• Question: Why isn't Pluto considered a planet?

    Asked by OneDirection to Heather, Helen, Hugh, Jane, Julian on 11 Nov 2014. This question was also asked by FelicityP, ZAK.T!, SHARON ROSE, Srutika, Chrissypops.
    • Photo: Jane MacArthur

      Jane MacArthur answered on 11 Nov 2014:

      Pluto was re-defined as a dwarf planet, as it did not satisfy the condition of having sufficient gravity to “clear its orbit” – ie, sufficient gravitational force to either attract objects to collide with itself, or to push them out of its orbit off into outer space.

      The IAU (International Astronomical Union) made this part of the definition of a planet in 2006, along with ‘must orbit the sun’ and ‘must be big enough for gravitational forces to make it into a round sphere’.

    • Photo: Julian Onions

      Julian Onions answered on 11 Nov 2014:

      Pluto has always been an oddity in the list of planets.
      It orbits outside Neptune most of the time, but sometimes comes inside. Its on a very elliptical orbit, and is very tiny (its smaller than our moon).

      So an astronomer, Mike Brown, from Caltech (where Sheldon Cooper works apparently) did a long examination of the outer solar system, in the early 2000s and started to find other planets that were similar size to Pluto also going around in orbits like it. One was even bigger than Pluto.
      So what to do, should we have another 10 planets added in, or redefine what a planet is. So we decided to go with the latter, and made a special category for Pluto, Ceres, MakeMake, Eris and so on called dwarf planets.

      Mike Brown has written a very interesting and entertaining book about it called
      “How I killed Pluto, and why it had it coming!”
      http://www.amazon.co.uk/How-Killed-Pluto-Why-Coming/dp/0385531109

    • Photo: Helen Johnson

      Helen Johnson answered on 11 Nov 2014:

      What Jane said – she got there quickly! 😉
      There was a meeting in 2006 which decided that for something to be a planet, it must a) be able to pull itself into a sphere under gravity (not like a lumpy asteroid or comet) and b) have cleared its orbit of other objects.
      There are lots of Pluto-sized objects out past Neptune – these ‘dwarf planets’ – and so they decided they needed to either make ALL of these planets, or demote Pluto 🙁

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