Nearly every schoolchild who has grown up in the latter part of the 20th century has been taught that Pluto was not only the furthest planet from the Sun, but also the smallest in our solar system. For now, science books and astronomical charts will have to be revised due to a decision made by the International Astronomical Union (IAU) during a meeting in 2006. The decision, which included eight days of zealous debate and a nearly split vote, resulted in Pluto being downgraded to a dwarf or minor planet status. It’s not so much that it was intentionally removed from the classification, but rather that the IAU outlined a new definition of what constitutes a classic planet, and Pluto no longer meets the standards. Now, the list of classic planets in our solar system includes Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune.
The new standards dictate that, to be a classic planet, an object must be a celestial body orbiting the sun, with enough mass to allow its gravity to form into a round shape. In addition, must be gravitationally dominant enough to prevent anything of similar size, barring its own satellites, from floating around the planet. The term that the IAU used was “clearing the neighborhood of its orbit,” which was one of the most important aspects of the definition that was debated.
Although Pluto is now a dwarf planet, it also belongs to a third class of “lesser” objects that orbit the sun. The term used to describe it is “smaller solar system body,” which can also be used to refer to comets and asteroids. One would think that, because Pluto was downgraded from being the smallest of the planets, it would at least be the largest of the dwarf planets. This is not the case, as UB313, an icy object found further out in the solar system, is larger.
Regardless of the hot debate that has been raging since the 1990s, the National Aeronautics and Space Administration (NASA) felt that Pluto rated further study. In early 2006, NASA dispatched the New Horizons craft to further investigate the dwarf planet, and it is expected to arrive sometime in 2015. Some astronomers who were unhappy with the IAU’s decision believe that the decision to downgrade it will be overturned by then.
The conference, which included approximately 2,500 astronomers representing 75 countries, cast the vote amidst strong opposition. Some astronomers lament the fact that only 5% of the world’s astronomers took part in the decision to change the definition and believe that it will not stand as a result. Strong opposition came from the family of Clyde Tombaugh, the American who discovered Pluto in 1930 in Flagstaff, Arizona, as well as many other astronomers worldwide. For some, the decision was the correct one, as they believe that the original classification watered down the definition of a planet.