowling balls (not counting the holes), ball bearings, planets are pretty smooth. Figuring the height difference between the deepest point in the ocean to the top of the tallest mountain is about 15 miles, the radius of the earth (about 3900 miles) is within about a tolerance of about ± 0.17%

I think they are massive enough to create enough pressure to always be mostly liquid. Except for the iron core. I could be wrong though.

Except that the elements created within lesser stars remain within the dead lesser stars. The carbon, oxygen and nitrogen that go to make up you were also created in supernovae.

Sorry guys. I appreciate you not making too big a deal out of it, even though it must really get right up your nose.

I’ll try to do better. Promise.



PS Should one contact the moderator in such a case (if so, how?) or does one just as ask by editing the message?