Why are all planets and stars round?

Something to do with gravity that pulls toward the center of the star/planet so a sphere-shaped would have a surface that's equal distance to the center??? So the gravitational pull on every single surface area would be the same?

12 Answers

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  • Paul
    Lv 5
    1 decade ago
    Best Answer

    Yeah technically like that guy said before me, they're not quite perfectly round. Nothing is perfect. But he's being kind of a jerk with that answer, because they're spherical for almost all purposes.

    You're pretty much right with your assessment. Gravity pulls on everything equally, and nature doesn't like things to be unequal, so all the bumps get smoothed out because whatever is NOT in place is subject to either more or less force than the other parts.

    Here's something to add though- nature prefers the spherical shape over any other shape. This is because this shape minimizes the surface area given a particular volume- to say it another way, you get the most volume from the least area of any shape with a sphere.

    The only way things are not spherical is if they are not massive enough for gravity to be strong enough to round them into spheres, because for small objects, gravity is not the dominant force, but for large objects it is. In fact, one of the criteria for being called a planet is that the object be massive enough to have made itself spherical. This one one of the reasons why asteroids, being aspherical, are not planets.

    One more thing, going back to the fact that nature prefers spheres. In a vacuum (such as space), any fluid particle will form into a sphere naturally because like I said, this shape minimizes area and therefore minimizes energy. Nature always minimizes energy when possible. There are plenty of videos out there of people in 0g floating around and pouring out water, which then immediately form into a bunch of small spheres floating around. Pretty cool.

    Hope this helps

  • 4 years ago

    Gravity is the cause. It pulls everything together. Asteroids have such small amount of material that the total gravitational pull is not strong enough to crush it into a ball. The Earth does have enough pull for that but even so, not enough to completely round it out. Our Earth is extremely large. The hills and valleys are tiny in comparison. So tiny in fact that those features are ignored whenever the shape of the Earth is discussed. You may have a 12-inch globe of the Earth at home. Most globes have raised areas to show the mountains. If these features were actually to scale, you would not be able to feel them with your fingertips, even the highest peak, Mount Everest! At this scale, if Mount Everest dropped in altitude from its peak directly down to sea level (it doesn't), it would feel like the thickness of a single sheet of paper. A neutron star is far more massive than our sun with a surface gravity so strong that it has no hills or valleys. The entire surface is smoother than a sheet of glass.

  • Kim
    Lv 4
    1 decade ago

    That's basically correct. Star sized objects are formed through accretion of interstellar gases. These quite naturally accrete in a spheroid shape, or if under the influence of rotation, an oblate spheroid with an equatorial diameter greater than the polar diameter. Planets are no exception but undergo slightly different evolutionary processes. Our earth for example was bombarded with things like meteors and comets. Still they created enough impact heat to keep the surface molten and pliable and easily molded by gravity. The interior was molten from the beginning, not only from the pressure of gravity which causes internal heating but also from the decay of radioactive materials in its heavy iron core. Small objects such as asteroids don't generate enough gravity to round themselves and were probably a part of larger objects at one time. Even as solid and round as the earth seems, it is slightly egg shaped from its rotation. Not nearly as much as the sun, but still with a equatorial diameter 25 miles greater than the polar diameter.

  • DLM
    Lv 7
    1 decade ago

    That's the most natural way for gravity to coalesce. If matter is attracted to matter via gravity, matter will pull itself to a central point. If there is enough matter (and because gravity is based on amount of mass, which is a measure of matter) the natural shape is to be spherical.

    As the first answerer stated, "they are not" is also partially true. The earths circumference at the equator is larger than around the poles. The spin of the stars and planets slightly causes a bulge or oblatness at the equator. (Jupiter is the best example of this, for it has a ten hour "day," meaning it is spinning incredibly fast.

    The best example i have to explain this is to think about yourself spinning around in circles. If you're going fast enough, it becomes increasingly difficult to keep your arms tight to your sides. Your inertia wants to throw your arms outward. Well, a planets spinning makes its equator bulge slightly for the same reason. Obviously, this effect is noticeable at the poles, because movement of rotation at the equator is a greater velocity than at the poles.

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  • 1 decade ago

    Gravity Wants everything to be a sphere becuse all points are just as far form the surface as another and the planets and stars that rotate get "fatter" they flatten at the poles Like Example Saturn You can see this in photos of it.

  • Anonymous
    1 decade ago

    A sphere is the most compact way to arrange mass in 3d, so that's the shape planets and stars settle into. However, most planets and stars aren't perfectly round. The centrigufal force caused by their rotation causes them to bulge slightly at the equator. Earth is no exception, we're about 21km wider around the equator than we are pole to pole.

  • 1 decade ago

    I guess so. because atoms have gravitational pull of other atoms. if a planet was a perfect sphere, then it would have perfect distribution of gravity on its surface.

    IF the planet is made up of the same material, then it would be perfect because atoms have different gravitational pull, depending on how many electrons or neutrons it has.

  • Ken E
    Lv 6
    1 decade ago

    Actually they are not. The Earth is more egg shaped than perfectly round or sphere shaped.

  • 1 decade ago

    Nearly spherical yes. Due to hydrostatic equilibrium, i.e., gravity. Stars, planets and dwarf planets such as pluto have enough mass to form this way, asteroids do not.

    http://ircamera.as.arizona.edu/NatSci102/text/hydr...

  • 1 decade ago

    They are pinky and the brain. Just kidding man, you i am confused just as much as you.

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