• Question: what is happening with pluto? Is it a dwarf planet? What defines a planet?

    Asked by 08cgrr to Donna, Jo, Mark, Stuart, timcraggs on 14 Jun 2010 in Categories: . This question was also asked by abigail9742, allstarsam7399, mouthgiddy, emilyx.
    • Photo: Mark Lancaster

      Mark Lancaster answered on 13 Jun 2010:

      Poor old Pluto – when I was a lad it was a bona-fide planet and now it isn’t. You are a planet if: 1) You’re in orbit around the Sun, 2) You’ve enough mass and you’re nearly round and 3) You don’t have loads of debris around you. Pluto failed (3) since it’s in the middle of the Kuiper belt (lots of orbiting rocks) at the edge of our solar system – if you satisfy (1) and (2) you get to be a “dwarf star” which Pluto is. So the solar system has 8 planets and 5 dwarf planets (including Pluto). It’s a rather arbitrary definition and has only been around since 2006 – it gives the “International Astronomical Union” the opportunity to have lots of meetings…

    • Photo: Joanna Buckley

      Joanna Buckley answered on 13 Jun 2010:

      Astronomers have decided that Pluto is no longer unique or big enough to be called a planet. Faced with loads of Pluto-like stuff in space, they were forced to dde-classify it. It’s a bit like letting someone into a club with trainers on. When you let one in, then it’ll be difficult not to let everyone in wearing trainers. Did I just mention Pluto and trainers in the same sentence?

      For thousands of years there were 6 planets then 1781 it was 7 and in 1846 it was 8. Then in 1930 someone found litle Pluto. My favourite Pluto fact is due to it’s naming.The name was chosen from thousands submitted and rather than being named after Mickey Mouse’s faithful friend, Pluto the cartoon character was created a year later in 1931. Am I sad knowing that?

      It’s not that unique because there a lot of objects of a similar size and distance away from the sun. It’s a bit less than a full-on planet but more than an asteriod, hence a dwarf planet. Since 2006 when Pluto was dropped from the planets, there are three things which define a planet.

      1. It must orbit the Sun.
      2. It must be large enough for gravity to mould it into a nearly spherical shape.
      3. It must have cleared the neighbourhood around its orbit.

      Do you have a pneumonic to remember the planets? Mine was…

      My Very Energetic Mother Just Swam Under North Pier

      Now Pier doesn’t exist and that sentence makes no sense! Can you think of a funny new one without Pluto?

    • Photo: Donna MacCallum

      Donna MacCallum answered on 14 Jun 2010:

      When I was at school Pluto was still a planet and we had to learn the names and order of the 9 planets of our solar system. However, according to sources, the International Astonomical Union altered the classification of Pluto from planet to dwarf planet in 2006 (there are another 2 dwarf planets in our solar system).

      Apparently a planet must be the dominant gravitational body in it’s orbit – which Pluto isn’t – therfore, Pluto was reclassified!

    • Photo: Tim Craggs

      Tim Craggs answered on 14 Jun 2010:

      These are excellent questions!

      Perhaps Mark can answer some of them. But I have a question for you: What is the difference between a planet, a comet and an asteroid?

      Tim

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