• Question: Why are all planets spherical? Why are they not any other shapes??

    Asked by farheen2904 to Anil, Blanka, Cees, Emma, Mike on 22 Jun 2012.
    • Photo: Michael Cook

      Michael Cook answered on 22 Jun 2012:

      I’m not much of an astrophysicist, so I can’t help much here, but I think it’s to do with gravitational forces. As planets form, all the tiny bits of matter that make up the planet collect together and create a gravitational force (the more massive something is, the greater its gravitational pull). This force pulls equally in all directions – that’s why gravity is the same in Australia as it is in England or Africa (approximately!).

      By pulling in all directions equally, the planet is shaped into a rounder shape (not perfectly round, as you say – spherical) because in a sphere any point on the surface is a similar distance to the centre.

      I wish I knew more about this, actually! Space is really fascinating.

    • Photo: Blanka Sengerova

      Blanka Sengerova answered on 24 Jun 2012:

      As Mike says, I have a feeling it’s to do with the gravitational force – anything too big would have too much of a pull on it and be crushed under its own weight. I thought a nice brief explanation was here: http://www.pa.msu.edu/sciencet/ask_st/031198.html

    • Photo: Emma Trantham

      Emma Trantham answered on 26 Jun 2012:

      I agree with Mike and Blanka – it’s because of gravity pulling in all directions exactly the same amount.

      I think the reason that the earth is not completely spherical is because it is rotating.

      If you imagine tying a ball to a piece of string and then swinging it round in the circle you will feel the ball pull away from you (in fact if you let go it will fly off!). The forces that are acting on the ball are also acting on the earth as it spins round and that makes the earth slightly fatter around the equator and slightly more flat at the poles.

      I assume the same is true of other planets but I don’t know. Perhaps it depends on the speed they rotate at?