• Question: how do planets look spherical

    Asked by 16hmkl to Kanta on 21 Sep 2017.
    • Photo: Kanta Dihal

      Kanta Dihal answered on 21 Sep 2017:

      I’m not an astrophysicist, but this is what I learned somewhere along the way. In order to be called a planet, a thing that floats around in space has to meet three criteria:
      1) it must orbit the Sun or another planet
      2) it must be massive enough for its own gravity to make it spherical
      3) it has cleaned its neighborhood of smaller objects.
      Number 3) is why Pluto is not a planet. Number 2) is what we’re looking for here: a planet is made of enough mass that gravity starts pulling from the outside of the planet to the centre. Gravity pulls equally hard on all sides, so the material the planet is made of becomes a nice and smooth sphere.

      BUT. Because planets rotate, they’re not *exactly* spherical. A rotating sphere bulges out at the equator, so Jupiter in particular is a bit thicker in the middle, due to the ‘centrifugal force’.