• Question: How come Pluto is not exactly a planet

    Asked by bobo to Gioia, Iain, Jo, Leo, Mariam on 23 Jun 2010 in Categories: . This question was also asked by jogliquorandlime, issiphee1.
    • Photo: Gioia Cherubini

      Gioia Cherubini answered on 14 Jun 2010:

      Because it’s too small! When in 2006 the term “planet” was defined by the International Astronomical Unit, the definition excluded Pluto as a planet and it’s now under the definition of “dwarf planet”.

    • Photo: Joanna Watson

      Joanna Watson answered on 14 Jun 2010:

      Hey Bobo, I’ve wondered that too. I saw in the news that it had been downgraded and wasn’t a planet anymore, but I don’t know very much about astronomy so I don’t know why they did it. I hope one of the other scientists has given a good answer because then I can learn something too!

    • Photo: Mariam Orme

      Mariam Orme answered on 20 Jun 2010:

      I don’t know much about planets, but I think Pluto was downgraded because it’s so small and because its orbit is a bit erratic (the planets revolve around the sun, while other heavenly bodies don’t – for example Earth’s moon revolves around Earth – so if Pluto doesn’t revolve around the sun properly, it can’t be counted as a planet).
      Hopefully one of the other scientists will give you a better explanation!

    • Photo: Leo Garcia

      Leo Garcia answered on 20 Jun 2010:

      It’s down to how scientists define what constitutes a ‘planet’. These are:

      1 The object must be in orbit around the Sun.
      2 The object must be massive enough to be a sphere by its own gravitational force. More specifically, its own gravity should pull it into a shape of hydrostatic equilibrium.
      3 It must have cleared the neighbourhood around its orbit.

      And Pluto does not satisfy the third point. There was, as you might have read in the news, plenty of debate over the decision to remove Pluto’s ‘planet’ status – and you can read more about it here:


    • Photo: Iain Moal

      Iain Moal answered on 23 Jun 2010:

      A celestial body counts as a planet because of its size. Because astonomers realised that if they counted Pluto as a planet, then they should count other things as planets too, they decided to declassify pluto as a planet.