Why is everthing in the universe spherical? Planets, moons, stars, comets, etc…
My nephew asked me this and I had nothing to give him. I am sure there is a law or theory on this one, but I couldnt find it. So rather than half-ass it, I hope someone can help me look smart 
thanks!
Large bodies (like planets and stars) are spherical for the same reason the sea’s surface is horizontal, and mountains can’t get bigger than a certain size: gravity. Because of gravity, things try to move the centre of a large object. But they can’t all get there, so they finish up in roughly a spherical shape. With respect to the earth, you have to remember that most of the earth is liquid, under a relatively thin solid crust, which would not be able to support very large mountains rising out of the general spherical shape.
Because they are all subject to the effects of gravity. Gravity pulls matter together in the shape of a sphere if there is enough of it, especially if it a liquid or a gas (most of the earth is a liquid). The earth is in fact obloid like most planets because it is spinning and the combination of gravity and centripetal forces mean that this is the the most stable arrangement.
I think this is roughly correct and I will post now before the pros turn up with a more thorough explanation.
A sphere is a minimum-energy shape. Since every point on the surface of a sphere is equally distant from the center, all points on the surface are at equal energy potentials with respect to each other.
For any given collection of objects, gravity will tend to pull them toward their collective centre (of gravity) - in the case of planets, these gravitational forces mostly exceed the structural properties of the material, so any corners just collapse under their own weight; once everything has fallen inwards to for a sphere, there’s nowhere else to go.
A lot of astronomical objects are not spherical (or obloid thanks Courgette), but they are either (spinning) composite things like galaxies or exploding.
There is quite a lot of exploding going on out there.
Because space is isotropic.
Forces propagate equally well in all directions, and the only form that is symmetric in all directions is the sphere.
If gravity were stronger in one direction than another, objects would not be spherical.
Not everything in the universe is spherical. Most asteroids and smaller moons are not. You and I are not. Small objects don’t generate an intense enough gravitational field to overcome the internal “stickiness” of their atoms to each other and pull themselves into a sphere. Large objects generate more intense gravitational fields, with greater variation with distance from the center of mass. Then, too, a larger object typically will either coalesce out of gas or form from the accretion of large rocky fragments which melt upon impact. Liquid and gas have less “internal stickiness” and more easily compress into a sphere.
I’m sure I learned this in science in high school…
but if gravity is strong enough to pull and hold massive material into a sphere, why aren’t we crushed into the sphere also?
did gravity lose its strength over the ages?
or is it some other force that let’s human walk and jump and not be crushed on the surface like pancakes.
No. Remember that when it formed, the Earth was so hot that it was molten. Additinally, humans (and even mountains) are small relative to the size of the Earth, and more importantly, the atomic and molecular bonds within them are stronger than the force of gravity acting on them, so they can hold their shape.
I’ll leave the other questions for someone else. This one isn’t as daft as it seems. I’m sure I’ve read some umming-and-erring about whether the gravitational constant can or change or is changing. Has the current fashion for dark matter and negative energy replaced the need to have a G that changes? This is a genuine question I’m a bit out of touch with op to date cosmological thinking.
As they always say in the popular science books, gravity is the weakest of all the four known fundamental forces. You can jump off the ground even with all the mass of the earth pulling against you.
But massive objects still exert massive forces and are very patient. Once in the grip of a gravitation field, objects need huge amounts of energy to leave. You can jump off the ground, but you can’t get into space without a gigantic rocket strapped to your ass.
So over millions and billions of years, massive objects will tend to move everything that is caught in their fields to the lowest energy state. A bulge will find itself subject to stress because of the varying levels of force on its constituent parts. Eventually, shearing and crumbling occurs, leveling the bulge.
Hard bodies like the earth can stand one or two percent of variation from a perfect sphere. Anything more than that is unstable over time.
Here we have four large bowls. In the first is water, in the second meringue, in the third sand, and in the fourth chunks of rock. Your task is to build a scale model mountain using each. When you pile up the water into a mountain, it will, of course, immediately smooth out into a level surface. (We presume you cannot freeze the water, for this experiment.) The meringue will come up into small tufts, and stay that way for a few days – but inevitably it too will flatten out. The sand will create a mound, but only up to a certain angle. Once you’ve built it, it’ll stay that way for any reasonable time, presuming no external force flattening it out, but it’ll only build up so high before it starts sliding down the side of your mountain and trying to go for a level surface. The rocks, on the other hand, will stay in the vertical arrangement you make with them for an indefinite period, again assuming no external influences.
The electromagnetic adhesion of each is different – the water has so little adhesion, relatively, that the force of gravity is paramount. The meringue and the sand have relatively more but not enough to produce an accurate and stable representation of the mountain. The rocks have the most EM adhesion and will not slump under the pull of gravity.
Now extrapolate that to planetary scale. There are limits to the degree to which EM adhesion can counteract gravity for every substance – a different limit for each. But ultimately you will find a point at which gravity will overcome EM adhesion and cause the substance to “flatten out” – in spatial terms, to turn into a sphere or reasonable facsimile.
Phobos is quite happy remaining peanut-shaped – it’s small enough that its own gravity is not sufficient to pull it into spherical form, and the EM adhesion of the rock which it is composed of can fight Phobotian gravity. Larger satellites, on the other hand, are massive enough that their gravity makes the rock on their surface much like the sand in our experiment – it’ll build up to a certain level, and then slump under the force of gravity. You could not make a mountain high enough that its summit would achieve Clarke (geosynchronous) orbit on any known celestial body, AFAIK. But most are capable of some surface relief, on a very small scale as compared to planetary diameter. (But note that the surface of a “cold” neutron star has such a high surface gravity that the tallest possible mountain is something like a centimeter high.)
how big was the material before it became a planet?
were they big asteroid chunks that fused together? or was is gaseous elements that gathered and formed into the ‘hard’ material it is now?
Yeah, tell your young nephew that, I’m sure it will make everything clear to him.
hahaha!
Thanks for all the information, you guys/gals rock!
How old are you? At a certain age you’ll start to realize that gravity does, in the end, win. Ultimately, we are all pancakes.
Even less. The tallest mountains are what…5 or so miles tall? Relative to the 24000 mile diameter of the Earth it’s an insignificant %. I heard once that as a % of diameter, the planets are some of the smoothing objects in the universe since Lando Calarisian.
Well, only about 6000 mile diameter – the 25000 mile figure is the circumference. But other than that nitpick, your point is right on!
Sphericity comes from statistics: If an object is formed by smaller bits coming together, then there is no “preferred” direction. To get a long, thin object, stuff would have to collect only in a certain direction. To get eg a cube, it would have to “know” when to stop clumping together along specific lines!