If space is a vacuum, why are planets and even stars have round shapes?

Answer 1

Space is not a vacum. The 'air pressure' is just very very low.

Planets and stars are round because the biggest force acting upon them is gravity. Gravity affects everything within its range equaly, and thus makes clumps of matter into round shapes.

Answer 2

The mathematical reason is this:

3/2 kT = GMm/R^2

Now planets were formed when they were still melting so T was the temperature needed for particles with mass (m) to melt the rock of the planet.. If particles had that amount of energy needed to reach T for melting to occur, then gravity pulling with the same Force in all directions would cause a spherical shape..

Phobos however ( one of Mars' moons) is not round because melting didn't occur when it was being formed

Using the formula you can deduce that the MASS a body needs to have ( when formed) in order to have a spherical shape in space must be equal or more than

3 x 10^21 kg

For the response below
"I disagree with the last respondant. If the body is large enough it can be pulled into spherical shape without being in a liquid state."

Basically you are saying what i'm saying.. a larger body means a larger mass..

It seems you didn't understand the formula

GMm/R^2 IS the force of gravity between a particle (m) and a massive body (M)...

Take Ceres... Is its mass less or more than 3 x 10^21 kg?


No body can gain a spherical shape if it doesn't melt first ( neglecting rotation about axis).. Everything was formed at high temperatures.. Phobos wasn't large enough ( heavy enough) to become molten

Answer 3

First of all, who said that objects in space (a vacuum) can't be round? A Vacuum just means there are no outside forces acting on that specific place.
Planets and stars have a round shape because as matter clumps together, the gravity created my this mass keeps pulling in matter towards its center, creating spherical and ellipsoidal shapes. Our Sun is a good example of this situation. The sun is made of hot gasses. They are constantly heating and expanding but at the same time, the mass of the sun creates the force of gravity that keeps pulling the gasses back towards its center. In this way, the Sun "pulses". In theory, eventually the gravitational pull of the sun will not be sufficient to keep the constantly-expanding gasses in such a tight spherical shape and it will eventually grow in size, enveloping near planets and other matter that happens to be in it's gravitational pull.
This theory also coincides with the theory that stars are always growing. The stars are classified into categories by the sizes they have accomplished, but that's a whole other lecture. :)

Answer 4

I disagree with the last respondant. If the body is large enough it can be pulled into spherical shape without being in a liquid state.

Example, Ceres belongs to the asteroid belt, and assumes a spherical shape, whereas all the smaller asteroids do not.

If the solid Earth can bulge at the equator due to rotational forces, then it surely can be affected by gravity. In fact even the moon pulls the solid Earth, making tidal differences of several inches.

I understand that even a solid body of greater than 1000 km will assume a spherical shape. I wonder if your formula applies to bodies in general - not just liquid ones.

Also Phobos and Deimos, the moons of Mars, are far too small to assume a spherical shape, like most of the asteroids and most of the smaller moons in the Solar System.

Answer 5

Large mass object (Planets, stars, moons) are the shape they are due to the gravitation pull of the object. The larger an object is the stronger the gravitational pull is all portions of the object. The gravitational pull is equal in all directions and therefore will force the mass to become ball shaped.

Most objects this size also have a rotational effect such as the earth. This rotation and the centifical forces generated by it creates a sphere shape. This is why the earth is large in circumfrance horizontally around the equator than vertically around the poles.

Hope this answers the question!

Answer 6

Vacuum does not attract matter. Matter attracted by itself adopts the more energy-effective shape which is a sphere.
We observe that vacuum sucks matter here on the Earth because of the difference in pressure created by the vacuum cleaner with regards to the atmospheric pressure created by the Earth gravity.

Answer 7

Space is not a complete vacuum but rather it has whats called microgravity, gravity causes the planet to retain it's shape but if you notice due to the rotation of the planet there is a slight buldge around teh equator.

Answer 8

Why would a vacuum have any effect on the shape of planets. Gravity will cause anything in a liquid or molten state to be spherical.

Answer 9

They are formed into a sphere by the inward pulling of their gravity. The pull is nearly equal from all angles so the resultant shape is a sphere. In truth there is a bulge at the equator of a spinning body. The faster the spin and the more flexable the material, the greater the bulge.

Answer 10

Gravity