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Last Post 3/25/2011 11:14 AM by  Kris Sigsbee
ongoing pluto debate
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3/25/2011 6:22 AM

    Ms. West's class would like to know if the planet pluto might be re-classified as a planet, now that its known to have three orbiting moons instead of just one? thank you very much..

    Tags: Pluto, Kuiper Belt, Major Planet, Sedna, Minor Planet

    Kris Sigsbee

    Basic Member

    Basic Member

    3/25/2011 11:14 AM

    Hello Ms. West's Class!

    I'm not sure that there really is much of an ongoing debate about Pluto's status among scientists any more.

    Technically, Pluto is still classified as a planet, but it is only a "minor planet" or "dwarf planet." The number of moons Pluto or any of the other objects in the solar system has does not directly enter into whether or not an object can be classified as a a planet or not. For example, Mercury and Venus do not have moons, but in modern times there never has been any question as to whether or not Mercury and Venus are major planets. One the other hand, we know that there are asteroids that have other asteroids orbiting around them like moons. If having a moon was the only criteria for calling something a major planet, then we would have to re-classify all of these asteroids as major planets and demote much larger objects like Mercury and Venus to minor planet status.

    One of the two main conditions for an object in the solar system to be considered a major planet is that it must have cleared its orbit of other objects. Even though Mercury and Venus do not have moons, there is not really a lot of other debris or large objects along their orbits. Mercury and Venus are also massive enough to be round under their own gravity, which is the second condition for being classified as a major planet. Therefore, Mercury and Venus are major planets even though they do not have moons. Pluto's orbit crosses the orbit of Neptune, and other trans-Neptunian objects, like Sedna, have been discovered that are nearly as large as Pluto. Pluto definitely has not cleared its orbital neigborhood of other objects, so it does not qualify to be a major planet.

    Even though the composition of an object does not really enter into the current official definition of major and minor planets, it is also something to think about in Pluto's case. We usually split the major planets in the solar system into two categories - the rocky planets and gas giants. Mercury, Venus, Earth, and Mars definitely belong in the rocky or Terrestrial planet category. Jupiter, Saturn, Neptune, and Uranus definitely belong to the gas giant or Jovian planet category. Pluto has never quite fit into either of these categories because it is much too small to be a gas giant and its density is much less than that of the Terrestrial planets. In fact, Pluto is believed to have more in common with Kuiper Belt objects than it does with the Terrestrial planets.

    You can read more about asteroids with "moons" here:



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