Why are planets spherical?


We take it for granted, but why are planets spherical and not rectangular? Or cylindrical? Not just the planets but most celestial bodies are spherical and any curious kid would like to know the simple science behind this.   

Turns out, it is because of gravity. Planets and stars have a very strong gravitational pull because these things are huge, like really huge. This gravitational force tries to pull pretty much everything to the center and as you can imagine, if everything needs to be as close to the center as possible, it has to be a sphere.

Cubical Planets (Courtesy: AAM)
An example might help convince ourselves. Lets say we set out to construct a tall building. To keep it standing, we need to build a very strong foundation with pillars supporting the structure to counter act the gravitational pull. If the foundation is not strong enough, gravity would pull the building down and bring it closer to the center of the planet (in other words the building collapses). This is pretty much what happens on a much larger scale with all planets and stars. Lets suppose that a planet started off as a cube. Now the corners of the cube can be seen as really tall buildings (a rock building hundreds of miles long). No foundation can ever be strong enough to support such a tall building and gravity would eventually pull these corners towards the center thus converting the cube into a sphere. 

An interesting thing to note here, is that not all celestial bodies are spherical. The smaller ones like asteroids which only span a few miles remain in various bizarre shapes because the gravitational pull within these bodies is not strong enough to overcome the mechanical strength. Below is an image of Phobos, the moon of Mars. It is not spherical because it does not have enough gravitational pull to squish itself into a sphere. 

Phobos: Mars moon is not big enough and thus doesn't have enough gravitation pull to be squished into a sphere
Image Courtesy: NASA
Final note, a little off topic - planets are almost spheres but not exactly spherical. They bulge a little bit at the center (the equator) which is believed to be because of the spinning.

Did the simple science for kids in this post make you a little smarter?
If you have any ideas / questions you want to learn about, please leave a note in the comments section. 
Would be glad to know if there are any inaccuracies in the article.

1 comment:

  1. Einstein theory on gravity is different.. Gravity is not a force..space warps

    ReplyDelete