Ceres and Pluto: Dwarf Planets as a New Way of Thinking about an Old Solar System

The decision by the International Astronomical Union (IAU) in 2006 to define the terms “planet” and  “dwarf planet” has caught the attention of the public and students from grade school to graduate school.  The IAU’s decision has not changed the structure of the solar system; it has merely presented a  different way of classifying the bodies that make it up. Planets are the Greek word for “wanderers” that  were known as lights that moved in the sky. This middle school activity, developed for NASA’s Dawn  mission, utilizes a researched-based instructional strategy called direct vocabulary instruction to help  students understand the new definitions of planet and dwarf planet.

Many of us have grown up with an understanding that our solar system is comprised of remnants from  its early formation 4.5 billion years ago, primarily: bodies such as the Sun, planets, asteroids and  comets; gas and dust, as well as a large volume of space. Many school children have learned the  names and locations of the planets relative to the Sun using a mnemonic such as My Very Exceptional  Mother Just Sat Upon Nine Porcupines (Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus,  Neptune, Pluto). The last of these bodies to be discovered, Pluto, has recently been reclassified as a  dwarf planet. Pluto’s reduced status has even resulted in a new term as we enter 2007: someone or  something has been “Plutoed” if that person or thing has been downsized in its prominence.

However, this is only part of the story. The new category of dwarf planet has brought attention to the  largest body in the asteroid belt (a region of space lying between Mars (1.5 AU) and Jupiter (5.2 AU),  where the great majority of the asteroids are found.), Ceres, one of the two targets for NASA’s Dawn  mission, which also has been reclassified as a dwarf planet. This has led Dawn Co-Investigator Dr.  Lucy McFadden to coin the term “Ceresed” to mean someone or something that has been promoted or
upgraded in its prominence.

The way objects are classified can change over time. For instance, Ceres was considered a planet for
nearly fifty years before it was “demoted” to an asteroid or minor planet about 150 years ago. Now
Ceres has been “promoted” to a dwarf planet. See James Hilton’s essay on this topi
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The publicity generated by the IAU in 2006 has created a teachable moment for schools around the  world to consider the definitions of “planet,” “dwarf planet,” and “asteroid” with the potential for a better  understanding of our solar system and how discoveries and our changing understanding over time  necessitates a change in the language we use to talk about it. This activity will engage students in  these new definitions through the use of direct vocabulary instruction. For a more complete discussion  of this instructional strategy, refer to Building Background Knowledge
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