Physical Cosmology

Why was Pluto demoted?
Answered by Science Channel
  • Science Channel

    Science Channel

  1. Pluto is one of the most interesting objects in our solar system. Like our solar system's planets, it has enough mass to maintain a nearly round shape. It's dense and rocky like Mercury, Venus, Earth and Mars. On the other hand, it's smaller than all of the other planets, even smaller than the Earth's moon. Pluto orbits the sun in an ellipse, like everything else in the system. Pluto's orbit, however, is exceptionally elliptical when compared with the planets. Its orbit also is a bit erratic. All the planetary orbits line up in the same plane except Pluto's, which is 17 degrees off-angle. It also crosses Neptune's orbit, which some scientists say makes it not a planet at all. Another quirky characteristic of Pluto is that one of its moons, Charon, is nearly half of Pluto's size, a phenomenon unmatched in the solar system.

    Because of all of these factors, Pluto has inspired a long-running debate among astronomers as to its designation as a planet. On August 24, 2006, the International Astronomical Union (IAU) passed two resolutions in hopes of ending the controversy. Resolution 5A defines a planet as an object in space that orbits the sun, is a nearly round rigid body and clears the neighborhood around its orbit. All other objects orbiting the sun are either dwarf planets or small solar-system bodies. Resolution 6A names Pluto as a dwarf planet.

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