Pluto was downgraded from being a planet to a dwarf planet or minor planet due to a new definition of what constitutes a classical planet by the International Astronomical Union. The new standards require a celestial body to be a round shape and gravitational dominant enough to keep anything of similar size from floating around it. Pluto now belongs to a third class of “minor” objects that orbit the sun. Some astronomers believe the decision will be reversed, while others think it was the correct one. NASA sent the New Horizons spacecraft to investigate Pluto further, and it is expected to arrive in 2015.
Almost every schoolboy growing up in the latter part of the 20th century was 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) at a meeting in 2012. The decision, which included eight days of zealous debate and a nearly split vote, led to Pluto being downgraded to dwarf or minor planet status. It’s not so much that it was intentionally removed from the classification, but rather that the IAU has outlined a new definition of what constitutes a classical 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 classical planet, an object must be a celestial body orbiting the sun, with enough mass to allow its gravity to assume a round shape. It also needs to be gravitational dominant enough to keep anything of similar size, excluding its own satellites, from floating around the planet. The term used by the IAU was “to clean up the vicinity of its orbit”, which was one of the most important aspects of the definition that was discussed.
Although Pluto is now a dwarf planet, it also belongs to a third class of “minor” objects that orbit the sun. The term used to describe it is “smallest solar system body,” which can also be used to refer to comets and asteroids. One would think that since Pluto has been downgraded from being the smallest of the planets, it would at least be the largest of the dwarf planets. This isn’t the case, as UB313, an icy object found further out in the solar system, is larger.
Regardless of the heated debate that has raged since the 1990s, the National Aeronautics and Space Administration (NASA) felt that Pluto considered further study. In early 2006, NASA sent the New Horizons spacecraft to investigate the dwarf planet further, and it is expected to arrive in 2015. Some astronomers who were unhappy with the IAU’s decision believe the decision to downgrade it will be reversed by then.
The conference, which included some 2,500 astronomers representing 75 countries, voted amid strong opposition. Some astronomers complain that only 5% of the world’s astronomers took part in the decision to change the definition and believe it won’t hold up 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 from around the world. For some, the decision was the correct one, as they believe the original classification watered down the definition of a planet.
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