Pluto Dropped From Planet List

Posted on August 24, 2006

In a controversial decision the IAU has dropped Pluto from the list of nine planets. The decision was made at the 2006 International Astronomical Union (IAU) General Assembly. From now on there will only be eight planets. Pluto will be demoted to a dwarf planet. The IAU released the following image that shows eight planets and three dwarf planets.

Eight Planets


Space.com reports that IAU decision means there will be three different categories for objects in our solar system.
  • Planets: The eight worlds from Mercury to Neptune.
  • Dwarf Planets: Pluto and any other round object that "has not cleared the neighborhood around its orbit, and is not a satellite."
  • Small Solar System Bodies: All other objects orbiting the Sun.
  • There is a great deal of controversy over the decision but some astronomers believe there was little choice but to change the description of a planet especially following the discovery of objects larger than Pluto like 2003 UB313. The recently discovery 2003 UB313 is now one of the dwarf planets. At the time of its discovery 2003 UB313 was considered the 10th planet.

    Space.com also reported that one group of astronomers particularly upset by the decision are the New Horizon team headed by Alan Stern.
    "I'm embarassed for astronomy," said Alan Stern, leader of NASA's New Horizon's mission to Pluto and a scientist at the Southwest Research Institute. "Less than 5 percent of the world's astronomers voted."

    "This definition stinks, for technical reasons," Stern told SPACE.com. He expects the astronomy community to overturn the decision. Other astronomers criticized the definition as ambiguous.
    You can read more about the New Horizons mission here. The spacecraft will still study Pluto whether it is a planet or a dwarf planet. Pluto was discovered by Clyde Tombaugh in 1930. Some of his ashes are on the New Horizons spacecraft.

    Image: IAU