August 24, 2006, is a day that will stand out in the minds of science teachers and planetarium fans worldwide. That's the day when Pluto lost its status as a planet, forcing a mad rush of revised lesson plans, new textbook printings, and incorrect trivia answers.
Pluto's reclassification resulted from shifts in standards within the scientific community. The International Astronomical Union (IAU), an international organization in charge of naming practices in space, held a vote on adopting new criteria to dictate what makes a planet. After several vigorous debates, new criteria were voted through by a majority of the IAU's members. A planet was subsequently defined as a celestial body that orbits the sun, exceeds a certain threshold of generating its own gravity, and has cleared the orbit of its immediate neighborhood.
Although Pluto passes the planet test for the first two criteria, it fails on the third. Pluto has not cleared out its own neighborhood. In fact, it's located in the middle of a large field of space debris called the Kuiper belt. The Kuiper Belt is located beyond Neptune, and it has several celestial bodies of comparable size to Pluto. Had the competing argument won out in the 2006 IAU debate, these bodies would be considered planets along with Pluto, and the Solar System would have 12 planets.
We all know what actually happened, however. Pluto was kicked out of the planet club and a new classification was invented: the "dwarf planet." Dwarf planets refer to any celestial body that fulfills the first two criteria of the IAU planet rules, but not the third. Now, these dwarf planets have only two paths back to astronomic credibility: either several asteroids or other rocks crash into them, allowing them to increase their overall mass and regain planet status, or they wait until the scientists of the IAU change the definition of what constitutes a planet again. Until then, Pluto stands as the sole example of a celestial body that experienced the highs of being a planet and the lows of being on the outside looking in.
對於全球的自然科學老師和星象儀迷而言，2006年8月24日是一個教人難忘的日子，因為這一天，冥王星正式從行星中除名，他們必須緊急修改教學計畫、更新教科書，並且修正繁瑣的試題解答。
冥王星之所以被重新歸類，起因於科學界重新界定了行星的定義。國際天文聯會（負責太空命名的國際組織），針對是否要採行認定行星的新標準，舉辦了一次投票。經過無數次的激辯之後，多數會員投票支持新的行星標準。他們最後將行星定義為：繞行太陽運轉的天體，必須能夠自己產生足夠的重力，並且清除鄰近軌道的其他天體。
冥王星雖然符合前兩項條件，卻不符合第三項，冥王星並沒有清除鄰近的天體。事實上，冥王星正位於一大片由太空碎片組成的古柏帶之中。古柏帶位於海王星之外，內部還有幾個與冥王星大小相當的天體。假如在國際天文聯會2006年的爭辯中，另一方的論點能夠勝出的話，這些天體會和冥王星一起並列為行星，太陽系就會有12個行星。
不過顯然事實並非如此，冥王星被踢出行星之列，並且設立了一個新的類別，叫做「矮行星」。只要符合國際天文聯會所制訂的前兩項條件，但不符合第三項的天體，就被稱為矮行星。如今，矮行星若要重拾其天文地位，唯有二途：一是必須被幾個小行星或岩石撞擊，增加它們的整體質量，才能重回行星地位；二是等待國際天文聯會的科學家們再次修改行星定義。屆時，冥王星將是唯一一個經歷過躋身行星的高峰，也嚐過被拒於門外的低谷這兩種大起大落的天體。
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