• Question: Why is Pluto considered a dwarf planet and then not part of the solar system?

    Asked by conn207 to Antonia, Douglas, Hugh, Matt, Tom on 14 Jun 2010 in Categories: . This question was also asked by hannah0090.
    • Photo: Matthew Hurley

      Matthew Hurley answered on 13 Jun 2010:

      Pluto isn’t in the solar system? I’m sorry conn207 – I don’t know.

    • Photo: Tom Hardy

      Tom Hardy answered on 14 Jun 2010:

      As with most areas of science there is a Society or Committee that governs and regulates the field. The Planetary Society and the International Astronomical Union are two such groups.

      They have set out some rules that decide what counts as a planet, “a “planet” is now defined as a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighbourhood around its orbit.”

      Don’t worry, I don’t understand all of that either but the key thing is that Pluto does not do all of those things mentioned above so it is NOT technically a planet so does not fall within out solar system.

      I’ve had to take the next bit from the International Astronomical Union website, but I wanted you to have the proper description of what a dwarf planet is so here goes:

      “A dwarf planet, according to the new definition, is a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, (c) has not cleared the neighbourhood around its orbit, and (d) is not a satellite.”

    • Photo: Douglas Blane

      Douglas Blane answered on 14 Jun 2010:

      The problems began when astronomers started discovering a whole load of rocky objects out at the far reaches of the solar system. One of these, called Eris, is bigger than Pluto.

      Then the International Astronomical Union decided in 2006 to define a planet for the first time. If they went for a definition that included Pluto it would also include a lot of these other objects. So you’d have had dozens of planets.

      They decided instead on a definition that wouldn’t include Pluto or any of the others. And that they would invent a new name for them – dwarf planets.

      The dwarf planets are still part of the solar system, though, because that includes everything held by the sun’s gravity. Even out beyond Pluto the sun’s gravity is still strong enough to stop things flying off into space

    • Photo: Antonia Hamilton

      Antonia Hamilton answered on 14 Jun 2010:

      I’m not an expert on planets, but I think the problem is that there are lots of pluto-sized rocks out there, and so it wasn’t fair to consider Pluto a planet but not any of the others. So poor Pluto got demoted.

Comments